Effects of habitat fragmentation by Toston Dam on genetic structure, natal origins, and homing of brown trout in the upper Missouri River

Abstract

Brown Trout became a key coldwater sportfish after their establishment in southwest Montana, where maintaining abundant Brown Trout populations is a priority for Fish, Wildlife & Parks. Toston Dam was built in 1940, blocking upstream fish passage, and creating a run-of-river, irrigation storage reservoir. The Upper Missouri River below Toston Dam was among 12 Montana rivers designated as Blue-Ribbon Trout Streams in 1969 but currently supports a Brown Trout population at low abundance. Biologists have been concerned with the decline in Brown Trout abundance in this reach of the Missouri River since the early 1990s. The effects of Toston Dam on Brown Trout in the Upper Missouri River sub-basin are not well understood. We therefore assessed how habitat fragmentation by Toston Dam affects Brown Trout in the Upper Missouri River to improve management of the species. Genetic analyses were used to assess whether Brown Trout above and below Toston Dam differ genetically. We used strontium isotopic and elemental ratios in water and otoliths to identify natal origins of Missouri River Brown Trout. We used radio telemetry to determine if migrating adults translocated above Toston Dam were attempting to return to Sixteenmile Creek, a presumed recruitment source for Brown Trout below Toston Dam. Results showed no genetic difference between Brown Trout above and below Toston Dam, but genetic structure within the sub-basin resembled patterns observed in native populations. Otolith microchemistry indicated only 11 of 80 Missouri River Brown Trout originated there. Fourteen fish were from tributary creeks, emphasizing the importance of tributaries for recruitment. Only three fish originated in Sixteenmile Creek, and just 1 of 60 telemetered fish ascended it, suggesting it is less important in sustaining Brown Trout below Toston Dam than previously thought. In contrast, the Gallatin River was important for Missouri River Brown Trout as 37 of 80 fish were from the Gallatin River and 32 telemetered fish ascended it. Brown Trout translocated above Toston Dam made extensive use of upstream waterways. Our multifaceted approach produced three complementary lines of evidence that will inform management aimed at improving connectivity, recruitment, and the sustainability of the Brown Trout fishery.