Changes in fisheries practices and Oystercatcher survival, recruitment and body mass in a marginal Cockle fishery

Small hand-gathering shellfisheries have not been associated with the major negative ecological impacts observed as a result of large-scale and long-term mechanical dredging of Mussel Mytilus edulis and Cockles Cerastoderma edule in the Dutch part of the Wadden Sea and the Wash Estuary in the UK. A hand-gathering Cockle fishery has been in existence for many years at Traeth Lafan, a large sandy area in north Wales designated as a Special Protection Area on account of the wintering population of Oystercatchers Haematopus ostralegus. In addition to the hand fishery, suction-dredging for Cockles took place in four winters between 1989/1990 and 1996/1997 and the amount of farmed Mussels increased dramatically from 1995/1996 onwards. Since 1980/1981, the fishery can be characterised into three periods of similar management: 1980/1981 to 1988/1989, 1991/1992 and 1992/1993 (period A, eleven winters, hand gathering Cockles only, low Mussel stocks), 1989/1990 to 1990/1991 and 1993/1994 (period B, three winters, suction dredging and hand gathering of Cockles, low Mussel stocks) and 1994/1995 to 2002/2003 (period C, suction dredging in one out of nine winters and high Mussel stocks). Adult Oystercatcher survival, juvenile recruitment and both adult and juvenile mass were lower in period B, the winters in which dredging occurred and Mussel stocks were low. Increases in bird numbers after the dredging periods were due to greater juvenile recruitment, most likely caused by adults moving from the Cockle beds to feed on the new Mussel resource provided by a large increase in commercial farming. Commercial dredging for shellfish, even on a small scale, will increase the risk of higher Oystercatcher mortality, especially if alternative food sources are not available. Mussel culture in the intertidal area adds new food supplies but internationally important numbers of birds should not have to rely on a food source of which the availability is likely to be determined by market forces.