Fall Chinook trap rates mean less fish to catch early, but more later

Since 1975, adult salmon (primarily Chinook and steelhead) have been trapped and sampled at Lower Granite Dam on the Snake River, located in southeast Washington. That will happen again this year, but more will be caught earlier in the fish’s annual run than compared to some years in the past.

During the fall Chinook salmon run, fish are collected and transported to either the Washington Department of Fish and Wildlife’s (WDFW’s) Lyons Ferry Hatchery or the Nez Perce Tribal Hatchery to fulfill broodstock program needs at each facility.

Broodstock refers to male and female salmon used to create the next generation of fish at the hatcheries. These two hatcheries provide 100% of the hatchery fall Chinook production in the Snake River Basin, and in total release about 5.6 million juveniles annually. These fish are produced to mitigate the loss of wild fall Chinook production in the Snake River Basin caused by the construction and operation of hydropower dams. Hatchery mitigation for fall Chinook in the Snake River is funded by Idaho Power Company (Hells Canyon, Oxbow, and Brownlee dams) the Lower Snake River Compensation Plan Program (Ice Harbor, Lower Monumental, Little Goose, and Lower Granite dams on the lower Snake River) and Bonneville Power Administration (the Federal Columbia River Power System).

Snake River fall Chinook are listed as threatened under the Endangered Species Act. In 1990, only 78 wild adult fall Chinook crossed Lower Granite Dam. The fall Chinook hatchery program is designed to supplement the wild fall Chinook population and has played a critical role in increasing the abundance of wild or natural fish (fish that were born and raised in the wild but could have either hatchery or natural origin parents). Returns of wild fall Chinook to the Snake River Basin in recent years have averaged 10,000 adults, a dramatic improvement from where the wild origin run was in the early 1990’s.

Fisheries managers in the Snake Basin[1] manage the fall Chinook hatchery program to use both hatchery and natural origin adults as broodstock. While both Lyons Ferry and Nez Perce Tribal hatcheries have adult traps, they catch predominantly hatchery fish. That is why these two hatcheries collect fall Chinook for broodstock at Lower Granite Dam. Both hatchery and natural origin fish can be caught there when they make their way up the fish ladder over the dam on their way back to their spawning grounds in the Snake River and its tributaries upstream of Lower Granite Dam.

The fall Chinook run at Lower Granite Dam starts soon, officially on August 18. That is when the fish crossing the dam start being counted and recorded as fall Chinook, and the numbers posted on the internet at the Fish Passage Center website.

At about the same time, but slightly delayed, summer steelhead also start their run through Lower Granite Dam. Management agencies within the Snake River Basin try to limit sampling and handling impacts to about 20 percent of the summer steelhead run, particularly wild steelhead, from Lower Granite Dam trapping operations at Lower Granite Dam. It can be tricky to trap a certain number of one species while both are going through the dam and ensure that we aren’t inadvertently impacting the other at the same time.

In years with high returns, the trap rates at Lower Granite Dam can be set low, hence the overall percent of each run handled is also low. However, with lower returns in both species the last few years, the fisheries managers have had to get a bit more creative.

Since fall Chinook show up at Lower Granite Dam in higher numbers earlier than the summer steelhead, a trapping strategy that seems to work for both species is to set the trap rate higher at the start of the fall Chinook run, then lower it as the summer steelhead run starts increasing. By implementing this strategy the last two years, we’ve been able to meet the fall Chinook broodstock target of including about 30% natural origin fish into the broodstock and not over impacting wild summer steelhead. With similar run sizes predicted in 2020, the plan is again to set the trap rate higher early (70%) and then drop the rate to 20% sometime around the first week of September. By doing this, we are trapping the majority of fall Chinook needed for broodstock in the early stages of that run and decreasing our chances of impacting the wild summer steelhead population over its entire run.

Another benefit of collecting a good number of fall Chinook for broodstock by early September is that older and larger fall Chinook (hatchery and natural origin) and more natural origin fall Chinook tend to return earlier in the season. By trapping at a higher rate to begin with, we get our hands on the priority target fish for the hatchery programs.

What does this mean for the fall Chinook anglers who fish in the Clarkston, Washington and Lewiston, Idaho area? The hatcheries need to collect approximately 3,600 adult fall Chinook for broodstock. Most of these fish will be collected from August 18 to early September. The run forecast for Snake River fall Chinook in 2020 is 18,150 adults (not including jacks — fish less than 22 inches). After the broodstock is collected for the hatcheries, there will still be almost 15,000 adult fall Chinook that pass over Lower Granite Dam this year, more than enough to provide good fishing opportunities for anglers in the area.

[1] Managers include Idaho Dept of Fish and Game, Oregon Dept of Fish and Wildlife, and the Confederated Tribe of the Umatilla Indian Reservation in addition to WDFW and NPT