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Idaho Fish and Game

Last fall's wild steelhead return exceeded expectations because of unusually small fish


Idaho "B" run fish are famous for their large size, but not all achieve it

Last year’s Idaho steelhead run received a lot of attention for the wrong reason. It was a low run year, and Fish and Game biologists did not initially see as many fish back as they would have liked, but they were pleasantly surprised in the spring. 

The wild run of large fish known as “B-runs” destined for the upper Clearwater, Middle Fork and South Fork of the Salmon rivers received even more attention because of a very low return based on window counts at dams as steelhead migrated up the Columbia and Snake rivers. 

The low return focused the attention and concerns of fisheries managers and anglers alike.  But data from last fall also suggested the return of wild “B-runs” wasn’t as low as window counts estimated, and information gathered during spring in spawning streams confirmed it. The run wasn't great by any stretch, but not catastrophic, either.

The actual spawners in wild B-run drainages exceeded a thousand fish despite dam counts showing less than 500. How could that be? It’s more complex than just counting fish and segregating them into categories. Here’s how it works, and what happened. 

Wild steelhead are divided into two categories  (A-run and B-run) based on length, which is useful for managing Columbia River fisheries, but a bit misleading when those fish return to Idaho streams. 

In general, B-run fish are older and larger than A-run fish, and B-runs also start their spawning migration from the ocean later in the fall than A-runs. With that being said, there is a lot of variation in Idaho's steelhead populations, and what we see in the wild does not always perfectly align with those simple categories. 

Lower Granite Dam on the Snake River is the last of eight dams steelhead cross prior to entering Idaho waters. People counting fish at the dam’s fish ladder classify B-run steelhead as those longer than 30.5 inches. 

Based strictly on fish size at Lower Granite window counts last fall, there were 452 B-runs, which was the lowest estimate since counts started in 1987. However, as it turned out, the window counts did not tell the whole story.

Idaho Fish and Game, along with federal NOAA fisheries staff, trap wild steelhead as they pass through Lower Granite Dam's fish ladders. Personnel tag the fish, measure their lengths, take samples for genetics and scrape off scales that determine how old each fish is.

Tiny electronic tags are also inserted into these fish, which are then detected upstream when the tagged steelhead swim into spawning tributaries. The age, length and genetic data from steelhead tagged at the dam can then be identified within a specific spawning population, giving fisheries managers a better understanding of the biology of these remarkable fish. 

Based on the tag detections over the last nine years, biologists know not all fish destined for the B-run rivers and streams are over 30.5 inches. Some are smaller, and spent only one year in the ocean, while some fish that spent two years in the ocean still don’t grow to 30.5 inches if they did not find enough food in the ocean to grow to that length.

Biologists saw a larger percentage of smaller steelhead returning to these B-run streams during spring. In fact, only 18 percent of the steelhead swimming past the detectors in B-run streams were over 30.5 inches. So, those fish were not identified as B-runs in the window counts at the dam.  

Only after the spawning season could Fish and Game account for that difference based on detections of fish at the instream detectors, so biologists were able to estimate that 1,000 to 2,000 wild steelhead spawned in Idaho’s B-run streams during spring.

To add perspective, there have been seven years since 1987 when the window counts at Lower Granite Dam have been between 1,000 and 2,000 wild B-run fish. These low return years likely also missed some smaller fish, but bottom line, Idaho has experienced some low wild return years in the past. And some of these low return years were followed by strong wild steelhead returns within a couple years when river and ocean conditions were more favorable to steelhead survival.

Even with the boost in B-runs compared to the window counts, it was still a low return for wild steelhead. But it’s good to know there were more wild steelhead spawning than reported last fall. The final estimate of steelhead spawning in all of Idaho’s wild steelhead drainages will come out later this year after all the fish are aged and the genetic analysis is completed.

It is too early to know how this year’s run will be because steelhead are currently leaving the ocean, and only a small number of early fish have reached Idaho. But Idaho’s wild steelhead have seen hard times before, and while Fish and Game expects improved future returns, its likely their resilience will be tested.

In the meantime, Idaho Fish and Game biologists will continue to use these identification tools to better understand the complexities of wild steelhead and manage them into the future.