South Fork Fish Counts

South Fork Snake River 2014 fall population estimates

The Idaho Department of Fish and Game continued annual fish population monitoring at the Conant and Lorenzo monitoring reaches of the South Fork Snake River in the fall of 2014 to evaluate the effectiveness of management actions on the river. Data from these surveys are used in decision making aimed at achieving the goals outlined in the state fish management plan which include preserving the genetic integrity and population viability of native Yellowstone Cutthroat Trout and reducing Rainbow Trout abundance to less than 10% of the species composition at the Conant monitoring site in the upper river. We monitor two sites annually - the Conant monitoring reach near the highway 26 bridge in Swan Valley and the Lorenzo monitoring reach in the lower river near the highway 20 bridge. This is an initial summary of our 2014 data from these two sites and may be subject to change with further review and analysis. In 2014, we generally observed fewer trout than we have captured in the previous couple years. Across the board, for all species at both monitoring sites, abundances decreased in 2014 relative to the previous few years when we were near all-time high abundances.


Total trout abundance at Conant decreased from 5,372 trout/mile in 2013 to 3,671 trout/mile in 2014. All three trout species (Yellowstone Cutthroat Trout [YCT], Rainbow Trout including hybrids [RBT], and Brown Trout [BNT]) experienced declines (Figure 1; Table 1). Cutthroat declined by 771 fish to 1,487 YCT per mile, and this decline was statistically significant (Figure 1). The lesser declines in Rainbow Trout and Brown Trout were not statistically significant. The Rainbow Trout estimate for 2014 at Conant was 1,418 RBT/mile and Rainbow Trout comprised 29% of the catch. The brown trout estimate at Conant in 2014 was 765 BNT/mile and comprised 32% of the catch. The ratio of Cutthroat Trout to Rainbow Trout is similar to what it was in 2010 and 2011, with nearly similar densities of both species.


At the Lorenzo monitoring reach, abundances of both Brown Trout and Yellowstone Cutthroat Trout were lower than those in 2013 and both declines were statistically significant. The Yellowstone Cutthroat Trout abundances decreased for the first time since the early 2000’s, decreasing by 293 fish per mile while Brown Trout abundances decreased by 559 fish per mile (Figure 2; Table 2). We captured 33 rainbow trout during our 2014 electrofishing surveys at the Lorenzo reach which represented 3% of the total catch and was similar to the previous five years. We captured 190 YCT and 1,078 BNT during the survey, representing 15% and 83% of the catch, respectively.

We will be continuing to analyze this year’s data in an attempt to identify the factors involved with these declines in abundance.