Predator-prey overlap induced Holling type III functionalresponse in the North Sea fish assemblage

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Understanding the response of predator populations to varying prey fields is a prerequisite to understand prey population dynamics and to correctly parameterise multi species stock assessment or ecosystem models. Previous analyses on the large scale feeding response of predator populations, however, came to unrealistic results for the North Sea. The observed feeding response types (e.g., negative prey switching) would lead to the extinction of prey populations when becoming scarce. We analysed the large scale response of North Sea cod (Gadus morhua) and whiting (Merlangius merlangus) populations to varying prey fields using Generalised Additive Models (GAMs). Thereby, we took changes in predator-prey overlap explicitly into account in contrast to previous analyses. The composition of the prey field and changes in predator prey overlap had significant effects on the diet composition in the final GAM explaining 65.0% of the variance. The existence of a large scale prey refuge at low prey abundances as proposed by the Holling type III functional response could be demonstrated from field data. The refuge was not only caused by an active prey switching behaviour of the predators. It was also caused by a passive change in the availability of prey due to changes in predator-prey overlap associated with changes in the prey abundance. In addition, a rapid increase in relative stomach contents was observed, if the prey populations passed the abundance threshold of the prey refuge. At even higher abundances a saturation effect in relative stomach contents was detected. These findings support the predator pit theory.