Gastric evacuation in cod: Prey-specific evacuation rates for use in North Sea, Baltic Sea and Barents Sea multi-species models

Abstract

Gastric evacuation experiments were performed on North Sea cod Gadus morhua. Seven fish species of different energy density, smelt, herring, sprat, sandeel, goby, whiting, and Norway pout, and one crustacean, brown shrimp were tested as prey. A general evacuation model, including predator weight, temperature, and meal size as variables, was fitted to the data on wet weights by means of non-linear regression technique. Additionally the extensive stomach evacuation data on Barents Sea cod from the literature were reanalysed to test for differences in gastric evacuation between stocks from different ecosystems. It was shown that the exclusion of meal size as an explanatory variable caused only minor reductions in the explained variance (0–3.2%) with only one exception (capelin) where the explained variance was reduced by 6.1%. When meal size was excluded from the model, the estimates for the curvature parameter were generally close to 0.5 and since the model can be applied in this form only to estimate field consumption rates meal size was excluded in further analyses. The parameters for curvature, predator weight, and temperature effect were similar for the different prey types and both data sets. The mean curvature parameter was estimated at 0.534 and not significantly different from 0.5. This confirmed the work performed with other gadoid predators. The exponent of the predator weight effect was estimated as 0.306, and the exponent for the temperature effect as 0.110 which corresponds to a Q 10=3.0. The estimates for the prey-specific parameter ranged from 0.00424 ( C. crangon) to 0.0141 (polychaetes). In case of fish prey a large part of the variation in these prey-specific evacuation parameters could be explained by differences in energy density of the prey.