Predicting food requirements of overwintering shorebird populations on the Solway Firth. A report to Scottish Natural Heritage and Marine Scotland.

Abstract

In this report we use a recently-developed spreadsheet model to predict the overwinter food requirements of two shorebird species, oystercatcher (Haematopus ostralegus) and red knot (Calidris canutus), within the Solway Firth. The model is based on the energy requirements of the birds together with the energy value of their shellfish food. The model predicts the quantity of shellfish required to maintain high survival rates, and hence avoid significant mortality events within the oystercatcher and knot populations. Knot were assumed to consume 5-14mm cockles (Cerastoderma edule L.), 5-24mm mussels (Mytilus edulis L.) and 8-16 mm tellin (Macoma balthica L.). Oystercatcher were assumed to consume >15mm cockles, 30-60mm mussels and >12mm tellin. The biomasses of invertebrate prey were derived from intertidal surveys of the site. The population sizes of the bird species were derived from Wetland Bird Survey (WeBS) core counts. Predictions were for the winter of 2013-2014. Shellfishing was assumed to exploit >28mm cockles. The food requirements of oystercatcher and knot were predicted for different combinations of food supply. All scenarios assumed that the birds could consume cockles, mussels and tellin. Alternative scenarios assumed that knot and oystercatcher could consume other food from upshore areas, or that oystercatcher could consume food from terrestrial habitats. Cockle and tellin biomasses were estimated within Solway Firth, and at Wigtown Bay, a site outside the area in which bird population sizes were estimated. Further scenarios therefore assumed that birds either could, or could not, consume food from Wigtown Bay. In each scenario the model initially predicted the amount of shellfish biomass not required by the birds. This was then converted into the biomass potentially available for fishing, accounting for the fact that the size range exploited by fishing did not overlap completely with that consumed by the birds. In the case of knot there was no overlap, and so the amount available to fishing was only calculated from the biomass of shellfish not required by oystercatcher. The model predicted that approximately 700 tonnes of >28mm cockles could potentially be exploited by shellfishing during the winter of 2013-2014, after taking into account the food requirements of the birds, excluding cockle and tellin biomass in Wigtown Bay, and assuming that oystercatcher consumed cockles, mussels, tellin and prey from upshore areas and terrestrial habitats. This was considered to be the most realistic scenario given that oystercatcher can potentially feed on terrestrial and upshore habitats, and given the distance between Wigtown and the area in which oystercatcher population size was estimated. The cockle, mussel and tellin surveys did not cover the entire extent of the Solway Firth, not recording cockles or tellin in English waters or mussels or the Scottish side, and so it is likely that a higher biomass of shellfish food is available to the birds in reality. However, without a more extensive survey it is not possible to quantify this. The spreadsheet model’s predictions for the winter of 2007-2008 were also compared with those of a more complex individual-based model that was developed for oystercatcher and knot in the Solway Firth based on shellfish biomass during 2005 to 2007. The individual-based model predicted that knot survival was 100% in all simulations for the winter of 2007-2008, consistent with the prediction of the spreadsheet model that 18038 tonnes of shellfish were not required by the birds during this winter. The spreadsheet model predicted that the oystercatcher population required all of the shellfish food available during the winter of 2007-2008. Similarly, the individual-based model predicted that oystercatcher were relatively sensitive to the amount of biomass removed by fishing during this winter. With a shellfishing Total Allowable Catch (TAC) set at 1000 tonnes there was a predicted reduction in survival and TACs set at 500, 750 and 1000 tonnes were predicted to reduce body mass. The spreadsheet model predicted that birds required all of the food during 2007-2008 and hence that any TAC would reduce survival. This demonstrates that the spreadsheet model is capable of producing broadly similar predictions to the more complex individual model, although the latter is more sensitive when stock levels are more critical.