Investigations of Bull Trout (Salvelinus confluentus) in the St. Mary River Drainage, Montana, Report for 2006

Mogen, Jim; Kaeding, Lynn

The bull trout (Salvelinus confluentus) inhabits mainly freshwater environments in western North America, primarily west of the Continental Divide. The historic decline of bull trout led to its classification as a “threatened” species, under the U.S. Endangered Species Act, in 1999. As part of that designation, bull trout were said to collectively exist as five “distinct population segments,” only one of which occurred east of the Continental Divide, in the St. Mary River and Belly River drainages in Montana and Alberta (Federal Register 64: 58910 [1 November 1999]). The study described here, conducted between 1997 and 2006, determined key characteristics of bull trout populations in St. Mary River tributaries in Montana, including locations of spawning areas, sizes of spawning stocks, and the extent that bull trout move among tributaries; identified factors that may unduly limit the populations; and recommended management actions to eliminate or ameliorate the effects of those factors. Electrofishing showed bull trout were widely distributed and often abundant in St. Mary River tributaries. Moreover, the species remained in all of the waters that it historically inhabited in the drainage in Montana. The occurrence of age-0 bull trout indicated recent spawning and reproduction in each creek in which the species was found, except lower Otatso and Divide creeks, and annual reproduction was indicated by multiple age-classes of young fish. In contrast, the occurrence of redds revealed major bull trout spawning areas in only Boulder and Kennedy creeks, as well as above the Slide Lakes in upper Otatso Creek. Recaptures of tagged fish revealed bull trout movements among most creeks, as well as movements both upstream and downstream over the St. Mary River Diversion dam and downstream over the rockslide that forms the Slide Lakes. Although both migratory and non-migratory bull trout remained in the St. Mary River drainage, migratory fish were most obvious because they were caught in traps or moved between creeks. Resident (i.e., non-migratory) bull trout also occurred in some creeks but were less conspicuous that migratory fish. Results of our study and concurrent investigations supported some previous U.S. Fish and Wildlife Service conclusions that bull trout in the St. Mary River drainage are negatively affected by operation of water-storage and delivery systems that are part of the Milk River Irrigation Project (Federal Register 64: 58910). A concurrent study, described in a separate report (Mogen and Kaeding 2002), showed that bull trout are entrained in the St. Mary Canal and thereby lost from the reproducing population. In addition, our results from radio telemetry, also described in separate reports (e.g., Mogen and Kaeding 2005b), suggested the acute reductions in discharge from Sherburne Dam in the fall produced low-flow conditions downstream in Swiftcurrent Creek that led to the death of bull trout. On the other hand, results from that study showed movements of bull trout past the St. Mary Diversion dam, which had previously been thought to be a major barrier to such movements. Timing of those movements is not precisely known but probably occurred when the dam was open, usually between October and April. Brook trout (S. fontinalis) constituted a small portion of the fish communities in St. Mary River tributaries. We found no evidence that brook trout had displaced bull trout or that hybridization with brook trout was an emerging threat to the bull trout genome.

Investigations of Bull Trout (Salvelinus confluentus) in the St. Mary River Drainage, Montana, Report for 2006

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