Pied oystercatcher, Haematopus longirostri
A model was developed incorporating the oystercatchers, cockles and shellfishing. The numbers of oystercatchers and the abundance and distribution of cockles were obtained from surveys of the lough. The model predicted that the current low intensity of shellfishing was unlikely to be increasing the mortality rate of oystercatchers, but that large increases in shellfishing intensity could increase mortality.
- The objective of this project was to predict the potential impact of cockle harvesting on the oystercatcher population in Strangford Lough. Although there is currently no licensed commercial cockle fishery in Strangford Lough, some commercial cockle harvesting occurred following a crash in the cockle population of the Wash, Eastern England. Strangford Lough supports an overwintering population of between 5000 and 6000 oystercatchers, the majority of which feed on cockles and thus may be affected by commercial cockle harvesting.
- The model used for this project was an individual behaviour-based model. The model incorporates the two aspects of shellfishing that are potentially detrimental to birds, depletion of their shellfish prey and disturbance. It follows individual birds and their behavioural responses to shellfishing, so it could be used to predict whether birds could compensate for the effects of shellfishing by feeding on alternative sources of food when cockle beds were not available. More importantly, it could predict the effects of different shellfishing regimes on bird body condition and mortality, crucial aspects of fitness in an overwintering population.
- A number of important parameter values in the model were derived from extrapolated data, e.g. the abundance of cockles in the Lough. There were no available data on the quality of alternative feeding areas, such as upshore mudflats and terrestrial fields, which are often important to birds. The quality of these areas was assumed to be comparable to that in other sites. All of the model’s predictions therefore depend on the reliability of the cockle data and the assumption of the quality of alternative feeding areas, and thus must be treated as provisional. Predictions that are more accurate could be produced if a complete survey of the cockle beds and other feeding areas were to be conducted.
- The model showed that the current estimated cockle stocks are sufficient to support the oystercatcher population of Strangford Lough, even under adverse circumstances such as a prolonged period of cold weather or a lack of alternative feeding areas. However, if the cockle beds declined in area, the birds would be more vulnerable and a large proportion might be in poor condition at the end of winter.
- Harvesting of cockles by hand-gathering was predicted to have less effect on the birds than harvesting using a tractor-towed dredge. This was particularly true if the dredge killed cockles below its minimum fishable size. The model predicted one-third of current cockle stocks could be harvested by hand-gathering without any effect on oystercatcher mortality or body condition.
- In comparison to allowing hand-gathering of cockles throughout the winter, restricting it to the second half of winter reduced its impact on the birds significantly. Conversely, restricting hand-gathering to the first half of winter substantially increased bird mortality at low shellfish harvests. Daily and weekly bans on handgathering reduced bird mortality, but were less effective than an early-season ban.
- The predictions of the current model, based largely on extrapolated data, show that Strangford Lough has the potential to support a licensed cockle fishery without affecting the oystercatchers that overwinter there. However, the predicted 2 oystercatcher mortality rate was highly sensitive to the cockle stocks, which should be surveyed completely to confirm this prediction.
- Suggestions are made as to how further research could remove some uncertainties in the current parameter values used in the model, enabling the model to predict more accurately the effects of different types and intensities of shellfishing on the oystercatcher population.
Funding and Collaboration
Environment and Heritage Service of Northern Ireland.
West, A.D., Stillman, R.A. and Portig, A., 2002. Modelling of the interaction between oystercatchers and shellfish in Strangford Lough, Northern Ireland, Centre for Ecology and Hydrology, Dorset.