Steelhead 101: Using redd counts to estimate escapement of steelhead
Last week we defined the terms run size and escapement. If you didn’t see the post, check it out. This week we will discuss how fisheries managers actually measure escapement for wild steelhead using redd counts, and some of the challenges they face in doing so.
We begin with escapement because it is usually measured first, and then run size is calculated based on fish that were killed indirectly (e.g. catch-and-release mortality) or directly (e.g. harvested). While escapement can be measured with several methods, our focus here is on redd counts because they are a fairly common method.
Redds are nests dug by steelhead in stream gravel, and as anglers know, redds can be easily observed when they are fresh and water clarity is good. Redds are typically counted by surveyors who walk the stream, or float in boats, but they may also be counted by helicopter or airplane. The challenge with redd counts is they require good stream clarity and lots of labor because steelhead spawn in a variety of habitats and over a period potentially ranging 6-8 months (November-July depending on the watershed). This means managers have to come up with sample designs that allow them to survey certain stream reaches within a watershed on a regular interval. Typically those reaches represent a small proportion of the watershed. There is simply too much habitat for a complete census.
From those surveys , managers can estimate how many steelhead redds are in a watershed in a given year. Next, they have to convert redd estimates to steelhead abundance estimates, which is not as easy as it sounds. Why? Because some female steelhead dig more than one redd. Further, even if we know the ratio of females to redds, sex ratios are not always equal in steelhead – particularly in inland rivers where there are often more females than males – so we can’t just assume that there is one male steelhead for every female. And, sex ratios can change from year to year, making it difficult to convert numbers of females to numbers of males – which is needed to arrive at a total escapement estimate.
Overall, steelhead fishery managers have to make assumptions about:
- The number of redds in unsurveyed stream sections
- The number of redds dug during times surveys were not conducted
- The number of redds per female
- The ratio of females to males.
As you can imagine, that is a tall order! Owing to these challenges, and others (such as poor stream visibility precluding reliable redd observation), there is often quite a bit of uncertainty in escapement estimates based on redd counts, even though this is often the best available method. Moreover, escapement estimates are not even conducted in many Northwest watersheds because of limited funding and labor to complete the field surveys.
The point is that while redd counts are an important tool for estimating steelhead escapement, this method (like all methods) has limitations and uncertainties — something to think about next time you see data on steelhead escapements. This is exactly why it is important to take a conservative approach to managing and fishing for wild steelhead on many rivers until we have conclusive data (such as we have on the venerable Skagit River in Washington) that wild steelhead escapement is robust and sustained over years.
Next week we will review some of the other methods used to estimate steelhead escapement.