Explaining diet composition of North Sea cod (Gadus morhua): prey size preference vs. prey availability

Abstract

The nature and significance of size preference for fish prey in the diet selection of North Sea cod (Gadus morhua L.) was analysed. The analysis combined information on size-specific abundance derived from bottom trawl surveys with prey size frequencies in cod stomachs from the International North Sea Stomach Database. To estimate the abundance of all potential fish prey in the sea, a length-based number spectrum was calculated, corrected for gear efficiency by the application of a species-specific correction factor, and weighted by local predator abundance to take the spatial–temporal overlap between cod and its prey into consideration. A prey size preference model for cod feeding on fish is presented. Results showed that the preferred predator-to-prey weight ratio is an exponentially increasing function of predator size and an exponentially decreasing function of the slope of the number spectrum. More than 75% of fish found in the stomachs of North Sea cod originated from the least preferred quartile of the prey size range, indicating that prey abundance is the main determinant of the diet composition.