Selection of prey size and prey species by 1-group cod Gadus morhua: Effects of satiation level and prey handling times

Abstract

We performed laboratory experiments to investigate feeding behaviour of 1-group Atlantic cod Gadus morhua and compared the results with stomach content records from wild 1-group cod feeding within a shallow bay on the west coast of Sweden. On the basis of the field observations, 3 prey species were investigated: juvenile shore crabs Carcinus maenas, brown shrimps Crangon crangon and 0-group plaice Pleuronectes platessa. When fed single species meals in the laboratory, satiation level was mostly independent of prey size, but cod consumed more plaice than shrimps and more shrimps than crabs. Once satiated on crabs, cod ate plaice if they were subsequently offered, but the reverse was not true. The time taken to ingest crabs was independent of the prey:cod (P:C) length ratio, cod length or stomach fullness, whereas plaice and shrimp ingestion times increased with P:C length ratio, and shrimp times also increased with stomach fullness. Consequently, the profitability of crabs increased up until the maximum edible size, whereas shrimp and plaice profitability peaked at P:C length ratios lower than the maximum edible sizes. For a given prey species, size selection in the field correlated closely with the size-dependent profitability relationships. Species selection had no apparent dependence upon handling-time profitability, species-dependent satiation level or gastric evacuation rate. Additional factors of probable importance include prey evasiveness, prey abundance, habitat patchiness and abiotic factors such as prevailing light conditions.