Biodiversity & Conservation

Dame (1996) suggested that dense beds of bivalve suspension feeders were important for pelagic-benthic coupling in estuarine ecosystems, resulting in increased rates of nutrient and organic carbon turnover and an overall increase in the productivity of the ecosystem. Newell (1988; cited in Dame, 1996) suggested that the Crassostrea edulis population in Chesapeake Bay were an important grazer of phytoplankton and that the destruction of the oyster reefs resulted in reduced grazing of the phytoplankton, spring blooms that increased turbidity and the risk of anoxia, and an increase in summer zooplankton and pelagic predators such as jelly fish and ctenophores, essentially changing aspects of the ecosystem. Similarly, the increase in nutrients and suspended sediments in Chesapeake Bay due to agricultural runoff and coastal development was exacerbated by the decline in the major filter feeding species, the oyster reefs (Dame, 1992).

Native oyster beds, although scarce, are probably of similar importance to their local ecosystems, as a major grazer of the phytoplankton, contributing to pelagic-benthic coupling, stabilizing sediment and providing substratum for numerous species in what might otherwise be bare sediment. The introduction of such hard substrata, therefore, markedly increases species diversity at a location.

British native oyster beds (characteristic of this biotope) were exploited in Roman times. The introduction of oyster dredging in the mid 19th century developed the oyster beds into one of Britain's largest fisheries, employing about 120,000 men around the coast in the 1880's. However, by the late 19th century stocks were beginning to be depleted so that by the 1950s the native oyster beds were regarded as scarce (Korringa, 1952; Yonge, 1960; Edwards, 1997). This biotope is still regarded as scarce today. Over-fishing, combined with reductions in water quality, cold winters (hence poor spat fall), flooding, the introduction of non-native competitors and pests (see sensitivity), outbreaks of disease and severe winters were blamed for the decline (Korringa, 1952; Yonge, 1960; Edwards, 1997). As a result, although 700 million oysters were consumed in London alone in 1864, the catch fell from 40 million in 1920 to 3 million in the 1960s, from which the catch has not recovered (Edwards, 1997). The European flat oyster (Ostrea edulis) fishery continues to use traditional techniques, however, most oysters reaching the market come from 'relays' or from mariculture (Edwards, 1997).

Native oyster fisheries are subject primarily to UK shellfisheries conservation legislation; the species is not named in any national or international nature conservation legislation or conventions. However, Ostrea edulis is included in a Species Action Plan under the UK Biodiversity Action Plan (Anon, 1999c). Commercial native and non-native oyster transplantation has been recorded as a dispersal mechanism for non-native species, including serious pests such as Crepidula fornicata and Urosalpinx cinerea (Anon, 1999c; Blanchard, 1997; Eno et al., 2000).