Effects of Stocking on Genetics of Wild Brook Trout Populations

The study was undertaken to evaluate the long-term genetic impact of maintenance stocking upon wild brook trout (Salvelinusfontinalis) populations in Wisconsin. Trout were collected from streams of the Wolf and Fox River drainages and from the Osceola State Trout Hatchery. The stocking histories of the streams ranged from unstocked to heavily stocked for many years. The planted fish consisted primarily of fingerling and catchable brook and brown trout (Salmo trutta). Blood plasma and whole-eye homogenate samples were analyzed electrophoretically for transferrin (Tf) and lactate dehydrogenase (Ldh-B2) systems. Esterase was monomorphic in all samples, but Tf and LDH displayed genetic polymorphism. The occurrence of several Tf A/A phenotypes among wild fish is notable because previous genetic studies considered the combination to be lethal. The hatchery stock was genetically distinct from most wild populations at both loci. Variation of Tf allelic frequencies among wild populations uggested an undisturbed natural geographic pattern. There were significant correlations between Ldh-B2 allelic frequencies and stream stocking histories, however, with the wild type allele decreasing in importance as stocking intensity increased. This relationship does not seem to reflect interbreeding between wild and hatchery trout. Rather, it may indicate alteration of selective pressures induced by ecological interactions between the two stocks. Evaluations of maintenance trout stocking programs have typically focused on survival and harvest of the stocked fish with little attention directed toward impacts on the native populations. Several recent studies suggest, however, that wild stream trout populations may be adversely affected by..stocking through increased angling pressures (Butler and Borgeson 1965), competitk)n with domesticated fish (Thuemler 1975; Vincent 1975), or by other behavioral interactions between the two stocks (Butler 1975). Various authors have speculated that native gene pools may be altered through interbreeding of wild and hatchery fish (Smith and Needham 1942; Miller 1957; Calaprice 1969). Although examples of genetic swamping through interspecific hybridization are •vell known among salmonids (Miller 1961), standard morphological analysis has generally •Journal Paper number J-8947 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project number 2075. • Present address: 219 Hodson Hall, 1980 Folwell Avenue, Department of Entomology, Fisheries and Wildlife, University of Minnesota, St. Paul, Minnesota 55108. proven inadequate to evaluate genetic interactions at the intraspecific level. Biochemical evidence of possible genetic exchange between •'ild and hatchery stocks has been presented by M•ller (1970) for Atlantic salmon (Salmo salar) and by Eckroat (1971, 1973) for brook trout (Salvelinusfontinalis), but in both situations the data were generated only incidentally and lacked support of stocking information. In this paper we compare wild and domesticated stocks of Wisconsin brook trout at several genetic loci as determined by electrophoresis of blood and whole-eye proteins. The study was designed to provide information on (1) levels of genetic variability in the two stocks; (2) spatial relationships of breeding units among wild populations; and (3) genetic impact, if any, of stocking upon the native brook