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

We performed laboratory experiments to investigate feeding behaviour of 1-g roup Atlantic cod Gadus morhua and compared the results with stomach conten t records from wild 1-group cod feeding within a shallow bay on the west co ast of Sweden. On the basis of the field observations, 3 prey species were investigated: juvenile shore crabs Carcinus maenas, brown shrimps Crangon c rangon and 0-group plaice Pleuronectes platessa. When fed single species me als in the laboratory, satiation level was mostly independent of prey size, but cod consumed more plaice than shrimps and more shrimps than crabs. Onc e 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 p laice and shrimp ingestion times increased with P:C length ratio, and shrim p times also increased with stomach fullness. Consequently, the profitabili ty of crabs increased up until the maximum edible size, whereas shrimp and plaice profitability peaked at P:C length ratios lower than the maximum edi ble sizes. For a given prey species, size selection in the field correlated closely with the size-dependent profitability relationships. Species selec tion 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 patc hiness and abiotic factors such as prevailing light conditions.