Biologists working to understand what is driving a slow kokanee recovery at Deadwood Reservoir

Crossing off suspects while searching for a smoking gun

Initially, plankton sampling in 2023 suggested that the kokanee population was limited by food availability.

"Kokanee feed exclusively on zooplankton, and the zooplankton community looked pretty dismal when we sampled it in 2023," D'Amico said. "When we repeated that process in 2024, the picture looked a lot less bleak-but still not great."

In 2023 and 2024, sampling at Deadwood also included kokanee gillnets, predator gillnets, and water chemistry analysis. Fisheries staff examined the stomach contents of rainbow trout and landlocked Chinook salmon and did not observe much kokanee predation.

"While there's undoubtedly some kokanee predation in Deadwood, we can say with confidence that it's not the driving factor," D'Amico said.

Water chemistry samples were submitted to two different laboratories for analysis. The takeaway from both labs was that Deadwood was a very sterile waterbody, which wasn't surprising: Most of the upper Payette River watershed has low nitrogen and phosphorus levels. These nutrients form the basic building blocks of the food web.

For both water chemistry and zooplankton, what biologists were missing was context.

"We just don't have quality data over a long period of time to know what 'normal' looks like for Deadwood," D'Amico said.

This year, biologists are continuing to collect water chemistry and zooplankton samples from Deadwood Reservoir. Over time, the data they collect will help provide some of that context.

Biologists are also investigating another potential factor this year: the survival of young kokanee. They are deploying water temperature sensors to better understand kokanee development timing and outmigration from tributaries to the reservoir. This could provide insight into the environment juvenile kokanee are experiencing during a vulnerable life stage.

Fisheries managers aren't waiting for all the answers

Fisheries managers are not standing idly waiting for a definitive answer on what is causing the struggling kokanee population because they're trying to provide fishing at a popular destination, not just solve a biological riddle.

Fish and Game reduced the bag limit at Deadwood Reservoir from 15 to 6 kokanee and stopped collecting eggs from the reservoir over the past three years to boost natural reproduction. They temporarily ceased stocking rainbow trout and stocked 100,000 more kokanee fingerlings from eggs collected from another location in Southwest Idaho to give Deadwood a boost.

Yet, despite these management actions, the kokanee population in Deadwood has been slow to rebound.

"Based on our most recent gillnetting survey, both the size and density of kokanee seem to be trending up, but they're still not where we would hope for them to be," D'Amico said.

What the future holds

There are reasons for optimism, as mentioned earlier, but substantial uncertainty remains regarding the kokanee population in Deadwood.