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 cod Gadus morhua, 4.44 for haddock Melanogrammus aeglefinus, 4.88 for European hake Merluccius merluccius, 5.00 for megrim Lepidorhombus whiffiagonis and 5.27 for whiting (Merlangius merlangus). The level of taxonomic resolution of the prey did not greatly affect mean TL predator values; an effect on variance was evident, low resolution masking intra-population variability in TL. Generalized additive models were used to explain the variability of predator TL caused by environmental variables (ICES subdivision, season) and predator characteristics (length). Significant year, location, season, and interaction effects were found for some species and with length at the scale of ICES subdivision. The species specific variability of TL could be due to spatio-temporal variations in prey availability and/or in predator selectivity following ontogenetic changes. Omnivorous fish TL was less affected by spatio-temporal variations. In addition, results showed that the omnivory index and TL variability provide dissimilar information on predator feeding strategy. Combining information on TL variability and omnivory allowed us to separate between within-individual and between-individual components contributing to trophic niche width and to identify the type of generalization of fish predators