South Fork of the Snake Trout Population Survey 2016:
South Fork Snake River 2016 fall population estimates: This report is from Idaho Department of Fish and Game Regional Fisheries Biologist, Brett High. If you have any questions or concerns, feel free to contact him at brett.high@idfg.idaho.
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 2016 to evaluate the effectiveness of management actions on the river. Data from these surveys are used in decision making to achieve 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 reach in the upper river. This is an initial summary of our 2016 data from these reaches and may be subject to change with further review and analysis.
Conant
Total trout abundance at Conant changed from 4,032 trout/mile in 2015 to 3,758 trout/mile in 2016, which is slightly below the ten year average (2006-2015) of 4,253 trout/mile. The 2016 estimate for Yellowstone Cutthroat Trout [YCT] was slightly below the 10 year average with 1,450 YCT/mile (10 year average = 1,758). The Rainbow Trout estimate (1,295 RBT/mile) was also below the 10 year average of 1,573 RBT/mile. The Brown Trout (BNT) estimate (1,013 BNT/mile), however, was slightly above the 10 year average of 922 BNT/mile (Figure 1 and Table 1).
We also estimated the abundance of Mountain Whitefish (MWF) at Conant after a fish kill event this year involving Whitefish and PKD (Proliferative Kidney Disease). Our most recent Mountain Whitefish estimate prior to 2016 was conducted in 2012. Here are the results from these two surveys at the Conant reach:
2012 – 11,427 MWF/mile (±8,968)
2016 – 15,793 MWF/mile (±2,993)
Survey results from 2016 suggest that reductions in the population of whitefish in the river resulting from the outbreak of PKD were not significant, and whitefish appear to be thriving in the South Fork.
Figure 1. Estimates of fish per mile for Yellowstone Cutthroat Trout (YCT), Rainbow Trout (including hybrids; RBT), and Brown Trout (BNT) at the Conant monitoring reach from 1982 through 2016 with 95% confidence intervals.
Table 1. Point estimates for Yellowstone Cutthroat Trout (YCT), Rainbow Trout and Rainbow x Cutthroat hybrid trout (RBT), and Brown Trout (BNT) per mile with respective 95% confidence intervals for the Conant monitoring reach of the South Fork Snake River from 1982 through 2016.
Lorenzo
Cutthroat estimates at the Lorenzo monitoring reach are near their ten year average, while Brown Trout estimates increased over 2015, and are now higher than their 10 year average. The Yellowstone Cutthroat Trout estimate for 2016 was 344 YCT/mile, similar to their 10-year average of 378 YCT/mile. We estimated there were 2,172 BNT/mile which exceeds the ten year average for BNT at Lorenzo of 1,588 BNT/mile (see Figure 2; Table 2). We captured 36 Rainbow Trout during our electrofishing survey at the Lorenzo reach which represented 2% of the total catch and was similar to previous surveys.
Figure 2. Estimates of fish per mile for Yellowstone Cutthroat Trout and Brown Trout at the Lorenzo monitoring reach of the South Fork Snake River from 1987 through 2016 with 95% confidence intervals.
Table 2. Point estimates for Yellowstone Cutthroat Trout (YCT) and Brown Trout (BNT) per mile with respective 95% confidence intervals for the Lorenzo monitoring reach of the South Fork Snake River from 1987 through 2016.
