Impacts of shellfisheries and nutrient inputs on waterbird communities in the Wash, England.
Abstract
Overexploited fisheries threaten many species that depend on the exploited resource. Shorebird populations are in decline globally and here we describe how changing shellfishery management and nutrient inputs have had dramatic influence on waterbird communities on an internationally important wetland. Cockle Cerastoderma edule and mussel Mytilus edulis fisheries conflict with shorebirds by removing prey and increasing mortality amongst non-target benthic invertebrates. Under intense dredging pressure, evidence suggests that benthic invertebrates such as worms, with rapid growth and short-generation times, should predominate over species such as bivalves, with slower growth and longer generation times. We investigated the change in the waterbird assemblage in the Wash, eastern England, between 1981-1982 and 2002-2003. This period was characterized by heavy fishing pressure on mussels and cockles, ultimately leading to a crash in the mussel stocks. During the study period, the waterbird assemblage underwent a gradual change from one dominated by those species with a high proportion of bivalves or 'other' prey (e.g. crustaceans, fish) in their diet to those with a higher proportion of worms. This gradual change was punctuated by major shifts, corresponding to three winters when oystercatcher Haemotopus ostralegus mortality was 5-13 times normal winter levels. Oystercatcher, knot Calidris canutus and shelduck Tadorna tadorna showed the highest levels of decline. Since the last major oystercatcher mortality event in 1996-1997, the assemblage has not shifted back to that observed prior to the major crash in the mussel stock in 1992. Changes in the waterbird assemblage were significantly related to mussel and cockle stock levels and, to a lesser extent, nutrient levels. Although correlative, evidence from this study indicates that fisheries caused shifts towards a waterbird community dominated by species with a high proportion of worms in their diet. Synthesis and applications. Mechanical shellfisheries directly conflict with the nature conservation interest of sites holding internationally important waterbird populations. Removal of the mussel beds in the Wash led to major shifts in the waterbird assemblage, with a shift towards worm-feeders. Setting annual quotas that provide sufficient food for shellfish-eating birds is essential to maintain the favourable status of this and other internationally important wetlands where shellfish are exploited.