Changes in commercially fished shellfish stocks and shorebird populations in the Wash, England

The Wash, in eastern England, supports internationally important populations of 11 species of shorebird. A major commercial shellfishery provides potential conflict between fishermen and nature conservation interests. During the 1990s, high fishing mortality and low recruitment substantially reduced the stocks of cockle (Cerastoderma edule) and mussel (Mytilus edulis). Population models, constructed from estimates of survival and recruitment, indicated that declines in the availability of cockles and mussels were associated with changes in oystercatcher (Haemotopus ostralegus) survival between 1970 and 1998, including three periods of mass mortality, and also the recruitment of juvenile birds to both oystercatcher and knot (Calidris canutus) populations. Emigration of knot may also have taken place. Cockle recruitment was dependent on climatic conditions, whereas mussel populations tended to be stable. The decline in mussel stocks, due to over-fishing, increased the vulnerability of the oystercatcher population to mass-mortality episodes in poor cockle years. The key to preventing major oystercatcher kills in future is to ensure sufficient mussels are available in poor cockle years. Recent cultivation of mussels in inter-tidal areas has been beneficial and is an important management tool for maintaining bird populations.