Genetic Impacts of Broodstock Selection Strategies for Wisconsin's Wild Brook Trout Stocking Program

Abstract

In 1995, the Wisconsin Department of Natural Resources began a wild trout propagation program to improve the efficacy of wild trout stocking. The program captures broodstock from a wild source population, spawns these fish in a hatchery, returns the broodstock to their source population, and stocks the offspring in other streams. Two critical decisions in designing supplemental propagation programs that rely upon wild broodstock are: 1) selecting a genetically appropriate brood source, and 2) determining a sustainable proportion of fish collected from the source population. Since 2000, Ash Creek (Richland Co., WI) has served as the sole source of broodstock for brook trout (Salvelinus fontinalis) in southwestern Wisconsin. Questions exist regarding 1) the genetic suitability of Ash Creek as a broodstock source, and 2) the genetic impact of annual removals approaching 50% or more of the adult brook trout in the population. To determine the suitability of Ash Creek as a wild source of broodstock, the genetic diversity of Ash Creek and 13 other southwestern Wisconsin brook trout populations were characterized using 12 microsatellite loci. High levels of genetic divergence were found among populations; however, the majority of divergence appeared to be unrelated to the populations’ geographical proximities to one another. The results suggest the surveyed brook trout populations are 1) small and reproductively isolated and strongly influenced by genetic drift, and/or 2) have been impacted by historical stocking events through the introduction of exogenous genes into the native gene pool. As a consequence, the determination of Ash Creek’s population as a genetically appropriate and regionally representative broodstock is difficult. However, the degree of genetic differentiation observed among all populations studied would suggest a single source of brook trout would be insufficient for propagation throughout the southwestern region of Wisconsin. To determine if annual broodstock removals have resulted in any realized or potential detrimental impacts to levels of genetic diversity in Ash Creek’s population, genetic and morphological (length and weight) measurements were taken from the in-hatchery (wild hatchery broodstock) and in-stream adults (post-broodstock removals), and their in-hatchery and in-stream young-of-year (YOY) counterparts over two years. Levels of genetic diversity among adult components (in-hatchery and in-stream trout) did not differ over two years indicating that despite the large proportion of adult fish removed (50 to >80% per year), the yearly in-stream adults maintained levels of genetic diversity. However, a size bias was observed with larger fish being used as broodfish. The broodstock removals resulted in no reductions in levels of genetic diversity within or among the yearly in-stream YOY components. Levels of genetic diversity among the in-hatchery and in-stream YOY components were similar, although, the effective number of breeders that produced the in-hatchery YOY components was approximately seven times larger than that of the in-stream YOY components. This skew likely reflects a combination of the reduction in the size of the number of in-stream breeders and a lower variance in family size within the hatchery. These data suggest current broodstock selection strategies have had no detectable short-term impacts on genetic diversity levels within Ash Creek. Several results of this study raise concerns that the long-term impact of such strategies will be detrimental. The removal of larger brook trout and subsequent reductions in the mean body size of Ash Creek’s in-stream breeding population will have negative consequences to the population’s reproductive potential (lower mean fecundity) and genetic integrity (lower mean number of breeders).