Striking a balance with Deadwood Reservoir kokanee

With June here and lots of sunny days ahead, it is a good time to begin planning extended fishing trips to your favorite summer fishing destinations. One backcountry destination that has grown popular in recent years, particularly with kokanee salmon anglers, is Deadwood Reservoir in Central Idaho southeast of Cascade.

If anglers are planning on taking a trip to Deadwood to chase kokanee this summer, they are reminded when the 2019-21 Idaho Fishing Seasons and Rules were adopted in Nov. 2018, the bag limit was reduced to 15, and the possession limit was reduced to 45.

Why the limit reduction?

Before understanding why the reductions were needed, it’s important to understand how Deadwood Reservoir is managed. Biologists manage its kokanee population with three goals in mind: providing a quality kokanee fishery, providing a trophy fishing opportunity for large trout and landlocked Chinook salmon, and providing a crucial egg source for hatchery-raised kokanee that are stocked in other lakes and reservoirs.

Kokanee reproduce naturally at Deadwood Reservoir using its numerous tributaries, which provide excellent spawning habitat. Fish managers began collecting kokanee eggs at Deadwood Reservoir in 1986, and have collected them there almost every year since.

“It’s our primary source for early run kokanee eggs,” said John Cassinelli, Southwest Region fisheries manager.

The reservoir’s kokanee population has provided up to 7 million eggs to Fish and Game hatcheries in a year. Managers stock the young, hatchery-reared kokanee in up to 20 other waters statewide. Check out the Kokanee Chronicle for more information about Fish and Game’s hatchery kokanee program.

At the same time, the reservoir is managed for both a quality kokanee fishery, and a trophy fishery for trout and landlocked Chinook salmon. This is a delicate balance, even without factoring in the needs of Idaho’s hatchery kokanee program.

Fisheries managers try to stay in the “sweet spot” that provides enough juvenile kokanee for trophy trout and landlocked Chinook salmon to feed on and grow large without reducing the average size of adult kokanee to the point that it discourages anglers from fishing at Deadwood, or affecting the egg take for hatcheries.

Because kokanee are very “density-dependent,” the fewer fish there are, the bigger they tend to get, but they can also overpopulate. When the kokanee are forced to compete for limited food, their growth can be stunted, and fish are smaller than anglers want.

Prolific natural reproduction often occurs in Deadwood Reservoir’s tributaries, so the reservoir has historically had an overabundance of smaller kokanee.

Managing for better kokanee fishing and providing for hatcheries

Managers have tried using various methods to control the Deadwood Reservoir kokanee population since the 1970’s. Since the 1980’s, they have used a weir on the Deadwood River — the reservoir’s primary tributary — to collect eggs for the hatchery program and to restrict the number of fish that go up and spawn naturally to manage kokanee numbers for better fishing.

“We control the number of females released above the Deadwood River weir to naturally spawn based on the size of females and number of eggs per fish,” Cassinelli said.

From 2006 to 2011, managers ramped up their efforts to to address the reservoir’s overabundant kokanee population, installing a weir on an additional Deadwood Reservoir tributary where kokanee were spawning. At the same time, they continued to restrict the number of natural spawners in the Deadwood River, which also helped limit the population.

These efforts proved successful in decreasing the kokanee population and increasing their average size. But they ended up being too successful, and the population fell below the number required to meet the statewide demands for early run kokanee eggs from 2015 to 2017. The department discontinued its egg collection at Deadwood Reservoir in 2017 after managers observed a continued downward trend in the population, but resumed it in 2018.

Bigger kokanee means more anglers, more fish harvested

With the lower numbers of kokanee and increased fish size in Deadwood in recent years, managers were concerned that the combination of larger kokanee, the increasing popularity of kokanee fishing, and the liberal bag limit would result in an unsustainable level of angler harvest. Prior studies not only showed that bigger kokanee attracted more anglers, but larger fish are more frequently caught, thus increasing angler success rates.

In the early summer of 2018, researchers conducted a population survey at Deadwood Reservoir, which gave them an estimate of the kokanee population. Researchers followed up with a creel survey later that summer, and found sport anglers harvested about 26,000 adult kokanee.

Based on the 2018 netting, about 54,000 adults were expected to return to the weir on the Deadwood River, but only about 20,000 adult kokanee returned in the fall of 2018, meaning sport anglers harvested up to 57 percent of adult kokanee prior to spawning.

While anglers were typically harvesting less than 15 fish per day, the average trip duration for anglers surveyed was six and a half days, and nearly a third of anglers who caught kokanee left the reservoir with more than 50 fish. Because of that, the reduced daily limit at Deadwood Reservoir was needed to address the possession limit as well.

“Sport angler harvest at Deadwood Reservoir was substantial,” Cassinelli said. “Based on what we saw from the creel survey last year, we estimate the reduction in the possession limit would reduce harvest at Deadwood Reservoir by about 20 percent.”

Reducing bag limits was a balance between meeting anglers’ desires, kokanee population and size objectives while also meeting hatchery production goals. It was scoped through the rule change process in 2018 and broadly supported by anglers with more than 82 percent of 600 anglers who commented on the plan supporting the reduction.
The good news

Despite kokanee limits being reduced at Deadwood Reservoir, the average fish size is good for anglers – about 13-15 inches on average – and the limit remains fairly liberal compared to other kokanee fisheries in Idaho, which was important to fish managers and anglers.

“It’s a balance,” Cassinelli said. “Deadwood Reservoir is secluded: it’s 35 miles of dirt road no matter what route you take. We still wanted there to be an incentive for people to make the trip up there.”

The kokanee population in Deadwood Reservoir also appears to be rebounding following the low numbers observed in 2017. The reservoir was stocked with more than 320,000 hatchery fingerling kokanee in the summer of 2017, and 67,000 in 2018, to boost the population recovery. The 2018 spawning population met egg demands for Fish and Game’s hatchery program, which was about 3 million kokanee eggs.

“Kokanee populations go through these cycles, and it seems like we’ve turned the corner at Deadwood Reservoir,” Cassinelli said. “Now, we’re heading back up in the other direction, and we don’t want to end up in a situation where we have a lot of small fish and a 15-fish limit. What we’re hoping to do is try to head that off and keep the population where it is right now with decent numbers of 12 to 14 inch kokanee, but not an under or overabundance.”