Distribution, movements and diet of macrocrustaceans on a Scottish sandy beach with particular reference to predation on juvenile fishes

Shrimp and crabs were sampled by beam trawl over tidal and diel cycles on a sandy beach on the west coast of Scotland in June and August. There were significant positive relationships between depth (0 to 5 m) and numbers of individuals and numbers of species caught. More species and individuals were caught at night than by day, but there was no evidence for the existence of distinct 'day' and 'night' communities. Differences in species composition and numbers caught reflected differences in the migratory and burying behaviour of individual species. The macrocrustaceans found on this beach are omnivorous predators feeding on a wide range of prey species. Predation by them is one cause of mortality for the populations of juvenile fishes on the beach. Two species, the shrimp Crangon crangon and the shore crab Carcinus maenas, account for most of such predation by macrocrustaceans, which affects both round and flatfish juveniles in varying proportions at different times. In particular, the intensity of predation was generally greatest at night for both species.