, Volume 191, Issue 1, pp 223-231

Predation pressure from above: observations on the activities of piscivorous birds at a shallow eutrophic lake

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Abstract

The foraging activities of piscivorous birds at a small eutrophic lake in southern Sweden were observed during the ice-free months of 1984. Species-specific patterns of abundance and distribution were apparent. Great crested grebes and red-breasted mergansers were present for periods of months and weeks, respectively, while grey herons, black-headed gulls and red kites made numerous shorter visits. Both cyprinids and percids were seen to be captured. Speculative calculations suggest that the amount of fish removed by the birds is significant when compared with that taken by the lake's piscivorous fish, constituting 34% of total consumption in summer and at 99% becoming by far the more important component in late autumn. The peculiar characteristics of this source of predation pressure and their implications for fish populations are discussed.