Effects of exotic amphipod invasions on fish diet in the Lower Rhine

To assess the importance and effect of recent exotic species invasions on components of food webs of common Lower Rhine biotopes (breakwaters and channel-connected sandpits), the diets of two important macrozoobenthivores, perch (Perca fluviatilis L.) and eel (Anguilla anguilla (L.)) were compared before and after invasions of exotic Amphipoda. The diet of small perch (6-12 cm) shifted between 1989 and 1994/1995 following the mass invasion of Corophium curvispinum SARS which, together with chironomid larvae and Gammarus tigrinus SEXTON, now dominates perch diet at breakwaters. In sandpits, perch diet shifted to one consisting mainly of C. eurvispinum and G. tigrinus. C. curvispinum is the most important prey of medium-sized eel (24-33 cm) in both biotopes. At breakwaters, G. tigrinus has become less frequent in their diet. In sandpits, these exotics and the larger exotic gammarid, Dikerogammarus villosus SOWINSKY, have become important food items for eel. Chironomidae decreased greatly in dietary importance. Measures of diet shift (percentage frequency similarity: PS), showed large, significant changes over time in the diet composition of each species in each biotope. PS values ranged from 72 % (Perch: Breakwater) to 18 % (Eel: Breakwater). Due to the changes in food supply caused by recent exotic invasions, dietary overlap between perch and eel at breakwaters decreased between 1989 and 1994, but increased in sandpits. Intraspecies comparisons of diet between biotopes also showed increased similarity over time, especially for small perch, due to the current food web importance of exotic amphipods in both biotopes.