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Ashton Reservoir Update

This July, we conducted a fisheries survey on Ashton Reservoir. Ashton Reservoir is located just north of Ashton, Idaho along the Henrys Fork Snake River, and is a popular location for anglers. Idaho Department of Fish and Game (IDFG) staff have previously conducted surveys on Ashton Reservoir in the 1985 (Maiolie 1987) and 2008 (Schoby et. al 2210). Our 2021 survey consisted of setting ten gill nets, and night electroshocking six random sites along the shoreline of the reservoir.

During our 2021 survey we captured at total of 1,683 fish. Catch was predominately Utah Chub (46%), followed by Utah Sucker (24%), Yellow Perch (22%), Redside Shiner (3%), Brown Trout (2%), Rainbow Trout (2%), Longnose Dace (0.6%), and kokanee (0.2%).


Gill nets

Similar to our overall catch, gill-net catch was dominated by Utah Chub (72%) and Utah Sucker (18%) which comprised over 89% of the total catch. Other species captured in the nets included Yellow Perch (6%), Rainbow Trout (3%), Brown Trout (2%), and kokanee (0.4%). Utah Chub and Utah Sucker dominated the biomass in 2021 similar to the 2008 and 1985 surveys, but Yellow Perch were not present in the reservoir in 1985. Species compositions between 2008 and 2021 were comparable. Yellow Perch were first observed during our 2008 survey and the population has increased over the past 13 years.



We conducted our electrofishing effort on the reservoir at night. This was because fish are more likely to be in shallower water after dark, and the fish are easier to capture in shallow water. We utilized boat-mounted electrofishing gear to help us catch fish near the shoreline. We use electricity to momentarily stun fish so it is easier for our staff to net them. The fish are transferred to a live well in the boat to recover where they are identified, measured, and released back into the reservoir.

By electrofishing the shoreline of the reservoir we are able to capture younger fish and smaller fish than our gill nets typically capture. Our catch was dominated by Yellow Perch (45%) which were mostly young-of-the-year. We also captured Utah Sucker (34%), Utah Chub (9%), Redside Shiner (8%), Brown Trout (2%), Longnose Dace (2%), and Rainbow Trout (0.5%). The average length of captured fish was larger in our gill-net catch compared to the electrofishing catch (Table 1).


Table 1. Average total length (inches) of each species of fish captured during our 2021 electrofishing and gill-net efforts. The double dashed line indicates no fish were captured.

The only species IDFG stocks in Ashton Reservoir are Rainbow Trout. Each year IDFG stocks Ashton Reservoir with approximately 40,000 Rainbow Trout greater than 6 inches. Our survey results indicate other species are reproducing and persisting in Ashton Reservoir. By including both gillnetting and electrofishing techniques in the survey, we are able to obtain a more complete look at the fishery. The fish populations in the Ashton Reservoir fishery are strong and should continue to provide anglers with a lot of opportunity.



Maiolie 1987. Ashton Reservoir fishery enhancement evaluation. Job completion report. Idaho Department of Fish and Game, Boise, Idaho.

Schoby, G., B. High, D. Keen, and D. Garren. 2010. Fishery management annual report, Upper Snake Region, 2008. Report No. 10-107. Idaho Department of Fish and Game, Boise, Idaho.