Genetic Differentiation and Hybridization between Stocked Hatchery and Native Brook Trout in Great Smoky Mountains National Park

Abstract

Starch gel electrophoresis was used to examine the protein products of 34 presumptive loci in nine populations of brook trout Salvelinusfontinalis from Great Smoky Mountains National Park and in two brook trout hatchery strains that were derived from northeastern U.S. populations and used extensively for stocking in the southern Appalachian Mountains. Five of the streams sampled had not been stocked and presumably contained only native brook trout. Three other streams contained native populations but also had been stocked, and the remaining stream originally was devoid of natives and presumably contained only introduced brook trout. Mean genetic similarity was high among the native populations (7 = 0.985 ± 0.017 SE) and among the hatcheryderived populations (7 = 0.986 ± 0.003). Mean genetic similarity between the native populations and hatchery stocks was 0.906 ± 0.024. These results are consistent with previous studies suggesting that native brook trout in the southeastern United States are taxonomically distinct from northeastern brook trout. Genotypes at diagnostic loci demonstrate that introgressive hybridization has occurred between hatchery and native trout in all three stocked populations. Average individual heterozygosity was lowest in the native populations (/)Q -0.025), highest in the hatchery stocks (//o = 0.112), and intermediate in the stocked populations (H Q = 0.053).

Genetic Characteristics of Southern and Northern Brook Trout (Salvelinus fontinalis) Populations at the Zone of Contact

“…Allele frequencies at the other markers were compared to those observed in northern and SABT populations characterized in previous studies [7][8][9][10][11][12][13][14][15][16]. Individual heterozygosity and polymorphism were calculated across five loci to assess levels of genetic diversity within each population [32].…”

“…The Grassy Branch population was characterized as southern Appalachian, and the Henshew Branch population was characterized as pure northern. Results from earlier genetic studies [8,11,14] and its geographic location suggest that the Holston River historically contained the southern Appalachian lineage, so the presence of the northern allele is likely due to stocking.…”

“…Genetic differences between the two lineages may be large enough to justify distinction at the subspecies level [7,8]. In addition, screening of allozyme [7][8][9][10][11][12][13][14][15][16], mitochondrial DNA [17][18][19], and microsatellite nuclear DNA [20,21] markers has uncovered smaller scale genetic variation throughout the geographic range of brook trout. Differentiation at smaller geographic scales may reflect different colonization histories, as well as differential effects of selective and non-selective population genetic processes.…”

“…Comparison of external microbial assemblages suggested that SABT exhibit greater ability to inhibit microbial growth in their epidermal mucus than do northern brook trout of hatchery ancestry [26]. Demonstration that SABT are genetically distinct from northern-origin hatchery stocks led management agencies to assess the heritage of populations within their jurisdiction, for example, in the Great Smoky Mountains National Park [8,13], Tennessee [11], North Carolina [12,16,27], and Georgia [10]. Molecular and adaptive differentiation may warrant management of brook trout populations or groups of populations as evolutionary significant units [28], although some of their population genetic differentiation may reflect stocking history.…”

Book Review

“…Discussion of geographic distribution of genetic variation did not mention such key driving factors as the recolonization of North America by fishes after deglaciation. Discussion of the effects of stocking hatchery brook trout did not mention the landmark work of McCracken's group (e.g., McCracken et al 1993), and of interspecific hybridization did not mention introgression of rainbow trout into cutthroat trout gene pools (Leary et al 1995). The concluding discussion of an integrated management strategy, describing designation of areas where different degrees of preservation or manipulation of aquatic gene pools would be practiced, omitted discussion of such key operational issues as how areas would be designated and subtleties of management.…”