Stock Identification of Naturalized Brown Trout in Lake Superior Tributaries: Differentiation Based on Allozyme Data

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Abstract
Eight collections of brown trout Salmo trutta from four Lake Superior tributary systems in Wisconsin (the speciesˈ principal range in the Lake Superior drainage) were analyzed electrophoretically for enzyme expression at 15 polymorphic loci. Collections included site‐specific samples from the Brule River, as well as anadromous and resident fish from the Brule and Sioux Rivers. Significant differences occurred among the eight samples at 6 of 12 possible locus comparisons (P < 0.01). Hierarchial analysis indicated that significant differences also occurred among the four drainages and among samples within drainages. Average genetic fixation values Fst indicated that the genetic variation observed among the four drainages represented approximately 1.9% of the total variation and was similar to the variation observed among samples within the Brule and Sioux drainages (1.6%). The average sample of brown trout contained about 96.5% of the genetic variation observed. Comparisons between anadromous and resident fish indicated significant genetic differences in the Brule River (P < 0.01) and nearly significant ones in the Sioux River (P < 0.07). Thus, brown trout from southwestern Lake Superior drainages appear to have a population structure organized by drainage system and by life history. Their level of stock differentiation was much less than that observed in native populations from Northern Ireland, and probably reflects the recent (1900s) introduction of brown trout to the Lake Superior watershed. Similarly, differentiation among New York State hatchery stocks was 2.2 times greater than that observed among wild Lake Superior populations. Genetic differences among Lake Superior brown trout may indicate a relatively rapid rate of population differentiation since introduction, or the partial preservation of the original genetic identifies of the different European sources that were stocked into the basin. Management actions such as stocking, regulations, and assessment should be chosen that will preserve the stock structure of brown trout in Lake Superior.