Feeding ecology of invasive age-0 white perch and native white bass after two decades of co-existence in Saginaw Bay, Lake Huron

The diets and energy content of sympatric populations of invasive age-0 white perch Morone americana and native age-0 white bass Morone chrysops were evaluated in Saginaw Bay, Lake Huron following >20 years of coexistence. Fish were collected during July-November in 2009 and 2010 to assess seasonal and interannual patterns of diet composition, diet similarity, feeding strategy and energy density for the two species. The diet composition by weight of age-0 white bass was dominated by various zooplankton taxa, fish, or emergent insects, depending on the month and year. Although fish occasionally comprised a large fraction of the diet biomass, they were eaten by <24% of white bass each month. The diet composition of age-0 white perch shifted from one dominated by chironomids and other benthic macroinvertebrates in 2009 to one largely consisting of Daphnia spp. in 2010. There was more overlap in standardized diet assemblages in 2010 than in 2009 due to the increased importance of Daphnia spp. in white perch diets in 2010. Contrary to expectations, complete separation of diets was not a requirement that enabled the long-term coexistence of invasive white perch and native white bass in Saginaw Bay. Both age-0 white bass and white perch had a mixed feeding strategy with varying degrees of specialization and generalization on different prey. The inter-annual variation in prey, i.e., higher densities of zooplankton in 2009 and chironomids in 2010, is directly opposite of the pattern observed in white perch diets, i.e., diets dominated by chironomids in 2009 and zooplankton in 2010. Energy density increased from July into autumn/fall for both species suggesting that food limitation was not severe.