Assessment of tributary potential for wild rainbow trout recruitment in Hebgen Reservoir, Montana
Watschke, Darin Allen
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Trout fisheries in Montana reservoirs are almost entirely maintained by stocking hatchery fish. An exception is Hebgen Reservoir, where wild rainbow trout Oncorhynchus mykiss were established in 1979. Continued, unexpectedly low gill net catch rates of rainbow trout led to the objective of this study, which was to assess tributary production of wild rainbow trout and identify potential limiting factors. A combination of redd surveys, adult, young-of-the-year (YOY; age-0), and juvenile (age- 1 and age-2) trapping, and measurements of water temperature and spawning and rearing habitat was used to assess spawning use and habitat characteristics of 11 tributaries, comprising 170 stream kilometers, in 2002 and 2003. A total of 5,642 redds were counted, suggesting the number of spawners was not limiting. Redd occurrence within individual habitat units was positively associated with spawning gravel densities, and negatively associated with rearing habitat density. At the tributary scale, redd abundance was positively associated with availability of both spawning and rearing habitat. Temperature also appeared to influence spawning as most production occurred in tributaries with May to July temperature averaging 8 to 10 .C. The majority (80%) of spawning occurred in only two of the 11 tributaries (Duck Creek and the South Fork of the Madison River). These tributaries contained a combination of abundant spawning and rearing habitat. Rainbow trout YOY production estimates exceeded 4.7 million in 2002 and 2003 combined and abundant YOY and age-1 and age-2 juvenile rainbow trout were captured during spring and summer outmigrations in two lake tributaries. Estimates of available spawning (7.0 ha) and rearing habitat (1.1 x 106 m3) suggest that tributary habitat does not limit the rainbow trout population of Hebgen Reservoir from reaching a self-sustaining level that would meet the recreational requirements of the sport fishery. However, high densities of YOY and juvenile brown trout Salmo trutta captured during outmigrant trapping of rainbow trout suggests that competition and predation may be affecting overwinter survival of rainbow trout in the tributaries. Protection and enhancement of tributary habitat in other reservoirs offers the potential for greater use of wild trout for maintenance of trout fisheries in lentic systems.