Investigating trophic‐level variability in Celtic Sea fish predators

The trophic level (TL) mean and variance, and the degree of omnivory for five Celtic Sea fish predators were estimated using a database of stomach content records characterized by a high level of taxonomic resolution. The predators occupied a high position in the food web,i.e.4·75 for Atlantic codGadus morhua, 4·44 for haddockMelanogrammus aeglefinus, 4·88 for European hakeMerluccius merluccius, 5·00 for megrimLepidorhombus whiffiagonisand 5·27 for whitingMerlangius merlangus. The level of taxonomic resolution of the prey did not greatly affect meanTLpredator values; an effect on variance was evident, low resolution masking intra‐population variability inTL. Generalized additive models (GAM) were used to explain the variability of predatorTLcaused by environmental variables (International Council for the Exploration of the Sea, ICES, division and season) and predator characteristics (total length,LT). Significant year, location season and interaction effects were found for some species and withLTat the scale of ICES subdivision. The species‐specific variability ofTLcould be due to spatio‐temporal variations in prey availability and in predator selectivity following ontogenetic changes. Omnivorous fishTLwas less affected by spatio‐temporal variations. In addition, results showed that the omnivory index andTLvariability provide dissimilar information on predator feeding strategy. Combining information onTLvariability and omnivory allowed between within‐individual and between‐individual components contributing to trophic niche width to be separated and the type of generalization of fish predators to be identified.