ShagsPhalacrocorax aristotelisat cage fish farms in Argyll, Western Scotland

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Abstract
The feeding ecology of Shags was investigated in Argyll, Western Scotland, with particular reference to cage fish farms. Most Shags were recorded during winter, presumably having dispersed from breeding colonies elsewhere. The proportion of adults increased during October and remained at about 75% until February before falling to zero in June. Shags apparently did not take fish from farm cages but attacked them through the netting. Losses to Shags were confined to the smallest stock and represented 0.1% of the 17 302 dead fish examined at one farm. Most of the fish eaten by Shags near farms were wild fishes that concentrate around the cages. The diet of birds differed between years apparently reflecting the local abundance of juvenile fish. Observations of feeding Shags appeared to underestimate the proportion of small (<5 g) fishes in the diet suggesting that birds swallowed some prey underwater. Thus intake rates based on surface observations were minimum values. Adult birds had consistently higher feeding success and intake rates than juvenil es both near a fish farm and in the natural habitat. There was no evidence that intake rat es of either adult or juvenile birds differed between the fish farm rind the natural habitat.