Size-selective predation by roach (Rutilus rutilus) on zebra mussel (Dreissena polymorpha): field stuides

Studies of predation by roach (Rutilus rutilus) on zebra mussel (Dreissena polymorpha) in a large, eutrophic lake showed that there was a clearly marked size threshold (∼ 160 mm SL) above which roach began to feed on mussels. Roach preying on various sizes of mussels selected them in proportions different from their abundance and accessibility in the habitat. The mean size of mussels ingested by roach of 220 mm and larger, which fed predominantly on Dreissena, closely followed the pattern expected for a constant ratio of mean prey size to mean predator mouth size = 0.59. To explain the size selection we applied an optimal foraging approach, based on the ability of different-sized fish to crush (cost) mussels of different sizes, and hence crushing resistance, and energy contents ('benefit'). We found that fish smaller than 160 mm, which showed no inclination to eat Dreissena, would only be able to take small mussels with a very high cost/benefit ratio. The real switch to Dreissena would be expected in fish of 230-240 mm that could take most of their prey from highly profitable, numerous, and easily accessible size classes while keeping the mean prey size at the optimal level relative to mean predator mouth size.