Earlier this year if you asked Idaho Fish and Game how the trout were doing in Island Park’s Warm River, you might have gotten a shoulder shrug.

That’s because the last time the river was surveyed was in the mid 1980s, nearly 40 years ago. To get a better handle on this unique spring-fed tributary of the Henrys Fork River, nine Fish and Game biologists and technicians conducted an abundance survey last month about a mile downstream from the Warm Springs that feeds into Warm River. Some of the findings of the survey surprised John Heckel, fisheries biologist.

“I was kind of surprised at the abundance of juvenile fish,” Heckel said. “It’s great rearing habitat. It’s super clean water. There are a lot of invertebrates so there’s a lot of bugs in there and then there’s a lot of cover with the weeds. There’s quite a bit of timber in there too and those kinds of in-stream habitats are great protection for juvenile fish. There were thousands of juveniles.”

The survey crew found an estimated 1,200 fish per mile using a recapture abundance calculation method. Using the same analysis broken down by species, biologists estimated 772 brown trout, 753 rainbow trout, and 55 mountain whitefish per mile, Fish and Game said in its survey report. Most of the fish were less than 8 inches long. A handful of brook trout also popped up in the survey, but in small numbers. Nongame species found included Paiute and mottled sculpin.

The staff used a towed barge electrofishing setup and two backpack electrofishers to sample the fish population.

“When you think of a typical trout fishery that is a spring (fed) creek, you often find some really big trophy sized trout in spring creeks, but we didn’t capture any that were over 18 inches,” Heckel said. “It doesn’t mean that they’re not in there somewhere, but I guess I was a little surprised we didn’t capture any bigger fish. That reach of river we surveyed does seem like a nursery area.”

Heckel believes mature brown and rainbow trout are migrating up Warm River from Henrys Fork to spawn and only a few big fish remain as permanent residents.

The crew also conducted a survey upstream of Warm Springs in the Pole Bridge Campground area where the stream is smaller, using backpack electrofishing.

“That was 100% brook trout at Pole Bridge,” Heckel said. “You’re getting more into the head waters at that point. So if folks want brook trout fishing, the upper Warm River has a lot of fishing.”

Heckel said Fish and Game doesn’t expect to let the river go another 40 years before its next abundance survey.

“It’ll be good to monitor this on some kind of a cycle,” he said. “We have to work that out. It probably wouldn’t be an annual survey because it’s not a large, well-known river that gets a ton of pressure, but it would be good monitoring so we have some up-to-date data to tell people and to monitor the health of the fishery.”

Heckel speculated that the river’s lack of access points along the nearby roads and its being in grizzly bear country may hold back some anglers.

“You know, that could be the reason why not many people fish it, is because of that grizzly bear presence,” he said. “We found a lot of scat on the road driving in there and on the bank of the river when we were doing the survey. So, it’s there. They’re in there pretty thick.”

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