MarLIN

information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Basking shark (Cetorhinus maximus)

Distribution data supplied by the Ocean Biogeographic Information System (OBIS). To interrogate UK data visit the NBN Atlas.

Summary

Description

The basking shark is the largest fish in British waters growing up to a maximum of 12 m long, its size is the most obvious distinguishing feature. Smaller specimens can be identified by the stout body, moon-shaped tail and the five long gill slits that run from the back behind the head to round under the throat. The gill arches carry a high number of gill rakers that act as a filter to catch the plankton upon which the fish feeds. The basking shark is slate grey to black dorsally, lighter ventrally, with light patches under the snout and on the belly. Filtered water is expelled through the greatly enlarged gill slits. Basking sharks generally live in open waters but migrate towards the shore in summer, when they can be seen 'basking', i.e., swimming slowly at the surface with the mouth wide open with the snout and dorsal fin visible above water.

Recorded distribution in Britain and Ireland

Usually sighted in the summer in areas such as western Ireland, western Scotland, the Clyde, the central Irish Sea, approaches to the Bristol Channel and the western English Channel.

Global distribution

Circum-globally distributed in temperate to boreal seas (Sims, 2008) and sighted occasionally in the tropics (e.g. the Galapagos and Hawaii).

Habitat

Pelagic and migratory. Often observed feeding along tidal fronts on the continental shelf and shelf edge.

Depth range

0 - 1264 m

Identifying features

  • Britain's largest fish, maximum length 10 - 12 m long.
  • Dorsal surface grey to black in colour, undersides paler.
  • Five long gill slits running from the back of the head to below the throat.
  • Long snout, especially in juveniles.
  • When feeding, characteristically cruises near the surface of the water with mouth gaping.

Additional information

Cetorhinus maximus live either solitarily or in shoals of up to approximately 400 individuals.

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Biology review

Taxonomy

PhylumChordata
ClassElasmobranchii
OrderLamniformes
FamilyCetorhinidae
GenusCetorhinus
Authority(Gunnerus, 1765)
Recent Synonyms

Biology

Typical abundanceLow density
Male size range10 - 12 m
Male size at maturity5 - 7 m
Female size range10 - 12 m
Female size at maturity8.1 - 9.8 m
Growth formPisciform
Growth rate0.4 m/year
Body flexibilityHigh (greater than 45 degrees)
MobilityMobile, Muscular contraction (body length), Swimmer
Characteristic feeding methodSearcher / forager, Swimming
Diet/food sourceHeterotroph, Planktotroph
Typically feeds onCalanoid copepods and other zooplankton.
SociabilityGregarious
Environmental positionPelagic, Water column
DependencyNo information found.
SupportsHost
Is the species harmful?No

Biology information

Feeding

The basking shark (Cetorhinus maximus) is an obligate ram feeder, using its gill rakers to filter zooplankton from the water. In the UK, its preferred prey species are likely to include Calanus helgolandicus (Speedie, 1999) and Calanus finmarchicus (Sims et al., 1997), although other species of calanoid crustacean may be preferred outside of the UK. The analysis of stomach contents has shown that, while copepods are the dominant prey species, fish eggs, fish larvae, cirripede and decapod larvae are also consumed (Matthews & Parker, 1950).

Parker & Boesman (1954) suggested that the basking shark would shed its gill-rakers during autumn and go through a period of winter hibernation, triggered by low prey abundance and the inability to derive enough energy for growth. However, arguments opposing this idea have been put forward by Sims (1999) and Sims et al. (2003). Current evidence suggests that the basking shark can utilize the low concentrations of zooplankton (down to ca 0.5-0.6 g m-3) found outside summer months (Sims, 1999; Sims et al., 2003). For example, a study on Cetorhinus maximus by Sims et al. (2003) recorded diving activity (down to between 750 and 1000 m) and long range movement during winter in some individuals, indicating that they do not remain motionless at the seabed. Therefore, it is likely that gill-raker shedding is not universal to all individuals and shedding and regrowth may happen asynchronously (Sims, 2008).

Prey detection

At small spatial scales, Sims & Quayle (1998) suggested that (as is the case with sea birds; Nevitt et al., 1995) Cetorhinus maximus may use olfactory cues to detect dimethyl sulphide, which is released by phytoplankton when they are grazed on by zooplankton. Basking sharks may also use electroreception via their electrosensory pores (ampullae of Lorenzini) to detect the electrical signals given off by the muscle movement of prey (Sims & Quayle, 1998). The ampullae of Lorenzini are concentrated around the snout (Kempster & Collin, 2011) suggesting their use to detect zooplankton distribution. This is supported by the reduced swimming speed of the basking shark during feeding (Sims, 2000), which would allow the shark to detect small-scale changes in prey activity (Kempster & Collin, 2011) whilst reducing drag-induced energetic costs.

Over various spatial and temporal scales, an adult basking shark demonstrates foraging patterns known as ‘Lévy walks’. This is the best search strategy to enable foraging on patchily distributed prey, whereby the predator is effectively a probabilistic or 'blind' hunter (Sims et al., 2008). The basking shark is also known to exhibit ‘yo-yo diving’ (diving from surface to depth repeatedly with little time at the top or bottom), which is an additional foraging strategy used more commonly in summer months when prey distribution is more patchy (Shepard et al., 2006; Witt et al., 2014).

Growth

Growth rates have been estimated at 0.4 m per year (Pauly, 1978; 2002), which is slower than initially assumed because some basking sharks lose their gill-rakers and cease feeding during the winter. However, some individuals show behaviour consistent with foraging during the winter (Sims et al., 2003), so actual growth rates may be slightly higher than estimated by Pauly (1978; 2002) (see Sims, 2008 for review).

Sociability

The basking shark is solitary predominantly but aggregations of 6 - 12 sharks can occur in areas of dense zooplankton abundance (Speedie, 1999), and in rare circumstances, groups may contain hundreds of individuals (Skomal et al., 2004). Aggregations of Cetorhinus maximus engaged in courtship behaviour are associated with thermal fronts. Sims et al. (2000) suggested that these areas of rich prey concentration are important to the species as breeding grounds.

Habitat preferences

Physiographic preferencesOpen coast
Biological zone preferencesOceanic, Pelagic
Substratum / habitat preferencesNot relevant
Tidal strength preferencesModerately Strong 1 to 3 knots (0.5-1.5 m/sec.), Weak < 1 knot (<0.5 m/sec.)
Wave exposure preferencesNot relevant
Salinity preferencesFull (30-40 psu)
Depth range0 - 1264 m
Other preferencesNone
Migration Pattern

Habitat Information

Migration

Migration in the basking shark (Cetorhinus maximus) is not fully understood.  However, a number of patterns have been identified. Firstly, the basking shark can travel large horizontal distances, such as across the Atlantic in extreme cases. For example, one tagged basking shark travelled a distance of 9,589 km, moving from the Isle of Man, UK to Newfoundland, Canada in 82 days (Gore et al., 2008). In a study of Cetorhinus maximus by Skomal et al. (2009), similarly large distances (approx. 9000 km) were estimated for the tracks of sharks moving southwards in the western Atlantic from Cape Cod, Massachusetts as far south as the mouth of the Amazon river. Southward migration can be justified by the need for food after seasonal declines in zooplankton abundance in the North. However, long distance transequatorial migration (Skomal et al., 2009) seems too energetically costly to be for feeding alone. It is thought that the stable environment of the tropics may provide the conditions required for reproduction (e.g. mating, gestation or nursing grounds).

In the UK, basking shark migration is relatively ambiguous but there is some evidence for a north to south seasonal migration in response to changing thermal conditions, with northerly movement in early summer and southerly movement later summer/autumn (Sims et al., 2003; 2008). In the UK, the basking shark may also undertake a seasonal west to east migration.

Diving Behaviour

In addition to horizontal movements, the basking shark also exhibits vertical migrations to a range of depths. Evidence indicates that Cetorhinus maximus commonly dives to depths within the range of 80 to 500 m (Francis & Duffy, 2002; Gore et al., 2008). The plasticity in diving patterns is thought to be a response to changes in prey abundance, although this has not been observed directly (Gore et al., 2008; Sims, 2008). The deepest recorded dive to 1,264 m was achieved by an 8.0 m female during her migration across the Atlantic (Gore et al., 2008). Cetorhinus maximus was also recorded at similar depths (up to 904 m) in New Zealand (Francis and Duffy, 2002) and the Bay of Biscay (between 750- 1000 m)(Sims et al., 2003).

Vertical basking shark migrations have been correlated with environmental variables such as tidal phase, lunar cycle and time of day (Shepard et al., 2006). In their study, the maximum depth reached was 192 m. Of all vertical movements, most studied are the diel vertical migrations (DVM) exhibited by the basking shark in response to the DVM of its zooplanktonic prey (Sims et al., 2005; Shepard et al., 2006; Witt et al., 2014). Sims et al. (2005) found that the whether the sharks exhibited normal or reverse DVM depended on the water mass under study. In deep stratified water, sharks assumed normal DVM coinciding with the DVM of the zooplankton. In tidal fronts, sharks exhibited reverse DVM that reflected the movement of copepod prey to avoid their planktonic predators (e.g. chaetognaths).

Life history

Adult characteristics

Reproductive typeGonochoristic (dioecious), Sexual
Reproductive frequency < Biannual
Fecundity (number of eggs)2-10
Generation time21-50 years
Age at maturity12-20 years
SeasonData deficient
Life span21-100 years

Larval characteristics

Larval/propagule typeNot relevant
Larval/juvenile development See additional information
Duration of larval stageNot relevant
Larval dispersal potential Not relevant
Larval settlement periodNot relevant

Life history information

Reproduction

Cetorhinus maximus bear live young (ovoviviparity) that hatch from eggs inside the uterus of the female (Matthews, 1950). Matthews (1950) and Compagno (1984) suggested that the young are nourished by the consumption of other eggs (oophagy or interuterine cannibalism) within the uterus, which explained the large number of eggs found in the single functioning ovary (Kunzlik, 1988).  However, Ali et al. (2012) suggested that oophagy would not be possible due to the large size of the egg capsules and the planktonic feeding method of the basking shark. Attempts to estimate gestation (pregnancy) period have resulted in a broad time scale, from 1 - 3.5 years (Parker & Stott, 1965; Compagno, 1984; Pauly, 2002; Sims et al., 2008, 2015), after which, about six pups are born (Sund, 1943). Young basking sharks are observed in the late summer, suggesting that they are born at this time. New-borns are between 1.5 and 2 m long at birth (Sund, 1943) and, after giving birth the females are thought to rest for 2-3 years before mating again. Only two pregnant females have ever been recorded in the literature (Sund, 1943; Ali et al., 2012). The lack of observations of pregnant females led Sims et al. (1997) to suggest that pregnant females did not surface, and spent time in deep offshore waters. The generation time of Cetorhinus maximus is estimated at 34 years (Sims et al., 2015).

Sexual maturity in males is attained at a size range between 4 - 7 m and about 12 - 16 years of age, and in females between 8.0 - 9.8 m at possibly 16 - 20 years of age (Compagno, 1984; 2002). However, Ali et al. (2012) reported a 6.9 m female basking shark (off the Syrian coast) believed to be at the beginning of gestation, which indicated that females might mature at smaller sizes in some cases.

Sexual segregation

Populations of the basking shark (Cetorhinus maximus) are often reported with male and female individuals occurring together, particularly in the summer (Mathews & Parker, 1950; Sims et al., 2000). However, female basking sharks were more abundant than males from surface fisheries off Scotland (Watkins, 1958) and Japan (Anon., 2002), while males were more common in subsurface nets around Newfoundland (Lien & Fawcett, 1986). It was suggested that the basking shark exhibits sexual segregation in surface activity (Lien & Fawcett, 1986; Bloomfield & Solandt, 2008).

Fecundity

Fecundity is thought to be very low in Cetorhinus maximus even when compared with other large ovoviviparous sharks (Compago, 1984; Sims, 2005).  The only observed basking shark birth was in Norway (in August 1936). The female basking shark was caught and gave birth to six pups whilst being towed (Sund, 1943). However, Ali et al. (2012) reported a second pregnant female with 34 egg cases, which suggests a higher fecundity, particularly since Ali et al. reported no sign of egg consumption by within the uterus (oophagy). Despite this, basking sharks born in any one year comprise less than 2.8% of the population in any given year (Sims, 2008).

Mating

Mating has not been observed directly. However, mating wounds (skin abrasion and cloacal wounds in the female) occur during the summer (Matthews, 1950). Courting behaviour has been observed between May and July along oceanographic and thermal fronts, where prey species are highly concentrated (Sims et al., 2000). Large aggregations of sharks engaged in courtship behaviour are found associated with thermal fronts, leading Sims et al. (2000) to suggest that these areas of rich prey concentration are important to the species as breeding grounds.

Sensitivity reviewHow is sensitivity assessed?

Resilience and recovery rates

The basking shark (Cetorhinus maximus) is the third largest fish in the world and one of only three filter-feeding sharks. As a member of the Order: Lamniformes, the basking shark shares similar life history strategies with its relatives in this group. It has a slow growth rate, estimated at 0.4 m per year (Pauly, 1978; 2002), partially attributed to the periodic loss of gill-rakers in some individuals, although not the entire population (Sims et al., 2003). The basking shark is long-lived, with a predicted lifespan of 40 - 50 years,  however, evidence is lacking to support a confident estimate (Garcia et al., 2008; Sims et al., 2015). Slow maturation rates are seen in the basking shark. In males, sexual maturity is attained at a size range between 5 - 7 m that is thought to be at about 12 - 16 years of age, and in females between 8.1 - 9.8 m at possibly 16 - 20 years in age (Compagno, 1984). However, Ali et al. (2012) reported a 6.9 m female basking shark (off the Syrian coast) believed to be at the beginning of gestation, which indicated that females might mature at smaller sizes in some cases.

Cetorhinus maximus are thought to pair and mate in early summer (Matthews 1950, Sims et al., 2000) after which, the gestation (pregnancy) period is 1 - 3.5 years (Parker & Stott, 1965; Compagno, 1984; Pauly, 2002; Sims et al., 2008, 2015). The basking shark probably bears live young, hatched from eggs within the uterus of the females (ovoviviparity) (Matthews, 1950). The method used to nourish the young within the uterus is debated, with evidence both for (Matthews, 1950; Compagno, 1984; Kunzlik, 1988) and against (Ali et al., 2012) the consumption of additional eggs in the uterus by the young basking sharks (oophagy/ interuterine cannibalism). Only two accounts of pregnancy in the basking shark have been published (Sund, 1943; Ali et al., 2012). In the first, a caught female gave birth to six pups suggesting a low fecundity. However, the findings of Ali et al. (2012) suggest a slightly higher fecundity, as a female was found with 34 egg cases (not all fertilized) at the beginning of gestation with no sign of oophagy. After giving birth the females are thought to rest for 2 years before mating again (Parker & Stott, 1965; Pauly, 2002; Compagno, 1984). Evidence indicates that basking shark recruitment is low, with basking sharks born in any one year comprising less than 2.8% of the population (Sims, 2008),  which is consistent with long maturation, slow growth rates and low fecundity of the basking shark.

Cetorhinus maximus experienced dramatic population loss caused by fisheries, that targeted the basking shark for its valuable liver oil and fins. Exploitation by fisheries began in the 1700s in Norwegian, Scottish and Irish waters, and ended in the mid 1800s after a decline in basking shark abundance. Within this time, landings were as high as 1000 individuals per year in Irish waters (ICES, 2016). The Norwegian fishery restarted in 1920, later to be joined by the Scottish (1940s) and the Irish (1947).  The Norwegians dominated the market by taking between 1266 and 4266 basking sharks per year (from 1959 to 1980), compared with lower numbers in Scottish (total estimate of 970 individuals, from 1946 to1953) and Irish waters (average of 1475 individuals per year, from 1951 to 1955). Although the extent and scale of these fisheries was not well recorded, in the 51 year period between 1946 and 1997, at least 105,730 sharks (mainly females) were likely to have been captured in the North East Atlantic (Sims, 2008) with peak landings (5266 metric tonnes) observed in 1979 (ICES, 2016). Most basking shark fisheries reported declines in landings before they closed down (Sims et al., 2015). The North East Atlantic fisheries experienced a large decline in basking shark total catch, with a total of 3680 t in 1977 compared with 119 t in the year 2000, before regulations were put in place (ICES, 2016). Sims et al. (2015) stated that the overall result of fishery efforts was thought to have reduced the basking shark population to less than half of its original size over the previous three generations (>100 years).

Regulations were put in place to control the exploitation of Cetorhinus maximus. In 2001, Norway reduced its basking shark landing quota to zero tonnes (Sims et al., 2015), and in 2006 ICES advised a zero total allowable catch (TAC), placing the basking shark on the Prohibited Species List.  ICES also advised that by-catch should be minimized (ICES, 2016). In addition, EU legislation prohibits Union fishing vessels from fishing basking sharks in all waters under Article 13 of the Council Regulation 2016/72 (ICES, 2016). This ban continues, however, dead or dying incidentally caught basking sharks can be landed but must be reported. In the UK, the basking shark has been protected since 1998 by the Wildlife and Countryside Act, Schedule 5 (ICES, 2016)

It is not known whether the basking shark population has recovered since protective measures were initiated in 2001 (Sims, 2008). However, public sightings schemes have provided some insight into the progress of the population. The longest ongoing basking shark public sighting scheme was initiated by the Marine Conservation Society (UK) in 1987 (Bloomfield & Solandt, 2006; Solandt & Ricks, 2009; Solandt & Chassin, 2013). By 2008, there were a total of 24,013 UK sightings recorded under this initiative (Sims, 2008).  The project highlights yearly trends in basking shark presence and individual length estimates per sighting provide information on growth patterns. In 2013, ca 3,000 basking shark individuals were recorded in over 1,000 sightings (Solandt & Chassin, 2013). A smaller public sightings scheme was established in Ireland (1993) to estimate the population of Cetorhinus maximus specifically in Irish waters. It reported a total of 425 individual basking sharks in one year of observation, encompassing all Irish coasts (Berrow & Heardman, 1994).

Additional UK Cetorhinus maximus population information is provided by Sims et al. (1997; 2008, unpublished data), in the form of basking sharks observed per unit time, which allows yearly comparisons of abundance within a small location (500 km2). This data showed that the years 1998 and 1999 had fewer sightings (0.01 and 0.02 sharks per hour, /hr), than the years prior (1995–1997: 0.10–0.35 /hr), and following (2000: 0.30 /hr and 2001: 0.14 /hr). This trend in surface swimming Cetorhinus maxmimus was positively correlated with the zooplankton data within this time, with more basking sharks reported during periods of higher zooplankton abundance (Sims, 2008). Overall, the surveys have provided some evidence for an improvement in the UK basking shark population. For instance, the average length of the animals recorded have been increasing in some schemes (Sims et al., 2015) and some have reported  increase in total abundance, but whether this is reflective of the basking shark population or an increase in public sightings efforts is unknown (Sims et al., 2015).

Hoelzel et al. (2006) studied the genetic diversity of the global population of the basking shark. In the study, Hoelzel et al. investigated the nucleotide and haplotypic (a group of alleles of different genes that are inherited together) diversity of a control region of mitochondrial DNA (mtDNA). Samples were taken from the tissue of stranded or incidentally caught basking sharks from the western North Atlantic, eastern North Atlantic, Mediterranean Sea, Indian Ocean and western Pacific. The results indicated both low nucleotide and haplotypic diversity, with only six identified haplotypes found across the samples. Hoelzel et al. (2006) estimated an effective population size of only 8,200 individuals. The low genetic variation observed in their samples was thought to be due to a bottleneck event in the Holocene epoch (within the last 11,500 years).

Resilience assessment. Cetorhinus maximus is a large, slow-growing, planktivorous shark, maturing at 12- 20 years of age depending on its sex. The generation time is presumed to be lengthy at 34 years (Sims et al., 2015) and females are thought to produce litters of around six pups (Sund, 1943). Each of these characteristics suggests that the basking shark population would be very slow to recover from major population loss, similar to the decline already experienced due to fisheries. The basking shark fishery remains closed (ICES, 2016) due to significant declines in landings between the years  1992 and  2000. Evidence in the UK, indicates some level of improvement in total abundance in some areas (based on public sighting schemes) of the North East Atlantic population after fishery closures. Unfortunately, no reliable estimate of population size prior to or after fishing efforts exists, making it difficult to calculate the population loss or the rate of recovery. However, Sims (2008) suggested that recruitment in the basking shark was low compared to other shark species, as the number of basking sharks born in any one year comprised less than 2.8% of the population. The recovery of the basking shark population is likely to be slow.

 Therefore, if the population were to suffer some mortality (that is ‘Medium’ resistance, <25% loss of population) then recovery may take up to 10 years and a resilience of ‘Medium’ (2-10 years) is recorded. However, if a pressure resulted in significant loss of population (‘Low’ resistance, loss of 25-75% of the population) then recovery could take over 10 years  and a resilience of ‘Low’ is recorded. Similarly, if the population suffered a severe loss (>75%) the resilience is likely to be ‘Very low’ (>25 years).  The resilience assessment is based on high quality evidence that is directly applicable to the species assessed and in general agreement about the rates of recovery and the recent declines in the natural population. However, there is little direct evidence to suggest that recovery has occurred in the past and a lack of understanding of the population dynamics of the species. Therefore, a precautionary confidence of Low is suggested for the resilience assessment. 

Hydrological Pressures

 ResistanceResilienceSensitivity
High High Not sensitive
Q: High
A: High
C: Medium
Q: High
A: High
C: High
Q: High
A: High
C: Medium

Cetorhinus maximus appears to be tolerant of a wide range of temperatures (Sims, 2008) ranging from 5.8 to 21°C. In the eastern Atlantic, a tagged shark demonstrated an apparent preference for temperatures of 15-17.5°C with 72% of temperature recordings falling within this range (Skomal et al., 2009). In the UK, the species can tolerate rapid changes in temperature associated with depth, on dives through the thermocline in stratified summer waters (Sims et al., 2003). A shark in this study experienced temperature gradients of up to 15°C in dives from 20 m depths to 100 m depths, reaching a maximum depth of 180 m in water with a temperature of 1°C (Sims et al., 2003).

Over large spatial scales, basking shark sightings are correlated with sea surface temperature (SST) and lagged SST, suggesting that basking shark distribution is related to the availability of climate-driven thermal habitat, which may affect prey abundance (Cotton et al., 2005). It is possible that apparent temperature preferences are a reflection of zooplankton distribution and tolerance. Therefore, while Cetorhinus maximus may not be directly affected by changes in temperature at the benchmark level, indirect effects may become apparent if changes in temperature alter prey abundance.

Basking sharks in the West Atlantic were reported to make transequatorial migrations, enter the warm waters of the tropics (Skomal et al., 2009) and occupy mesopelagic depths. However, one individual was found beached and moribund in waters of 24°C, suggesting the species usually avoids warmer waters (Sims, 2008).

Sensitivity assessment.  Basking sharks are found in temperate and tropical waters and are exposed to rapid temperature fluctuations (vertical temperature gradients of up to 15°C over ca 100 m) associated with swimming in surface and deep waters (up to 1264 m) (Sims et al., 2003; Sims, 2008; Gore et al., 2008). Although they may avoid warm waters (ca 24°C) their mobility would allow them to avoid localised warming at the level of the benchmark. Therefore, resistance is assessed as High. Hence, resilience is also High (by default) and sensitivity is assessed as Not sensitive.

High High Not sensitive
Q: High
A: High
C: Medium
Q: High
A: High
C: High
Q: High
A: High
C: Medium

Cetorhinus maximus appears to be tolerant of a wide range of temperatures (Sims, 2008) ranging from 5.8 to 21°C. In the eastern Atlantic, a tagged shark demonstrated an apparent preference for temperatures of 15-17.5°C with 72% of temperature recordings falling within this range (Skomal et al., 2009). In the UK, the species is recorded from water temperatures of 8-16°C and can tolerate rapid changes in temperature associated with depth on dives through the thermocline in stratified summer waters (Sims et al., 2003). A shark in this study experienced temperature gradients of up to 15 °C in dives from 20 m depths to 100 m depths, reaching a maximum depth of 180 m in water with a temperature of 1°C (Sims et al., 2003).

Over large spatial scales basking shark sightings are correlated with sea surface temperature (SST) and lagged SST, suggesting that basking shark distribution is related to the availability of climate-driven thermal habitat. An example of possible temperature driven migration was reported by Skomal et al. (2004) when a shark began moving southwards after water temperature dropped below the shark’s thermal preference (15.8 – 12.7°C). SST is known to impact plankton blooming which would subsequently affect prey abundance (Cotton et al., 2005). It is possible that apparent shark temperature preferences are a reflection of zooplankton distribution and tolerance, rather than the physiological requirements of the shark. Therefore, while Cetorhinus maximus may not be directly affected by a change in temperature at the benchmark level, indirect effects may become apparent if changes in temperature reduce prey abundance.

Sensitivity assessment.  Basking sharks are found in temperate and tropical waters and are exposed to rapid temperature fluctuations (vertical temperature gradients of up to 15°C over ca 100 m) associated with swimming in surface and deep waters (up to 1264 m). Their mobility would allow them to avoid localised warming at the level of the benchmark. Therefore, resistance is assessed as High. Hence, resilience is also High (by default) and sensitivity is recorded as Not sensitive.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Sensitivity assessment. Cetorhinus maximus is a fully marine species. It has a broad geographic range suggesting that the basking shark is able to cope with varying salinity levels. Supporting evidence is provided by studies that correlated environmental variables with basking shark distribution and found that their distribution could not be predicted by salinity levels alone (Soldo et al., 2008; Lucifora et al., 2015). In the case of hypersaline conditions, it is likely that the highly mobile Cetorhinus maximus would move to an area of normal salinity. Therefore, resistance is assessed as High. Hence, resilience is also High (by default) and sensitivity is assessed as Not sensitive.

High High Not sensitive
Q: High
A: High
C: Medium
Q: High
A: High
C: High
Q: High
A: High
C: Medium

In New Zealand, the basking shark Cetorhinus maximus was reported to enter the brackish Lake Ellesmere (Ryan, 1974; Dogshun, 1980; Francis & Duffy, 2002). A single 5 m shark was recorded in the lake (Ryan, 1974) and in September 1979, numerous sharks were found in the same lake, with a maximum of 21 sharks observed in one day (Dogshun, 1980).  They were thought to enter the coastal lake via the seaward entrance of a gravel spit. Lake Ellesmere is known to exhibit variable salinity, both spatially and temporally. In 1979 at the time of the shark encounters, the salinity at the entrance of the lake was thought to be 18 parts per thousand (ppt) (Ward et al., 1996). The basking sharks were presumably attracted by the high concentrations of zooplankton within the lake (Francis & Duffy, 2002).

Sensitivity assessment. The salinity change at the benchmark level is a decrease in one MNCR salinity category. Cetorhinus maximus is normally exposed to full salinity (30-40 ppt) and a reduction to variable salinity (18-40) did not have a negative impact on the health of the basking shark, in Lake Ellesmere. Also, as a highly mobile species, Cetorhinus maximus would be able to move away from any localised changes in salinity if they were to reach intolerable levels. Therefore, resistance is assessed as High. Hence, resilience is also High (by default) and sensitivity is assessed as Not sensitive.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Cetorhinus maximus is highly mobile so can move to areas with favourable feeding conditions, and is unlikely to be affected by local changes in water flow. A study by Witt et al. (2014) indicated that basking sharks spent most time in areas of low to moderate tidal speeds (mean 0.3 m/s) but the standard deviation of this value was from 0.06 - 1.0 m/s, which suggested that they can cope with varying tidal speeds. Therefore, a localised change of 0.1-0.2 m/s is unlikely to be significant, and resistance is assessed as High. Hence, resilience is also High (by default) and sensitivity is recorded as Not sensitive.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Sensitivity assessment. Changes in emergence are not relevant to the pelagic Cetorhinus maximus which is restricted to the open ocean.
High High Not sensitive
Q: High
A: Medium
C: Medium
Q: High
A: High
C: High
Q: High
A: Medium
C: Medium

As a mobile pelagic shark, with the ability to dive to depths of up to 1264 m (Gore et al., 2008), it is unlikely that the basking shark will be impacted by small-scale changes in near shore wave height. However, wave exposure caused by stormy weather may have an effect.  There are very few records of basking shark sightings during stormy weather, partly because of the logistical difficulties involved, but also because the increased mixing of the water causes a breakdown of the coastal fronts, so zooplankton is more widely distributed, and not aggregated near the surface. Although there may be small energy losses resulting from reduced efficiency of feeding, it is likely that basking sharks can dive to greater depths to continue feeding. Sims et al. (2003) showed how basking sharks continue to forage in the winter when prey are concentrated at depth rather than at the surface.  A similar behavioural change may occur in stormy weather.

During calm weather in the summer, the water column becomes stratified and dense aggregations of zooplankton form along coastal fronts. This may be beneficial to Cetorhinus maximus due to increased feeding efficiency on the highly concentrated plankton. Therefore, a decrease in wave action may be of benefit.

Sensitivity assessment.  Cetorhinus maximus is likely to avoid storms by diving to a greater depth to feed. In addition, at the benchmark level, a change of 3-5% of significant wave height is only a small change and is unlikely to affect the basking shark, especially in the open ocean. Therefore, resistance is assessed as High and resilience is High (by default). Hence, sensitivity is assessed as Not sensitive. 

Chemical Pressures

 ResistanceResilienceSensitivity
Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

No specific accounts of transition element or organo-metal contamination in Cetorhinus maximus have been found. However, Cadmium and Lead were detected in the tissue of six different shark species in the eastern Mediterranean, whilst a component of antifouling paints: Tributyltin (TBT), was detected in blue shark kidneys (Watts et al., 2001). Though little is known about the impacts of these chemicals on the health of sharks, Watts et al. (2001) stated ‘they are likely to cause severe damage to basic biological functions’.

As a filter-feeder, Cetorhinus maximus ­is also vulnerable to the indirect consumption of toxic substances via contaminated prey (zooplankton) however; there are currently no accounts of this in the scientific literature.

Nevertheless, the species is assessed as Not sensitive at the pressure benchmark, that assumes compliance with all relevant environmental protection standards.
Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

There is little information available in the scientific literature about the impacts of Hydrocarbon and PAH contamination on the basking shark/ However PCBs along with MEHP (plasticizer) and DDTs (toxic chemicals that adsorb onto the surface of plastics) were found incorporated into incidentally caught basking shark tissue in the Channel of Sicily, in the south Mediterranean (Fossi et al., 2014b). This study also found MEHP in Euphausia kronii (krill), samples; a prey species for the basking shark, which indicated that some component of the chemical ingestion was indirect.

Nevertheless, the species is assessed as Not sensitive at the pressure benchmark, that assumes compliance with all relevant environmental protection standards.

Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Little is known about the impact of synthetic compounds on Cetorhinus maximus. However, PCB was detected in basking shark tissue (Zitko et al., 1972; Fossi et al., 2014b). (See ‘Litter’ for more evidence of PCBs).

Nevertheless, the species is assessed as Not sensitive at the pressure benchmark, that assumes compliance with all relevant environmental protection standards.

No evidence (NEv) Not relevant (NR) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
No evidence was found. 
Not relevant (NR) Not relevant (NR) Not sensitive
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

The species is assessed as Not sensitive at the pressure benchmark, that assumes compliance with all relevant environmental protection standards.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

No information could be found on Cetorhinus maximus ability to tolerate hypoxia, but as the species is large and pelagic, it is unlikely to be able to tolerate low levels of oxygen. However, as a highly mobile species, Cetorhinus maximus would be able to move to an area with preferable oxygen levels. Therefore, resistance has been assessed as High, recovery is High (by default) and sensitivity is assessed as Not Sensitive.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

No information on the effect of nutrient enrichment or algal blooms was found. However, basking sharks are known to utilise oceanographic fronts that host an abundant food source (zooplankton) supported by the diffusion of nutrients from cold mixed water to warmer water and the subsequent growth of phytoplankton (Sims, 2008). As Cetorhinus maximus feeds on zooplankton, an increase in phytoplankton may increase the available food supply, not only in fronts but in other areas of enhanced nutrients. However, hypoxia caused by eutrophication may cause the basking shark to move to a more desirable area if the nutrient load rapidly increases.

Sensitivity assessment. Cetorhinus maximus is unlikely to be negatively impacted by nutrient enrichment at the benchmark level as it will lead to an increase in the food source. However, if nutrient levels lead to toxic blooms or hypoxia the, the highly mobile basking shark is likely to move to a more desirable area. Therefore, resistance has been assessed as High, recovery is High (by default) and sensitivity is assessed as Not Sensitive.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

No information was found on the specific effect of organic enrichment on Cetorhinus maximus. However, as a filter feeder, an increase in organic enrichment would likely affect the basking shark indirectly by influencing primary productivity and, therefore, prey abundance. Additionally, the potential for gill-raker clogging associated with increased suspended solids is low due to the method of filter-feeding used (cross-step filtration), which is thought to concentrate particles away from the gills using vortical flow to resuspend the particles that might otherwise clog the gill-rakers (Sanderson et al., 2016). Therefore, resistance has been assessed as High, recovery is High (by default) and sensitivity is assessed as Not Sensitive.

Physical Pressures

 ResistanceResilienceSensitivity
High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

There is currently no information available on the direct impact of saline habitat loss on Cetorhinus maximus. However, the basking shark relies on certain areas for important life history events, and their loss or obstruction could lead to a decrease in food supply or reproduction in the local area. Areas of importance include migratory pathways, such as the Irish Sea and the Firth of Clyde (Sims et al., 2003; Solandt & Chassin, 2013), and locations associated with reproductive activity such as oceanic fronts e.g. tidal fronts in the English Channel and the Ushant Front, Brittany (Sims et al., 2003). While unlikely, it is theoretically possible that major climatic change that changed oceanic currents could result in loss of the frontal systems frequented by the basking shark in UK waters. Similarly, it is theoretically possible that major engineering projects (e.g. barrages) in coastal seas could obstruct access to the frontal systems they use at present. However, Cetorhinus maximus is highly mobile with a broad geographic range so that any loss in reproductive success or food supply is likely to be temporary as the animals find new frontal systems to frequent.

Sensitivity assessment.  Therefore, resistance is assessed as High and resilience is High (by default). Hence, sensitivity is assessed as Not sensitive

 

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus, which is restricted to open water.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus, which is restricted to open water.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus, which is restricted to open water.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus, which is restricted to open water.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus, which is restricted to open water.

High High Not sensitive
Q: Medium
A: Medium
C: Medium
Q: High
A: High
C: High
Q: Medium
A: Medium
C: Medium

An increase in suspended solids in may affect the basking shark in numerous ways. Firstly, turbid waters attenuate light more rapidly than clear waters, which may result in a reduction in zooplankton prey. Reduced light intensity may lead to a decline in zooplankton by inhibiting the growth of their food source (phytoplankton). However, the basking shark is known to actively forage for zooplankton (Sims & Quayle, 1998), which would allow the shark to move to a more productive area. Depending on the scale of the food shortage and the distance travelled in order to find food, the basking shark is likely to experience some level of energy loss in turbid waters.

There are records of the basking shark occurring in turbid regions. An individual was tracked in the vicinity of the Amazon river mouth for approximately one month (Skomal et al., 2009). Additionally, the basking shark has been known to  penetrate estuaries in some cases (Knickle et al., 2017)

There is an increased risk of gill-raker clogging in turbid waters. However, there are no reports in the scientific literature of Cetorhinus maximus suffering from this problem. In addition, Sanderson et al. (2016) presented a model which showed how the basking shark might avoid gill-raker clogging by a particular filter feeding method (vortical cross- step filtration). This method is thought to concentrate particles away from the gills using vortical flow to resuspend particles that may otherwise clog the gill-rakers (Sanderson et al., 2016).

Sensitivity assessment. The turbidity change at the benchmark level is a change in one rank on the WFD scale for one year. As a highly mobile species, Cetorhinus maximus would be able to move away from any localised changes in turbidity if they were to reach intolerable levels. Energy losses may occur if the increase in turbidity occurs over a broad geographic range, as Cetorhinus maximus would be required to travel further to find food. Therefore, resistance is assessed as High. Hence, resilience is also High (by default) and sensitivity is assessed as Not sensitive.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus which is restricted to open water.

Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

Changes to the seabed are not relevant to the pelagic Cetorhinus maximus which is restricted to open water.

Medium Medium Medium
Q: Medium
A: Medium
C: Medium
Q: Low
A: NR
C: NR
Q: Low
A: Low
C: Low

The broad geographic range of Cetorhinus maximus means the likelihood of an encounter with marine litter is high.

Evidence supports a high abundance of marine plastics and microplastics in areas of Cetorhinus maximus activity. These include the North Atlantic and the North Sea and the Pelagos Sanctuary in the Mediterranean (a specific feeding area for the basking shark) where plastics are increasing (Thompson et al., 2004; Fossi et al., 2012; Panti et al., 2015). In the Mediterranean, Fossi et al. (2014a) calculated that the basking shark (swimming speed 0.85 m/s, mouth gape 0.4 m2) theoretically ingested 13,110 microplastic items per day, suggesting vulnerability to contamination by both plastics and their associated contaminants.

Fossi et al. (2014b) reported microplastic chemical consumption by basking sharks. They found MEHP (plasticizer) along with PCBs and DDTs (toxic chemicals that adsorb onto the surface of plastics) incorporated into incidentally caught basking shark tissue in the Channel of Sicily, south Mediterranean. They also found MEHP in Euphausia kronii (krill), samples; a prey species for the basking shark, which indicated that some component of the plastic contaminant ingestion was indirect.

Additionally, large sized litter such as discarded (ghost) fishing gear is a major threat to the basking shark. The United Nations Environment Programme (UNEP) and the Food and Agriculture Organization of the United Nations (FAO) have estimated that at least 640,000 tonnes of fishing gear are left in our oceans each year (World Animal Protection, 2014). The basking shark has been reported as a victim of entanglement in fishing gear in areas such as Canada (DFO, 2009). In the North East Atlantic, there are anecdotal reports of the basking shark being incidentally caught in gillnet and trawl fishing gear (ICES, 2016). In 1993, 28 records of sharks entangled in fishing gear were reported in the Irish Sea (Berrow & Heardman, 1994) and at least 22% of the sharks died as a result of the entanglement. Furthermore, the Marine Conservation Society (MCS) reported 63 sharks suffering from ship strike or entanglement in fishing gear between 1992- 2013 (Solandt & Chassin, 2013). Unfortunately, as a result of the zero total allowable catch (TAC) and the requirement of EU fishing industry to discard all incidentally caught basking sharks, there is little recorded information about these incidents. It is also difficult to quantify the impacts (ICES,2016).

Sensitivity assessment. More information about the impacts of litter on the health of Cetorhinus maximus, is required to make a confident assessment of their sensitivity. However, the basking shark clearly interacts with plastics of various sizes. Fossi et al. (2014a) suggest that the basking shark could ingest a large volume of microplastics per day in the Mediterranean and there is evidence to show the incorporation of microplastic associated chemicals into Cetorhinus maximus tissue. The effects of this are unknown, however, the ingestion of plastics and associated chemicals has proven detrimental to the health of other marine macrofauna, including sea birds, sea turtles and whales (Derraik, 2002), so they are likely to have a similar effect on the basking shark.

Entanglement by marine litter is also a threat to the basking shark, and although the scale of impact is currently unknown, Berrow and Heardman (1994) indicated that mortality (at least 22%) was experienced in entangled sharks in the Irish Sea. Therefore, resistance is assessed as Medium, hence resilience is likely to be Medium due to slow growth rates and low fecundity, and sensitivity is assessed as Medium.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Electromagnetic detection is well recorded in elasmobranch species and is thought to be a method of both prey detection and navigation (Kalmijn, 1971, 1982; Meyer et al., 2005).

Electric:

Cetorhinus maximus is thought to forage for zooplankton using passive electroreception, performed by the electrosensory pores focussed on its snout (Kempster & Collin, 2011). Zooplankton produce weak electric fields (up to 0.1V m-1, Kempster & Collin, 2011).  Therefore, if the basking shark is able to detect these signals, it is probable that it will also detect electric fields at the benchmark level.

Little is known about the direct impact of changing electric fields on Cetorhinus maximus. Gill & Kimber (2005) stated that electric fields may cause an attractive or avoidance response in some shark species. Kalmijn (1982) suggests that elasmobranchs (including Mustelus canis and Prionace glauca) are generally attracted to electric fields in the range 0.005 to 1mV cm-1 and avoid those around 10 mV cm-1 (the benchmark level), due to the field being perceived as prey or a threat.

Magnetic:

Less is known about the detection of magnetic fields by Cetorhinus maximus. Other sharks (Carcharhinus plumbeus and Sphyrna lewini) are attracted to magnetic fields in the range of 25-100 mT over <7m (Meyer et al., 2005). However, the magnetic field used in this study was far more intense than the benchmark level (10µT).

Sensitivity assessment. There is little direct evidence of the impact of electromagnetic fields on the basking shark. However, if the behaviour of Cetorhinus maximus reflects that of other sharks (Mustelus canis, Prionace glauca, Carcharhinus plumbeus and Sphyrna lewini) it may be attracted or repelled by fields at different strengths. The basking shark can probably detect electric fields at the benchmark level (1V m-1), and if it reflects the behaviour of Mustelus canis and Prionace glauca, a field of this strength may elicit an avoidance response.  There is no evidence to show the direct impact of a magnetic field of 10 µT on Cetorhinus maximus. But magnetic fields have been shown to attract sharks and might, therefore, affect the behaviour of the basking shark.

Cetorhinus maximus is highly mobile, allowing it to move away from areas of strong electric and magnetic fields. Any temporary attractive or avoidance responses caused by fields at the benchmark level are likely to result in little more than small-scale energy loss. Therefore, resistance is assessed as High. Hence, resilience is also High (by default) and sensitivity is assessed as Not sensitive.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

There is little available information on sound detection in Cetorhinus maximus. However, elasmobranchs have been shown to be sensitive to noise (Myrberg, 2001). Additionally, if disturbed by boats the basking shark has been reported to dive and move away from the area (Bloomfield & Solandt, 2008), although it is unknown whether this is caused by the noise, visual presence or a combination of the two.

Nelson & Gruber (1963) found that some sharks (Including Carcharhinus leucas, Sphyrna sp., Negaprion brevirostris and Galeocerdo cuvieri) could be strongly attracted to rapidly and irregularly pulsed sounds (mirroring the vibrations caused by struggling prey) at below 60 Hz. Sudden loud noises of low frequency have been shown to elicit an avoidance response in most fish (Vella et al., 2001). Similar findings by Myrberg et al. (1978) show avoidance behaviour in some sharks in response to rapidly changing sounds and sudden onset of transmission of an intense sound (impulsive sounds).

Sensitivity assessment. There is no direct evidence of sound causing basking shark mortality or stress.  But the behaviour of other sharks can be altered by sound.  The applicability of these behaviours to Cetorhinus maximus needs further study, particularly considering its feeding strategy as a filter-feeder (no need to detect struggling prey). Moreover, if sound at the benchmark level, elicited an attractive or avoidance response in the basking shark, it would be likely to result in little more than small-scale energy loss.

Therefore, Cetorhinus maximus is probably resistant to noise at the benchmark level so resistance is assessed as High.  Hence, resilience is also High (by default) and sensitivity is assessed as Not sensitive.  However, the applicability of the behaviour seen in other shark species to Cetorhinus maximus needs further study, particularly considering its feeding strategy as a filter-feeder (no need to detect struggling prey), and the confidence in the assessment is ‘Low’. 

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

There are no reports of Cetorhinus maximus being disturbed by light pollution. In addition, they have very small eyes in proportion to their body (SharkTrust, 2010). Moreover, if temporarily disturbed by high light levels, Cetorhinus maximus is highly mobile and able to move towards more preferable conditions. Therefore, resistance is assessed as High. Hence, resilience is also High (by default) and sensitivity is recorded as Not sensitive.

Medium Medium Medium
Q: Medium
A: Medium
C: Medium
Q: Low
A: NR
C: NR
Q: Low
A: Low
C: Low

There is currently no information available on the direct impact of saline habitat loss on Cetorhinus maximus. However, the understanding that the basking shark relies on certain areas for important life history events indicates that obstruction due to an offshore wind farm, wave or tidal device arrays or mariculture infrastructure could lead to a decrease in food supply or reproduction in the local area. Areas of importance include migratory pathways, such as the Irish Sea and the Firth of Clyde (Sims et al., 2003; Solandt & Chassin, 2013), and locations associated with reproductive activity such as oceanic fronts e.g. tidal fronts in the English Channel and the Ushant Front, Brittany (Sims et al., 2003). It is theoretically possible that major engineering projects (e.g. barrages) in coastal seas could obstruct access to the frontal systems they use at present. However, Cetorhinus maximus is highly mobile with a broad geographic range so that any loss in reproductive success or food supply is likely to be temporary as the animals find new frontal systems to frequent.

The basking shark is thought to be sensitive to entanglement in fishing gear. Cetorhinus maximus has been reported as a victim of entanglement in fishing gear in areas such as Canada (DFO, 2009). In the North East Atlantic, there are anecdotal reports of the basking shark being incidentally caught in gill net and trawl fishing gear (ICES, 2016). In 1993, 28 records of sharks entangled in fishing gear were reported in the Irish Sea (Berrow & Heardman, 1994) and at least 22% of the sharks died as a result of the entanglement. Unfortunately, as a result of the zero total allowable catch (TAC) and the requirement of EU fishing industry to discard all incidentally caught basking sharks, there is little recorded information about these incidents. It is also difficult to quantify the impacts (ICES, 2016).

Sensitivity assessment. At the benchmark level, if the body of water was obstructed by an offshore wind farm, wave or tidal device arrays or mariculture infrastructure, it is likely that the highly mobile Cetorhinus maximus would be able to swim around the obstruction and continue its migration along another route, resulting in little more than small-scale energy loss. However, fixed fishing gear (see benchmark, e.g. gill nets) have been reported to cause mortalities in the basking shark. Berrow & Heardman (1994) indicated that high mortality (at least 22%) was experienced in entangled sharks in the Irish Sea. Therefore, resistance is assessed as Medium, recovery is likely to be Medium due to slow growth rates and low fecundity, and sensitivity is recorded as Medium.

Medium Medium Medium
Q: Medium
A: Medium
C: Medium
Q: Low
A: NR
C: NR
Q: Low
A: Low
C: Low

Fish passage studies have identified the risk of collision with underwater objects for migratory fish species as high (Dadswell & Rulifson, 1994). As a mobile and broadly distributed species, Cetorhinus maximus could encounter anthropogenic objects (such as vessels and marine infrastructure) that may cause collisions. Because of their habit of feeding very close to the surface and at slow speeds (Sims, 2000), the basking shark is at risk from a collision with boat traffic. Compagno (1984) suggested that Cetorhinus maximus is tolerant to the presence of boats and divers, suggesting that it may not manoeuvre away from oncoming objects.

There have been numerous accounts of basking shark collisions, particularly ship-strikes (Kelly et al., 2004; Speedie & Johnson, 2008). The Marine Conservation Society (MCS) reported 63 sharks suffering from ship strike or entanglement in fishing gear between 1992 and 2013 (Solandt & Chassin, 2013). Despite having tough skin covered in dermal denticles, there is evidence of ship-strike causing scarring or injury (Darling and Keogh, 1994; Bloomfield & Solandt, 2008) and in more severe cases, fatal wounds (Chilton & Speedie, 2008).

Sensitivity assessment. It is difficult to quantify the impact of collisions on the basking shark. However, with 63 reported collisions over 21 years (Solandt & Chassin, 2013) the occurrence appears to be relatively low. However, in severe cases collisions can cause mortality (Chilton & Speedie, 2008). As a result, resistance has been assessed as Medium, recovery is likely to be Medium and sensitivity is Medium.

High High Not sensitive
Q: Low
A: NR
C: NR
Q: High
A: High
C: High
Q: Low
A: Low
C: Low

Cetorhinus maximus is thought to be relatively tolerant of visual presence (Speedie & Johnson, 2008; Compagno, 1984). However, if disturbed by boats, the basking shark has been reported to dive and move away from the area (Bloomfield & Solandt, 2008). Although it is unknown whether this is caused by the noise or visual presence of the boat, or a combination of the two.

The Marine Conservation Society (MCS) received anecdotal accounts of behavioural changes in the basking shark, from experienced wildlife observers in the Isle of Man.  In the accounts, courtship behaviour appeared to be disturbed by an approaching motorised craft, at a range of 1km (Bloomfiled & Solandt, 2008). On the other hand, at Gwennap Head, Seawatch Southwest wildlife observers (2007) began to see a change in behaviour when vessels came within 10 m (Bloomfiled & Solandt, 2008). Observations from Gwennap Head by Seawatch Southwest wildlife observers in 2007 reported that the sharks only showed altered behaviour when vessels approached very close to them (within 10 m). The impact of visual disturbance may be magnified in tourist areas where disturbance by boat traffic and marine tourism activities (Speedie and Johnson, 2008) is more frequent.

Sensitivity assessment. Visual disturbance appears to elicit a variety of reactions in the basking shark. Different outcomes documented include diving and moving away from the disturbance, disruption of courtship behaviour (Bloomfield & Solandt, 2008) or staying relatively undisturbed (Compagno, 1984). It may be that the impact is dependent on the extent of visual disturbance. However, it is possible that disturbance could lead to energy-loss caused by moving away from the area and reduced reproductive output if disturbed during courtship. Therefore, resistance has been assessed as High, recovery is High (by default) and sensitivity is assessed as Not Sensitive.

Biological Pressures

 ResistanceResilienceSensitivity
Not relevant (NR) Not relevant (NR) Not relevant (NR)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Sensitivity assessment. Not relevant - the basking shark is not subject to genetic modification or translocation for any commercial or conservation purposes.  
No evidence (NEv) No evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

There were no reports of direct impacts of invasive species on the success of Cetorhinus maximus.  However, there is the potential for indirect threats caused by invasive species lower down in the food chain. As a zooplanktivore, any invasive species impacting on the zooplankton assemblage is likely to indirectly impact the basking shark by altering food availability. An example is Mnemiopsis leidyi, an invasive ctenophore native to North and South America feeds on zooplankton (Colin et al., 2010). This species has already been found in the Mediterranean, Baltic and North Seas, however little is known about its future impacts.  

Sensitivity assessment. There is currently no evidence to suggest that the basking shark is affected by the introduction of invasive species. However, this may require re-evaluation as more information becomes available. 

No evidence (NEv) No evidence (NEv) No evidence (NEv)
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR
Q: NR
A: NR
C: NR

The first account of disease in Cetorhinus maximus was reported by Dagleish et al. (2010). The study was based on the post-mortem of a juvenile male basking shark found on Musselburgh beach, East Lothian, UK, in October 2007. Pyogranulomatous meningoencephalitis was found alongside multifocal, myocarditis (damage and inflammation of heart muscle) with myocyte necrosis (muscle cell death), oedema (fluid build up in body cavities) and haemorrhage. The exact cause of the disease was not found. However, the evidence suggested an infectious origin (possibly caused by bacteria) (Dagleish et al., 2010). The results could not conclude whether the meningoencephalitis was the cause of repeat live-stranding and subsequent death of the shark. The specimen was found in south-east Scotland, where it is uncommon, which was likely due to the impact of the disease on navigation (Dagleish et al., 2010).

There have been multiple accounts of basking shark associated parasites. Matthews & Parker (1950) reported the presence of three types of parasitic copepod: Dinematura producta (now Dinemoura producta), Caligus rapax and Nemesis lamna on Cetorhinus maximus.  These were found on the surface of the skin and the gills. The effects of the copepods attachment varied from minor skin erosion (Dinemoura product) to extensive gill damage (Nemesis lamna). Lampreys are also commonly found attached to basking sharks with little apparent damage (Matthews & Parker, 1950).

Further, there have been accounts of blood flukes (Hyperandrotrema cetorhini) or endoparasitic flat worms, found in the heart of Cetorhinus maximus (Orélis-Ribeiro et al., 2013). Despite little information about their impacts on basking sharks, they have been known to cause inflammation and a decrease in physiological and mechanical efficiency the infected organs in other fishes (Bullard & Overstreet, 2002).

Sensitivity assessment. As this is the first finding of disease in Cetorhinus maximus it is difficult to assess the sensitivity of the species to diseases. This account indicates that once infected, the basking shark may experience deteriorating health and consequent death. Additionally, with concerns to parasites, most parasites found on the basking shark are seemingly benign. Except for blood flukes (Hyperandrotrema cetorhini) which are suspected to cause inflammation in the infected organ (Bullard & Overstreet, 2002).

Based on current information, Cetorhinus maximus does not seem to be hugely under threat by disease. However, more information is needed to make a confident assessment. Therefore, No evidence is recorded. 

Low Low High
Q: High
A: High
C: High
Q: Low
A: NR
C: NR
Q: Low
A: Low
C: Low

Historically, the basking shark has been fished throughout its range, most commonly by harpoon (for reviews of fishing methods see: Kunzlik, 1988; Fairfax, 1998). Exploitation by fisheries (for its valuable liver oil and fins) in the North East Atlantic began in the 1700s in Norwegian, Scottish and Irish waters, and ended in the mid-1800s after a decline in basking shark abundance. In this period, landings were as high as 1000 individuals per year in Irish waters (ICES, 2016). The Norwegian fishery restarted in 1920, later to be joined by the Scottish (1940s) and the Irish (1947).  The Norwegians dominated the market by taking between 1266 and 4266 basking sharks per year (from the years 1959-1980), compared with lower numbers in Scottish (total estimate of 970 individuals, from the years 1946-1953) and Irish waters (average of 1475 individuals per year, from the years 1951-1955). Although the extent and scale of these fisheries is not well recorded, in the 51 year period between 1946 and 1997, at least 105,730 sharks (mainly females) are likely to have been captured in the northeast Atlantic (Sims, 2008) with peak landings (5266 metric tonnes, t) observed in 1979 (ICES, 2016).

Most basking shark fisheries reported declines in landings before they were terminated (Sims et al., 2015). North East Atlantic fisheries experienced a large decline in basking shark total catch with a total of 3680 t in 1977, compared with only 119 t in 2000 (ICES, 2016). Sims et al. (2015) stated that the overall result of fishery efforts was thought to have reduced the basking shark population to less than half of its original size over the previous three generation spans (>100 years).

Regulations have been put in place to control the exploitation of Cetorhinus maximus. In 2001, Norway reduced its basking shark landing quota to zero tonnes (Sims et al., 2015), and in 2006 ICES advised a zero total allowable catch (TAC) placing the basking shark on the Prohibited Species List.  It was also recommended that by-catch should be minimized (ICES, 2016). In addition, EU legislation prohibits Union fishing vessels from fishing basking sharks in all waters under Article 13 of the Council Regulation 2016/72 (ICES, 2016). This ban continues, however dead or dying incidentally caught basking sharks can be landed, but must be reported. In the UK, the basking shark has been protected since 1998 (ICES, 2016).

It is not known whether the basking shark population has recovered since protective measures were initiated (Sims, 2008), however, there is some evidence for improvement. Public sighting schemes in some locations (e.g. Irish Waters) have seen an increase in total abundance, but whether this reflects the basking shark population or an increase in public sightings efforts is unknown (Sims et al., 2015).

In addition, though many countries have banned the targeted removal of Cetorhinus maximus, it is likely that the increased demand for shark fin due to human consumption puts basking sharks at risk in less regulated areas (Sims, 2008) especially since its fins are amongst the most lucrative on the international market (Fowler, 2009). 

Sensitivity assessment. Although the impact of historic fisheries on Cetorhinus maximus populations cannot be exactly quantified, the basking shark population has experienced great losses over time. Sims et al. (2015) estimated that the overall result of fishery efforts reduced the basking shark population to less than half of its original size over less than 100 years. Despite some signs of improvement (e.g. increases in public sightings in Irish waters), with a generation span of ca 34 years, the basking shark is unlikely to have fully recovered from this loss. Therefore, resistance is assessed as Low, recovery is Low and sensitivity is assessed as High. 

Medium Medium Medium
Q: Medium
A: Medium
C: Medium
Q: Low
A: NR
C: NR
Q: Low
A: Low
C: Low

The basking shark was reported to be a victim of entanglement in fishing gear in areas such as Canada (DFO, 2009). In the North East Atlantic, there are anecdotal reports of the basking shark being incidentally caught in gillnet and trawl fishing gear (ICES, 2016). In 1993, 28 records of sharks entangled in fishing gear were reported in the Irish Sea (Berrow & Heardman, 1994) and at least 22% of the sharks died as a result of the entanglement. Furthermore, the Marine Conservation Society (MCS) reported 63 sharks suffering from ship strike or entanglement in fishing gear between 1992- 2013 (Solandt & Chassin, 2013). Basking sharks are also accidentally caught by towed gear (Francis & Duffy, 2002). Small numbers (130 individuals over 21 years) of incidentally caught basking sharks continue to be reported in the UK (Witt et al., 2012). Unfortunately, as a result of the zero total allowable catch (TAC) and the requirement of EU fishing industry to discard all incidentally caught basking sharks, there is little recorded information about these incidents. It is also difficult to quantify the impacts (ICES, 2016).

Finally, the high value of shark fins to the Asian market may result in basking sharks which are found alive being killed instead of released (Bloomfield & Solandt, 2006) although there are currently no records of the practice taking place in the UK (Bloomfield & Solandt, 2008).

Sensitivity assessment. Although the impact of accidental removal by fisheries and discarded (ghost) fishing gear on Cetorhinus maximus populations cannot be quantified, fishing gear poses a threat to this species.  A threat that is presumably still present, with small numbers (130 individuals over 21 years) of incidentally caught basking reported in the UK (Witt et al., 2012). Berrow & Heardman (1994) indicated that high mortality (at least 22%) was experienced in entangled sharks in the Irish Sea. Therefore, resistance is assessed as Medium, and resilience Medium due to slow growth rates, long generation times and low fecundity. Sensitivity is therefore assessed as Medium.

Importance review

Policy/legislation

Berne ConventionAppendix II
Wildlife & Countryside ActSchedule 5, section 9
UK Biodiversity Action Plan Priority
Species of principal importance (England)
Species of principal importance (Wales)
Scottish Biodiversity List
OSPAR Annex V
IUCN Red ListVulnerable (VU)
Features of Conservation Importance (England & Wales)
Priority Marine Features (Scotland)
Convention on Migratory SpeciesAppendix I or II

Status

Non-native

Importance information

Population size

There are currently no accurate estimates of the global population size of the basking shark (Cetorhinus maximus) (Sims, 2008).  The longest ongoing basking shark public sighting scheme was initiated by the Marine Conservation Society (UK) in 1987 (Bloomfield & Solandt, 2006; Solandt & Ricks, 2009; Solandt & Chassin, 2013). By 2008, there were a total of 24,013 UK sightings recorded under this initiative (Sims, 2008).  The project provides insight into yearly trends in basking shark presence and individual length estimates per sighting provide information on growth patterns. In 2013, ca 3,000 basking shark individuals were recorded in over 1,000 sightings (Solandt & Chassin, 2013). A smaller public sightings scheme was established in Ireland (1993) to estimate the population of Cetorhinus maximus specifically in Irish waters. It reported a total of 425 individual basking sharks in one year of observation, encompassing all Irish coasts (Berrow & Heardman, 1994).

Additional UK Cetorhinus maximus population information is provided by Sims et al. (1997; 2008, unpublished data), in the form of basking sharks observed per unit time, which allows yearly comparisons of abundance within a small location (500 km2). These data showed that the years 1998 and 1999 had fewer sightings (0.01 and 0.02 sharks per hour, /hr), than the years prior (1995–1997: 0.10–0.35 /hr ), and following (2000: 0.30 /hr and 2001: 0.14 /hr). This trend in surface swimming Cetorhinus maximus was positively correlated with the zooplankton data within this time, with more basking sharks reported during periods of higher zooplankton abundance (Sims, 2008).

Hoelzel et al. (2006) studied the genetic diversity of the global population of the basking shark. In the study, Hoelzel et al. investigated the nucleotide and haplotypic (a group of alleles of different genes that are inherited together) diversity of a control region of mitochondrial DNA (mtDNA). Samples were taken from the tissue of stranded or incidentally caught basking sharks from the western North Atlantic, eastern North Atlantic, Mediterranean Sea, Indian Ocean and western Pacific. The results indicated both low nucleotide and haplotypic diversity, with only six identified haplotypes found across the samples. Hoelzel et al. (2006) estimated an effective population size of only 8,200 individuals. The low genetic variation observed in their samples was thought to be due to a bottleneck event in the Holocene epoch (within the last 11,500 years).

Many local Cetorhinus maximus populations have declined due to fishing efforts including the North East Atlantic population (ICES, 2016). For example, it is thought that more than half of the European population was lost over 3 generation spans (Sims et al., 2015). Despite this, the current global population status is considered stable by the IUCN (Sims et al., 2015) and some public sighting schemes have seen an increase in total abundance. However, it is not known whether this is reflective of the basking shark population or an increase in public sightings efforts (Sims et al., 2015).

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Citation

This review can be cited as:

Wilson, C.M. & Wilding, C.M. 2017. Cetorhinus maximus Basking shark. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. Available from: http://www.marlin.ac.uk/species/detail/1438

Last Updated: 07/09/2017