Abstract
ABSTRACT: Predator–prey switching may stabilise predator–prey interactions, promote co-exis-tence of prey that share a common predator, and increase the overall stability of homogeneous sys-tems of interacting species. This study presents an investigation of prey switching of 2 major NorthSea fish predators, cod Gadus morhua and whiting Merlangius merlangus . Relative food compositionderived from analysis of more than 36000 stomachs is compared to the relative density of fish preyreflected by trawl survey catches, and generalised linear models are used to examine how preyswitching is affected by predator length and prey species and length. Possible effects of stomachsample size and predator density are also investigated. Prey preference is a decreasing function ofthe relative density of the prey (negative prey switching), in particular for large cod (40 to 100 cm).This was neither the result of variations in stomach sample size, nor of changes in local predatorabundance. If not counteracted by compensatory changes in spatial overlap or total food intake, neg-ative switching is likely to destabilise the interactions between cod and whiting and their fish prey. KEY WORDS: Prey switching · Piscivorous fishes · North Sea · Ecosystem stability
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