SIZE-SELECTIVE PREDATION OF 0-GROUP FLATFISHES ON A SCOTTISH COASTAL NURSERY GROUND

Fish predation on O-group flatfish (plaice Pleuronectes platessa, dab Limanda Limanda and flounder Pleuronectes flesus) populations on Tralee Beach, Scotland (56 degrees 31' N, 5 degrees 29' W) was studied by stomach content analysis during the summers of 1992 and 1993. I-group grey gurnard Eutrigla gurnardus, poor-cod Trisopterus minutus and whiting Merlangius merlangus and O-group cod Gadus morhua were the major fish predators of the O-group flatfishes. Flatfishes > 45 mm total length appeared to have a refuge in size from predation by the fishes sampled. Predation by fishes was shown to be negatively size-selective, concentrated on the smaller size-classes of flatfishes. Smaller individuals were eaten by a greater taxonomic variety and a larger size range of predators than larger flatfishes. Smaller flatfishes were therefore vulnerable to a larger number of predators. The size distributions of predators affected the pattern of predation because only the larger individuals ate flatfishes. It is therefore proposed that the degree of overlap in the size distributions of predators and prey determine the magnitude of predation. Growth rates and the timing of settlement in any one year will therefore have a considerable influence on predation, with the potential to result in a 'match or mismatch' of predator and prey size distributions.