Eco-morphology as a predictor of fish diets in a changing North Sea

We demonstrate the use of an eco-morphological method, quantifying the capacity of predatory fish species to utilize different prey types. Because this method focuses on the feeding capacity of predators and not only on their actual diets, this method potentially allows for the prediction of changes in food utilization in a changing environment and not only for detailed understanding of the actual interactions between predator and prey. To test the method we predicted the utilization of 31 different prey items in 9 species of gadoids and 12 species of flatfishes from the North Sea, through the following steps. 1) Categorisation of prey types in terms of physical (e.g. size, material properties) and chemical (e.g. carbohydrates / protein contents) properties that pose demands and selection pressure to their predators. This resulted in a functional, rather than in a taxonomic characterisation of prey types. 2) Identification of the required behaviour and morphological structures of the feeding, locomotory, and sensory apparatuses of predators to deal effectively with the demands of each food type. This resulted in a morphological profile for a specialist predator for each prey type. 3) Quantification of the same morphological structures in each fish species by detailed measurements, resulting in morphological profiles of each species. 4) Comparison of the specialist and species profiles. The extent to which the species profile fitted the specialist profiles resulted in a quantitative measure expressing the ability of each species to utilize a particular prey type: a hypothetical food niche (HFN). 5) Validation of the HFNs of all species by contrasting them with actual diets. We found that the gadoid and flatfish species could be separated in five “predatory groups”: large-mouthed flatfish, small-mouthed flatfish, soles, predatory gadoids and omnivorous gadoids. The utilization of fast, relatively large prey was predicted better than the utilization of slow or sessile prey that is well hidden, hard to crack or otherwise “tough to handle”. Overall predictions succeed in separating different feeding guilds, but in some cases do not succeed in distinguishing between species. Knowledge on feeding behaviour on slow and sedentary benthic prey is a limiting factor in this stage of methodology development.