Steelhead 101: Using weirs to estimate adult steelhead escapement

In Oregon, Science Friday by Nick Chambers

Recently we have described how scientists use redd counts and snorkel surveys to estimate steelhead escapement. This week we focus on weirs, a totally different way of counting steelhead.

Weir on Elwha River. photo credit WDFW

Rather than sending out surveyors to sample stretches of stream where they count redds or fish, the operation of a weir is much simpler. A weir basically blocks the river and forces fish to pass through a small opening where they must enter a trap. The fish are then captured in the trap, and counted and sampled as desired.


There are a number of benefits to using a weir for this purpose. First, fish have to go through the weir to get upstream, so it provides an accurate count of fish without extrapolating to areas and times that were not surveyed. Second, all fish that pass the weir are captured in a trap. This ensures that biologists can collect data on sex and size, in addition to collecting samples of DNA and scales — something not possible with snorkel surveys or redd counts. Third, weirs can also collect steelhead that have spawned and are swimming back downstream to the ocean. This allows biologists to potentially estimate the proportion of fish attempting to repeat spawn.


Steelhead being sampled at a weir


As with any method, there are tradeoffs and limitations associated with weirs. For example, weirs are often removed or modified prior to flood events so they are not damaged. As a result, they miss fish that pass during those times. In addition, weirs may inhibit fish from passing during low flow periods, in which case they may need to be removed. Last, weirs require quite a bit of maintenance and labor. They collect a lot of material that floats down the river, everything from sticks to tires, and that material must be constantly removed and accounted for. Similarly, the fish traps must be checked on a regular basis to ensure the fish are healthy and to eliminate predation by mammals that may see the trap as their personal McDonald’s.


Estimating escapement using weir data is simpler than for snorkel and redd surveys, but still has challenges. The biggest is that biologists must estimate how many fish are missed during high water periods when the weir is not operating. That is not an easy task. Nonetheless, extrapolation beyond that is fairly limited, so if there are good estimates for how many fish the weir misses, then there is a lower level of uncertainty in determining the size of escapement.
Weirs are not used as commonly as redd counts and snorkel surveys, but once in place, they can be operated consistently for long periods of time. And perhaps this is the biggest benefit to operating a weir: they essentially act like a dam and have the potential to provide accurate fish counts over successive years. In turn, that data can be compared to other types of monitoring, such as redd counts and snorkel surveys. This allows biologists to calibrate the latter, something not possible unless a weir is in place. So, weirs are an important tool for monitoring steelhead and their utility goes beyond counting fish to include sampling of fish and calibration of other types of surveys that occur above the weir.