Effect of Flow Fluctuations on Brown Trout in the Beaverhead River, Montana

Abstract

The effects of variable flows on standing crops of brown trout (Salmo trutta) in the Beaverhead, River below Clark Canyon Reservoir, Montana, were studied from 1966 to 1980. Large flow fluctuations during or prior to peak spawning in mid to late October appear to have hindered reproduction, thus influencing the recruitment of age-I+ (yearling) brown trout. Flow decreases of 6.59-18.08 m 3/s (54-74%) followed by or preceded by increases of 6.71-14.52 m 3/s (80-369%) occurred during spawning periods that yielded five of the six poor yearling estimates (39-164 yearlings/1,967 m of stream). Fluctuations lasted 3-23 d, with each decrease and increase taking 2-3 and 1-6 d, respectively, to complete. The nine spawning periods that yielded estimates of 333-1,255 yearlings/1,967 m of stream were devoid of these fluctuations. The poor yearling crops, all of which were found in the years prior to 1974, in turn, limited the total standing crops of brown trout during much of the study. Relationships between yearling numbers and the magnitude of the spawning, incubation, rearing, and potential stranding flows were not evident. The disruption of spawning is the most likely means by which flow fluctuations could have hindered reproduction.