Publication Abstract

Fisheries and bird predation as components of cockle mortality in the Burry Inlet, South Wales

M.C. Bell, R.C.A Bannister, P. Walker, P.W. Atkinson and N.A. Clark

The Burry Inlet in south Wales has supported a regionally important fishery for cockles for more than a century. Statistical analysis of survey data shows that annual variation in abundance is driven almost entirely by variation in settlement strength in autumn. Annual mortality varies between age-classes, falling from 65% during the first year of life to 57% subsequently, but variation between calendar years is relatively unimportant in determining overall abundance levels. The Burry Inlet also supports internationally important concentrations of shorebirds, notably oystercatcher (Haematopus ostralegus), an important predator of adult cockles. A model of fishing and bird predation is constructed, using data on cockle stock abundance, fishery removals, and bird numbers and incorporating information on seasonal patterns of cockle growth and flesh content and feeding requirements of birds. The model is used to obtain estimates of the numbers of cockles at size and age taken by birds and fishermen. The results show that there is a relatively small overlap between birds and fishing in the sizes of cockles taken – birds take a relatively large number, but small biomass of smaller cockles. Fishing and bird predation appear to be additive to each other as sources of mortality, but to be compensated for by changes in other (unknown) sources of mortality. After accounting for bird predation, fishing would appear not to increase overall cockle mortality provided that it takes no more than 30–40% of the overall abundance. The applicability of this finding to other cockle fisheries is discussed.

Reference:

M.C. Bell, R.C.A Bannister, P. Walker, P.W. Atkinson and N.A. Clark, 2001. Fisheries and bird predation as components of cockle mortality in the Burry Inlet, South Wales. ICES CM 2001/J:02. 20pp.