Changes in the number of oystercatchers Haematopus ostralegus wintering in the Burry Inlet in relation to the biomass of cockles Cerastoderma edule and its commercial exploitation

Abstract

1. The Burry Inlet estuary in South Wales supports internationally important numbers of oystercatchers Haematopus ostralegus during the winter, where they feed predominantly on cockles Cerastoderma edule. The cockle population has been fished commercially for over a century. 2. The depletion of cockle stocks by commercial fisheries and disturbance to feeding birds has caused conflicts between fishing and nature conservation interests in Europe. Therefore, the aim of our study was to investigate the relationship between oystercatcher numbers, the biomass of cockles present at the start of the winter, and the biomass of cockles landed by the fishery during the winter. Our analyses are based on data from 11 winters, covering the period 1982/83–1992/93. 3. The abundance of oystercatchers during winter (measured as total bird‐days during September to February inclusive) was not significantly related to the biomass of cockles at the start of the winter, or the biomass landed by the fishery. Winter oystercatcher abundance in the Burry Inlet was correlated, however, with the number of birds wintering in the UK. 4. The abundance of oystercatchers during spring (measured as total bird‐days during March and April) was positively correlated with the biomass of cockles at the start of the winter, and negatively correlated with the biomass landed by the fishery over the winter. The most likely explanation for this is that birds disperse from the Burry Inlet earlier in spring when the biomass of cockles at the start of the winter is small and/or the biomass landed by the fishery is large. 5. The most likely causal explanation for this dispersal is that overwinter cockle losses due to predation, fishing, and other sources of mortality cause increased prey depletion when the biomass of cockles at the start of the winter is small. An increase in fishery landings during winter could therefore cause a reduction in oystercatcher abundance during spring. Considerably higher levels of fishing effort in future could conceivably cause a reduction in the abundance of oystercatchers during winter, although, at present, there is no evidence to suggest that fishing affects oystercatcher abundance at this time. 6. The Burry Inlet cockle fishery is a low intensity fishery, removing < 25% of the available stock, and using traditional fishing methods such as hand gathering. Even at these low levels of fishing effort oystercatcher abundance was reduced during spring. The introduction of more efficient modern fishing methods, such as tractor or suction dredging could therefore cause a decline in the abundance of oystercatchers within the estuary, if the level of exploitation increased as a result. Current levels of exploitation therefore need to be maintained. Reliable predictive models are now required for quantifying the impact of fishery management on the abundance of birds and cockles. A precautionary approach to management is required at other sites where a lack of data means that it is difficult to assess the impact of fishing effort on oystercatcher or cockle abundance.