Press Release

Fishing has slowed at Mormon Reservoir, but no signs of winter kill found

After two great years of fishing, it appears angler harvest greatly limited trout carryover into Spring 2019

After two years of spectacular trout fishing at Mormon Reservoir, the trout population has likely returned to what it has traditionally been. Based on sonar data, underwater camera drifts, and reports from anglers, it appears that there are now fewer fish in the reservoir

Anglers who read about the resurgence of the Mormon Reservoir trout fishery might be wondering how the reservoir went from holding big trout, and lots of them, to not holding as many fish.

Mormon Reservoir
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IDFG

While winterkill has been an issue at Mormon in the past, It doesn’t appear that it is responsible for the current decline of trout in the fishery. Idaho Fish and Game biologists have observed very few dead fish around the perimeter of the reservoir, and efforts to document dead trout on the bottom of the reservoir using video equipment did not reveal any data to suggest a major winterkill event occurred.

So if there wasn’t a substantial die-off of fish over the winter, where did those fish go prior to this spring? It’s likely many ended up in anglers’ coolers, which was expected after two years of extraordinary catch rates.

“Although we can’t entirely rule out some level of winterkill, the evidence seems to indicate that last year’s harvest greatly limited the number of trout for carryover into the spring of 2019,” said Joe Thiessen, an Idaho Fish and Game fisheries biologist in the Magic Valley Region.

It may be disheartening for those who missed out on the stellar trout fishing in 2017 and 2018, but angler harvest is exactly what the trout in Mormon Reservoir are there for. Basically, as catch rates increased, so did the number of anglers, and many harvested the bounty of big trout that had built up after Fish and Game opportunistically stocked nearly 35,000 catchable trout in the reservoir in 2015. 

There were not enough fish available to continue stocking at that high rate because conditions improved at other reservoirs when that were unsuitable for trout due to drought in 2015. 

Mormon is currently managed as a put-and-take trout fishery, as there is no natural reproduction in the reservoir and there is frequently low trout carryover through the winter, particularly in low water storage years — which is beyond Fish and Game’s control.

But managers are evaluating the current regulations for the next three-year season setting cycle in 2021. If conditions at Mormon continue to consistently allow trout to overwinter, it’s possible the management strategy there could change.

In the meantime, the good news is that fishery managers have continued to plant some catchable trout, and lots of fingerlings in the reservoir. You can see stocking information in the Idaho Fishing Planner.

Thiessen said that with limited competition, those fish will grow fast. If they survive winter, like they have in recent years, we could see a bunch of big trout in Mormon Reservoir again within a few years.