Predation of 0-group flatfishes by 0-group cod:handling times and size selection

Abstract

Behavioural aspects of capture and ingestion of 0-group dab Llmanda limanda and plaice Pleuronectes platessa by 0-group cod Gadus morhua were examined in laboratory expenments. The ingestion time of plaice was longer than that of d a b of the same length and g a p e limitation was observed at a lowcr prey:predator length ratio for plaice (0.39) than for d a b (0.44). These observations can be explained by prcy morphology because the relatlve body width of plaice is greater than that of dab. Selectivr predation on smaller size-classes of flatfishes observed In the field could not be accounted for by gape limitation. Pursuit, ingestion, and handling (pursult + lngestion) times were pos-~tively related to flatfish size, and negatively related to cod size. Protitab~l~ty of flatfishes (weight gained per unit handling time) declined monotonically with Increasing prcy slze for the range of slze ratios of cod and flatfishes used in the experiments. There was therefore qualitative agreenlcnt between profitability and the select~on of smaller size-classes of flatfishes observed in the field. Capture efficiency and the number of times flatfishes were released during lngestion increased with increasing relative prey size. It is therefore suggested that negative slze-selection by cod in the field could result from the probabilities of both capture and ingestion after capture, In addition to active behavioural selection.