Steelhead 101: Estimating steelhead run size

In Oregon, Science Friday by Nick Chambers

Recently we have described the various methods used by biologists and resource managers to estimate steelhead escapement, which is the number of fish that escape and survive fisheries (all forms of angling) to actually spawn in a watershed.


Run size is the total number of steelhead that return to a watershed each year. In order to estimate run size managers need to have a good sense of how many fish escaped and spawned in the wild and how many fish were harvested in the fishery. The number of fish harvested includes the number of steelhead that were intentionally killed, such as those killed by anglers or fish caught in commercial fisheries via nets, as well as the number of fish that were unintentionally harvested (e.g. catch-and-release (C&R) mortality).

The number of steelhead that are intentionally harvested is provided annually and may also include a net drop-out rate, which is an estimate of how many fish escaped or dropped out of nets and perished. To estimate C&R mortality managers need to know how many fish were caught by anglers. This estimate is typically derived from results of creel surveys, which are often conducted by state fishery biologists. The C&R rate applied to the creel survey varies. For example, on the Olympic Peninsula resource managers apply a C&R rate of 10% to the total creel-survey catch. On the other hand, a rate of 5-7% is often applied to creel surveys in fisheries throughout the Columbia River system.


Regardless of the rate, the estimated total mortality associated with C&R fisheries is used to estimate the amount of unintentional harvest due to angling. Total run size is then estimated by summing the escapement data with the data for direct harvest, net drop-out, and C&R mortality.


While these methods have some margin for error, they are often used to make decisions on how steelhead angling quotas and regulations should be adjusted. The methods above are an example of the many criteria which need to be accounted for when calculating run size, however many rivers do not have escapement or run size calculated on an annual basis for wild steelhead populations. Similarly, not all areas have wild steelhead harvest so the specific metrics for these calculations vary from state to state.

Run size is one of the most basic ways to measure any fish population and is an important tool used to track population health and trends. Wild Steelheaders United is committed to improving the science of steelhead management, and will continue to work with the steelhead angling community to improve the ways we can help fisheries managers collect, analyze, and make decisions based on steelhead field data.