A PRELIMINARY ASSESSMENT OF THE QUANTITIES OF SHETLAND SANDEELS TAKEN BY SEABIRDS, SEALS, PREDATORY FISH AND THE INDUSTRIAL FISHERY IN 1981-83

Shetland holds a very high concentration of seabirds that feed to a large extent on sandeels Ammodytes marinus. The available data allow an assessment of the quantity of sandeels consumed by seabirds each year 1981–83, and sensitivity analysis suggests that this figure is accurate to about ±30%. Only very approximate estimates of sandeel consumption by seals and by predatory fish can be made. From 1981 to 1983 the Shetland fishery catch averaged 45,000 tonnes of sandeels, with an estimated natural consumption of 47,000 tonnes by seabirds (predominantly Guillemots Uria aalge and Fulmars Fulmarus glacialis). Based on sensible guesses and the little data available, consumption may have been about 25,000 tonnes by predatory fish and 9,000 tonnes by seals. The figures suggest that natural mortality of the Shetland sandeel stock in 1981–83 was considerably in excess of the figure used in VPA stock assessments. Further information is required for key variables in order to permit better estimates of sandeel stock dynamics. In particular, we need better data on numbers and diets of seals in Shetland, numbers and diets of predatory fish, diets and foraging distributions of Fulmars, especially out with the chick‐rearing period. In future, sandeel biomass assessments for Shetland should incorporate estimates of predation by seabirds as this is clearly an important part of total mortality for the stock. Copyright © 1990, Wiley Blackwell. All rights reserved