Towards a simplified approach for assessing bird food requirements on shellfisheries. A report to the Welsh Government.

Abstract

In northwest Europe conflicts have routinely occurred between economic and conservation interests regarding shellfish such as cockles and mussels. The harvest of these species is economically important, but shellfish also constitute the main overwinter food supply of the oystercatcher Haematopus ostralegus. In this report we describe attempts to produced a simplified modelling approach to predict the quantities of shellfish which need to be left unharvested in order to ensure high overwinter survival of oystercatcher. We review oystercatcher diet and prey selection in order to quantify the dependence of this species on shellfish, and determine the size ranges of shellfish which the birds consume. We also review the food requirements of oystercatchers, based on their energetic needs and the nutritional quality of shellfish. In general the data agree well with those used in previous oystercatcher modelling studies. However, there is a possibility that the daily energy requirements, calculated from an all bird allometric equation, may yield an underestimate of oystercatcher food requirements. A comparison of the physiological food requirements, i.e. the quantity directly consumed, and the ecological food requirements, i.e. the quantity required to avoid high mortality, indicated that the ecological food requirement was between 2.0 and 7.8 times greater, with the value depending on the proportion of cockles Cerastoderma edule and mussels Mytilus edulis in a site. These ratios are calculated from empirical data on oystercatcher survival and the predictions of individual-based models predicting the relationship between mortality rate and the abundance of the food supply. Data from the Burry Inlet indicated that the mean ecological food requirement was 3.3 times greater at this site. We describe a simplified spreadsheet model, which we used to predict the food requirements of the oystercatcher population of the Burry Inlet, and thus the quantity of shellfish which must be left unharvested in order to maintain low mortality rate. The model is based on parameter values derived from the literature reviews in this study, including the energy requirements of the birds, the energy content of shellfish, the minimum size of cockles and mussels consumed, and the ratio of the ecological and physiological requirements. We describe the assumptions and limitations of the model, and compare the model with more detailed individual-based models that can be used to predict the mortality rate of shorebirds in relation to the amount of food available.