Size limitation on zebra mussels consumed by freshwater drum may preclude the effectiveness of drum as a biological controller

The septa lengths of bivalve shells were used to estimate shell lengths of the largest zebra mussels (Dreissena polymorpha) crushed and consumed by freshwater drum (Aplodinotus grunniens) to determine if size limitation could preclude the effectiveness of drum as a biological controller of the zebra mussel. We examined gut samples of drum (273 to 542 mm long) collected from western Lake Erie in 1991, found the largest mussel (shell length = 21.4 mm) in the 11th largest drum (TL = 405 mm), and observed a reduction of mussel size in larger drum. The lack of a relationship between mussel size and drum size for larger specimens suggests that either drum prefer smaller mussels or the gape between the upper and lower pharyngeal teeth restricts drum feeding to zebra mussels of limited size. Although drum may reduce zebra mussel populations, because of the apparent size limitation of prey it is unlikely that drum would be fully effective as a biological controller; thus, this fish should not be introduced beyond its native range for that purpose.