Oystercatcher, Haematopus ostralegus
Knot Calidris canutus
The Morecambe Bay Special Protection Area (SPA) is one of the largest estuarine systems in the UK and supports nationally and internationally important populations of wildfowl and shorebirds. The flats usually contain abundant stocks of cockles Cerastoderma edulewhich, along with mussels Mytilus edulis, are the preferred prey of oystercatchers Haematopus ostraleguswintering in the bay. In recent years, the cockle population has declined to a fraction of what it used to be and there is concern about the potential effects of this decline on oystercatcher and knot Calidris canutuspopulations that overwinter in the bay.
The aim of this project was to bring together existing information about shellfish populations and bird populations in the bay and use this information to parameterise an individual-based model of oystercatcher and knot populations in Morecambe Bay. The model would then be used to run simulations of various different scenarios to give some insight into the potential effects of the decline in cockle stocks and the extent to which bird populations rely on those stocks for their survival.
The Morecambe Bay model was created in the knowledge that the shellfish data available, particularly with respect to mussels, was not comprehensive and up to date. Bearing that in mind, the predictions in this report should not be taken as an accurate representation of real-world conditions, but more an exploration of how the data that is available can be used to inform debate about shellfishing practices. They can also indicate where there is a need for further research.
The model predicted that shellfish densities observed in the bay were sufficient to support the observed oystercatcher population in both 2005 and 2009, provided that supplementary food was included. Without supplementary food during 2009, large oystercatcher mortalities were predicted and this has not been observed on the ground. It therefore seems likely that oystercatchers currently require, and are exploiting, food supplies over and above the cockle, mussel and Macomastocks recorded during the shellfishery surveys. At the cockle and mussel densities seen in 2005, no such supplementary food source was needed for oystercatchers in the model to survive the winter in good condition, so in years of high cockle abundance a knowledge of alternative sources of food for this species is less important.
Knot mortalities were predicted to be 100% when the model contained cockles, mussels and the observed abundance ofMacoma. This implies that the knot population is unlikely to be relying on prey from just the recorded shellfish beds. Given that the bay is a dynamic system and spat settlements are often ephemeral, this is perhaps to be expected. Doubling Macomadensity tended to increase knot survival, indicating the quantity of additional food that the birds may be exploiting.
Overall, the simulations show that the decline in cockle stocks between 2005 and 2009 is potentially large enough to affect the body condition and survival of oystercatchers in Morecambe Bay. The greatest unknown is the extent and availability of potential sources of food outside the established cockle beds. While the model predicted that oystercatchers would have no need of these alternative food supplies in a good cockle year, they are potentially very important in years when the cockle stocks are low. They also show that knot are likely to be exploiting food supplies outside the established cockle beds in both good and bad cockle years. Bearing this in mind, establishing a greater understanding of potential food supplies outside the cockle beds would provide a more accurate estimate of the importance of cockle stocks to the oystercatcher and knot populations of Morecambe Bay and increase confidence in the predictions of the model.
Funding and Collaboration
West, A. and Stillman, R., 2010. A single year study to determine the capacity of Morecambe Bay European marine site to support oystercatcher, using shellfish resource modelling techniques, Bournemouth University, Bournemouth.