Fatty acid signatures in plasma of captive herring gulls as indicators of demersal or pelagic fish diet

Current fishing practices provide scavenging seabirds with discarded demersal fish. In order to study the impact of fishery management measures and alterations in the availability of discards on seabird populations, accurate information on the birds' diet is essential. Studies of pellets and prey remains provide a biased picture and tend to be limited to seabird breeding seasons. Studying biochemical markers in bird tissues can complement other methods. In this study, captive herring gulls <i>Larus argentatus</i> were fed alternating diets of North Atlantic demersal and pelagic fish, and changes in the fatty acid signatures (FAS) of the birds' plasma were followed. The large differences in FAS of demersal and pelagic fish caused several clearly detectable differences in the plasma FAS of the gulls fed on these fish. A change from demersal to pelagic fish or vice versa could be detected in plasma FAS within 5 d of switching the diet, and transient changes in the gull's nutritional status did not disturb the use of the FAS. Relatively high levels of branched-chain 17:0, 18:1n-7 and 20:4n-6 are indicative of a diet of demersal fish, and high levels of 14:0, 22:1n-11, 20:1n-9, 18:2n-6, 18:3n-3 and 18:4n-3 occur when gulls eat pelagic fish. These reference FAS can be used to estimate the importance of demersal and pelagic fish in the recent diet of scavenging seabirds