Preliminary results from the 2021 spring suppression effort on rainbow trout in the South Fork Snake River

As scheduled, Fish and Game staff completed 24 days of electrofishing from April 19 through May 27. We maintained our plan by shocking Monday through Thursday at four reaches described in Table 1. We typically launched the electrofishing boats by 8:30 a.m. and concluded daily removals by 3:00 p.m., though we started later (10:00 a.m.) on Mondays to avoid crowding with boat anglers below Palisades Dam.

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Pat Kennedy Idaho Fish and Game

We typically operated two electrofishing boats, except on nine days we experimented with operating three electrofishing boats. Experimenting with more boats may help us understand if we can decrease the duration of future spring suppression efforts and increase catch by deploying more boats over a shorter period. The number of Rainbow Trout needing to be removed annually will be 30% of the abundance of Rainbow Trout estimated at the Conant monitoring site we survey annually in the fall.

SFSR Electrofishing
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James Brower Idaho Fish and Game

Overall, we averaged 1.4 Rainbow Trout per minute of shocking, averaged six hours and 17 minutes of electrofishing per day, and shocked for approximately 9,000 minutes (150 hrs.). This resulted in the removal of 10,654 Rainbow Trout. Now we have good estimates of catch-per-unit-effort (1.4 fish/min.) so during the planning of future spring Rainbow Trout suppression efforts, we can accurately estimate the time needed to achieve our goal. We weighed and measured 578 (5.4%) Rainbow Trout, and they averaged 15.7 inches and 1.6 lbs. Rainbow Trout were largest in sections 1 and 2 (Table 1), where they averaged 16.1 inches and 1.7 lbs.

We transported Rainbow Trout removed from the South Fork Snake River to four locations on the Henrys Fork Snake River, three public ponds, and to the upper Big Lost River (n = 923). Rainbow Trout were stocked at Jim Moore Pond (n = 1,095), Trail Creek Pond (n = 581), and Louis Pond (n = 647) in Swan Valley. In the Henrys Fork, we released Rainbow Trout at Del Rio Bridge (n = 1,622), the railroad trestle bridge downstream of St. Anthony (n = 1,969), Warm Slough Campground (n = 1,986), and Beaver Dick Park (n = 1,423). Transport mortality was low and averaged 2.0%. We were excited to relocate Rainbow Trout from the South Fork Snake River to other fisheries and to supplement Rainbow Trout populations where abundance of Rainbow Trout has decreased in the lower Henrys Fork and upper Big Lost. We tagged 100 Rainbow Trout released at Beaver Dick Park with T-bar anchor tags (Figure 1). These tags will help us determine how translocated fish benefit anglers fishing the receiving fisheries, and determine how far these transplanted fish move after relocation into new waters. If you encounter one of these tags, please report it following the instructions on the tag, or at


Figure 1. Examples of a T-bar anchor tag used by IDFG’s “Tag, You’re It” program.

T-Bar Tag
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Our goal was to cause the population of Rainbow Trout to decline more than what natural reproduction could replace. We estimated that we needed to remove approximately 12,000 Rainbow Trout, which we conservatively estimated to be 30% of the Rainbow Trout population in the upper South Fork Snake River. We anticipated that removing that many trout via electrofishing was going to be a difficult goal to achieve, and we fell slightly short of that goal. Thankfully, the Rainbow Trout Harvest Incentive Program has helped to achieve the goal of removing 30% of the population! To date, anglers have submitted 1,393 Rainbow Trout heads to the program. Angler harvest, combined with suppression, confirms that 12,047 Rainbow Trout have been removed from the South Fork. We know that some anglers do not turn their harvested Rainbow Trout into the incentive program, so our count of 1,393 is a minimum number harvested, and we still have another six months of harvest for anglers to contribute. This emphasizes the important contribution from the anglers that contribute to the Rainbow Trout Harvest Incentive Program and is a poignant example of how these anglers contribute to Cutthroat Trout management on the South Fork. These anglers also provided over 100 lbs. of filets to local food banks. These contributions from our anglers not only benefit Yellowstone Cutthroat Trout, but the local community as well. We appreciate these contributions! To date, these anglers have been rewarded $3,300 with one angler catching a fish with a $1,000 tag!



Related Story - Rainbow trout suppression on the South Fork Snake River