• Physical factors such as the nature of the substratum and its suitability for burrowing (Bailey & Chapman, 1983).
• A change in climate could lead to variable recruitment through changes in mortality rates of early life stages, for example, differences in sea temperature and wind induced wave action might affect the survival of larval Nephrops either directly or by regulating food supply.
• Patchy settlement of larvae (Bailey & Chapman, 1983).
• Different levels of fishing effort / intensity (Bailey & Chapman, 1983). However variations in fishing effort may only be partly responsible for variations in abundance (Tully & Hillis, 1995) as differences were observed in some areas before the pressure of significant fisheries (Thomas, 1965a).

Sociability
Although Nephrops norvegicus are essentially solitary animals, multiple occupancy can occur in the burrows (Marrs, pers. comm.).

Growth
Like other crustaceans, Nephrops norvegicus must moult, shedding their hard exoskeleton, to grow. Nephrops contain no annually marked structures, such as the otoliths found in fish, so the estimation of growth rates, age and maximum age has proved to be particularly difficult in this species.

Growth (and fecundity) in Nephrops norvegicus are known to vary geographically and have been shown to be negatively correlated with burrow density (Tuck et al., 1997). Thus, growth rate appears to be density-dependent, and is also thought to be related to food availability. For example, Tuck et al. (1997) found growth was correlated with infaunal biomass. This suggests that nutritional stress occurs in populations with slower growing individuals. Growth of Nephrops may also be influenced at high densities through social behaviour changes (Cobb et al., 1982). Parslow-Williams et al. (2001) found evidence that nutritional limitation was occurring in Nephrops norvegicus from a site in the Clyde Sea with a high population density, compared to another site with a low density of individuals.

Information on the growth rate of lobsters is very limited (Thomas,1965c). Despite being one of the most studied decapods, the area of age and growth estimation is still one for which there is no standard methodology (Castro, 1995). A number of studies have estimated the growth rate of Nephrops norvegicus:

• Thomas (1960; cited in Thomas, 1965c) estimated an average growth rate of 5.7% for males and 6.2% for female Nephrops based on eight moults that occurred in an aquarium;
• Thomas (1965c) estimated an average moult increase in carapace length of 7.1% for male Nephrops. Thomas (1965c) suggested that increases in length decreased as male Nephrops grew but this was not observed in female Nephrops (Thomas, 1965c);
• whereas, Höglund (1942; cited in Farmer, 1975) stated that individuals 10-15 cm in total length grew by 4 mm per moult;
• however, under natural conditions Barnes & Bagenal (1951) concluded that Nephrops moult at least once a year but gave no data for the rates of growth.

Size
Nephrops norvegicus grows to a maximum total length of 25 cm (including the tail, carapace and clawed legs), although is normally between 18-20 cm (Fish & Fish, 1996). The generally recognized standard measurement for Nephrops norvegicus is carapace length (CL). The maximum recorded CL of Nephrops was 80 mm (Marine Institute, 2001). However, in recent years Nephrops with carapaces larger than 60 mm are rare (Marine Institute, 2001).

Environmental position
Nephrops norvegicus construct extensive shallow and branching burrows in soft sediments such as fine or silty mud at depths of 20-800 m. Burrows may be up to 10 cm in diameter, over a metre long and penetrate the sediment to a depth of 20-30 cm (Rice & Chapman, 1981). Nephrops norvegicus usually remain within their burrows by day and emerge at sunset to forage during the night but in deeper water this activity is reversed and individuals are more active by day (Chapman & Rice, 1971). In laboratory conditions, large males are less inclined to make burrows than females and small males, which may account for the higher proportion of large males that are caught in small catches (Andersen, 1962; cited in Farmer, 1974a, b).

Feeding
Representatives of most invertebrate phyla have been found in the foregut of Nephrops norvegicus. Most studies show that Nephrops norvegicus feeds primarily on crustaceans but also molluscs and to a lesser extent polychaetes and echinoderms (Parslow-Williams et al., 2002). Although a crustacean diet has a lower energy content that some of the other faunal groups, they do provide a source of essential minerals such as calcium (Ennis, 1973). Any differences in diet appear to be due more to changes in prey abundance than to prey preference (Parslow-Williams et al., 2002) indicating that the species is an opportunistic predator. The size range of prey eaten by Nephrops norvegicus was investigated by Thomas & Davidson (1962) who found that the minimum food particle size ingested was 1 mm, and the maximum for hard particles such as bits of shells was 5 mm. They also found that larger soft bodied organisms like polychaetes could also be ingested if taken in lengthways.

Loo et al. (1993) suggested that Nephrops could filter feed, allowing Nephrops to extend the size range of its food items. Farmer (1974d) reported that the expodites of the various mouth parts of Nephrops in most cases bore plumose setae, which when waved continuously produced water currents. Farmer (1974d) suggested that this behaviour was for cleaning suspended food particles away from the mouth. During another study Nephrops norvegicus were kept in small tanks containing fluorescently-marked food particles comprised of the brine shrimp Artemia salina. These food particles were found in the gills, stomach and intestine of Nephrops. Loo et al. (1993) suggested that this provided effective evidence of filter-feeding. However, this feeding may be more 'micro-raptorial' rather than strictly filter-feeding (Parslow-Williams et al., 2002). During periods of food scarcity, females spend a prolonged period in their burrows and suspension feeding is thought to occur (Loo et al., 1993).

Diel variation in feeding rates have been observed indirectly in Nephrops norvegicus. Catch rates of lobsters has always varied depending on the time of day with peak catches at dawn and dusk. Only those lobsters that have emerged from their burrows are caught in the trawls and, since it is assumed that they emerge mostly to forage (Chapman, 1980), catch patterns indicate diel patterns of feeding behaviour. The stomach contents of animals sampled by Parslow-Williams et al., (2002) indicated a feeding peak around dawn but not around dusk, although animals were out of their burrows. Sampling is complicated by the fact that satiated animals will return to their burrows and be unavailable for capture.

Supports which species
The rare British Fries' goby Lesueurigobius friesii shares the burrows of Nephrops norvegicus. The minute Cycliophoran Symbion pandora is a unique sessile animal less than 1 mm long that was found in the mouth parts of Nephrops collected in Denmark and the first of its kind to be described. Symbion pandora has a basal attachment disc and an anterior ciliated food gathering organ (Conway Morris, 1995).

Barnes & Bagenal (1951) recorded large numbers of Balanus crenatus living on Nephrops norvegicus in the Clyde area. The following species have been observed on specimens of Nephrops norvegicus from the Irish Sea: Triticella koreni, Balanus crenatus, Electra pilosa, Eudendrium capillare, Sabella pavonina, Serpula vermicularis and a forminiferan probably Cyclogyra sp. (Farmer, 1972; cited in Farmer, 1975). The polychaete Histriobdella homari has been observed on the pleopods of two Nephrops norvegicus from the Irish Sea and Clyde Sea (Briggs et al., 1997).

Predators
Nephrops norvegicus is preyed upon by numerous white fish some of which are listed below.

• In Scotland the stomach contents of cod was examined. Results showed that 80% of cod had Nephrops norvegicus amongst their stomach contents (Thomas, 1965b).
• Nephrops was also found in 52% of the thornback ray Raja clavata that were sampled (Thomas 1965b).
• In the Clyde, Nephrops was found in 51% of the small spotted catshark (dogfish) Scyliorhinus canicula that were sampled (Gordon & De Silva, 1980).