Predator–prey interactions between brown trout Salmo trutta and native and introduced ampbipods; tbeir implications for fisb diets

An attempt to improve the food base for brown trout Salmo trutta in Northern Ireland was made in 195859 by deliberately introducing English Gammarus pule.x into several Irish rivers. In addition, another amphipod Crangonyx pseudogracilis, was later accidently introduced into Irish waters. Our study represents the first attempt to examine the trophic interactions between a native fish predator ( S. trutta ) and an array of these native ( Gammarus dueheni celticus ) and introduced ( G. pulex and C pseudogracilis ) amphipods. Feeding experiments, involving young brown trout predators and amphipod prey, revealed that the fish actively selected C. pseudogracilis relative to two alternative Gammarus prey species. Although the trout encountered the Gammarus species more than C pseudogracilis, they were eaten less than Crangonyx. Difficulties in handling and ingestion of Gammarus by trout may be a key component of the preference for the smaller, more easily handled Crangonyx. The microdistribution of the species was altered by the fish, due to predation being greater in particular microhabitats. Our study showed that the introduction of the herbivorous C. pseudogracilis into Irish freshwaters may represent a useful addition to fish diets, particularly for small and or juvenile fish. The repercussions of the deliberate introduction of G. pule.x are less clear. It may improve feeding for fish. but only if it can coexist with indigenous macromvertebrates and thus ultimately improve the range and quantity of possible food items in predator diets. Alternatively, being highly predatory towards other macroinvertebrates including G. d. celticus and C. pseudogracilis, G. pule.x may be deleterious to the diversity of the resident benthic community and hence reduce the diversity of prey available to fish predators.