Piscine predation on juvenile fishes on a Scottish sandy beach

Predation by larger fishes is a major cause of mortality for the populations of juvenile fishes on a sandy beach on the west coast of Scotland. Predation was concentrated on the most numerous species (0-group Pleuronectes platessa) in June but with the decline in numbers and growth in size of this species, the fish predators had changed their diet in August to feed principally on small sandeels (Ammodytidae). Six major predatory species had taken 95 per cent of all fish found in predators' stomachs and could be roughly divided into three categories according to their piscivorous tendencies and their abundance. (1) Common species in which fishes formed the main item of their diet (Gadus morhua). (2) Species in which fishes formed only a relatively minor constituent of the diet but which, by virtue of their abundance, potentially had a significant predatory impact (I-group P. platessa). (3) less abundant species whose diet consisted principally of fishes. The intensity of predation was generally greatest at night for G. morhua but during the day for I-group P. platessa. The distributions and movements of the dominant prey species can be interpreted in the light of their predator-prey relationships. The most common benthic species, 0-group P. platessa, move off- and onshore with the ebb and flow of the tides and concentrate in shallow mater at night. Such movements would have the effect of maintaining the majority of the population in depths where predation pressure from nocturnally onshore-migrating gadoids and other predators is likely to be minimized. (C) 1996 The Fisheries of the British Isles.