Predictors of White Perch Population Size Structure and Density: What Drives Population Dynamics of Invasive Fish?

As white perch Morone americana populations continue to expand westward from their native range in Atlantic coastal rivers and estuaries, understanding their invasion potential and population dynamics inland will be critical for the development of management strategies.   Midwestern and southeastern reservoirs and rivers, as well as the Great Lakes, are plagued with overly abundant white perch that may contribute to the decline of sport fish via competition with native species or by consuming eggs.  In North Carolina reservoirs, white perch populations typically either become highly abundant, with slow individual growth rates and diminished maximum size, or they remain at relatively low abundance and achieve maximum sizes comparable to native populations.  We studied small (less than 700 ha) municipal reservoirs in the Piedmont region of North Carolina to quantify abundance, growth, feeding habits, trophic structure, and maturity schedules of four white perch populations.  Two of the populations had high white perch abundance with small individuals (von Bertalanffy L_inf of 165 and 166 mm) and two populations had low white perch abundance with large individuals (L_inf of 225 and 285 mm).  Fish in all populations were relatively long lived, with maximum ages of at least 13 years.  Where white perch abundance was low, body condition was better than reservoirs with higher white perch abundance.  Interestingly, we found that white perch in the high abundance lakes ate mostly zooplankton and invertebrates, whereas those in the low abundance lakes switched to a diet of fish during summer, indicating that timing of, or potential for, piscivory might play an important role in determining maximum body size.  Ongoing stable isotope analysis will quantify trophic position of white perch and other predators in these lakes, and will elucidate more about the ways that competition and predation might differentially influence population size structure.  Ultimately, quantifying the underlying mechanisms associated with these alternative population states should provide natural resource managers with the ability to predict the impacts of newly introduced white perch populations and develop appropriate management strategies.