Ontogenetic diet shifts and interrupted piscivory in introduced largemouth bass (Micropterus salmoides)

The ecology of largemouth bass (Micropterus salmoides) is one of the best known for freshwater fish, though largely through studies within its native range (North America). I studied the habitat and diet of a bass population introduced into a Mediterranean lake. The bass displayed strong ontogenetic diet shifts as follows: young-of-the-year <25 mm fed on microcrustaceans; fish 25-75 mm, on amphipods and insects; fish 100-225 mm, on a freshwater shrimp, small fish and insects; fish 250-300 mm, on shrimp or crayfish; and fish >300 mm, on crayfish and large fish. The diet showed several differences from most previous studies: importance of freshwater shrimp instead of insects, low piscivory, and a delay in the ontogenetic shift to piscivory. Moreover, the ontogenetic shift to piscivory was interrupted at 250-300 mm, with consumption of shrimp and crayfish. This interruption of piscivory has been largely unreported and seems a consequence of the size-structure and species composition of the fish assemblage. A review of the literature suggests that piscivory by largemouth bass might be generally lower in populations introduced outside North America.