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

Behavioural aspects of capture and ingestion of O-group dab Limanda Limanda and plaice Pleuronectes platessa by O-group cod Gadus morhua were examined in laboratory experiments. The ingestion time of plaice was longer than that of dab of the same length and gape Limitation was observed at a lower prey:predator length ratio for plaice (0.39) than for dab (0.44). These observations can be explained by prey morphology because the relative body width of plaice is greater than that of dab. Selective predation on smaller size-classes of flatfishes observed in the held could not be accounted for by gape limitation. Pursuit, ingestion, and handling (pursuit + ingestion) times were positively related to flatfish size, and negatively related to cod size. Profitability of flatfishes (weight gained per unit handling time) declined monotonically with increasing prey size for the range of size ratios of cod and flatfishes used in the experiments. There was therefore qualitative agreement between profitability and the selection of smaller size-classes of flatfishes observed in the field. Capture efficiency and the number of times flatfishes were released during ingestion increased with increasing relative prey size. It is therefore suggested that negative size-selection by cod in the field could result from the probabilities of both capture and ingestion after capture, in addition to active behavioural selection.