Why So Few Returning Fish Isn’t Going to Lead to Reintroduction Success

Portland General Electric’s (PGE) goal for success of the Pelton-Round Butte Project reintroduction project is to establish self-sustaining, naturally reproducing runs of spring Chinook, summer steelhead and sockeye salmon. If this goal is actually achieved, mitigation for lost adult fish returns from hatchery production will no longer be needed. Natural reproduction by fish in natural habitat made accessible from reintroduction will fill that need.

After eight years of adult returns of fish from upstream reintroduction efforts (and nearly completed returns of spring Chinook and sockeye for 2020), that measure of success is going to be impossible to achieve with the level of adult returns from reintroduction to date. As you can see from the accompanying graph, in only one year did adult returns to the Pelton Trap at river mile 100 for any of the three species exceed 100 individuals and most returns were considerably lower than that! And perhaps more telling is that the overall trend in adult returns is not up, an indication that success is not just around the corner.

There are several reasons why low numbers of adult returns the project has realized will not result in successful reintroduction in tributaries upstream from Lake Billy Chinook.

First, there have been so few reintroduction-origin salmon and steelhead returning that there just is not enough mating activity for sufficient reproduction to drive increasing numbers of fish. If you assume that roughly half of any given return is made up of females, it is impossible under any assumed fecundity, egg deposition and egg to smolt survival scenario to increase adult return numbers. It is important to remember that the return numbers reported by PGE and shown here (with the exception of sockeye for which there is no hatchery production) are largely the result of massive releases of hatchery fry and smolt and not natural reproduction! 

Second, there is a concept in the study of population dynamics and particularly in the study of fish populations called depensation or, individually, depensatory mortality. In fish stocks, a decrease in the size of the breeding population (or in this case, a breeding population too small to start with!) will lead to reduced egg, fry and smolt production leading to population reproduction failure and subsequent local extinction. 

Simply put, in small populations there just are not enough individuals present to successfully find each other and reproduce. In the case of the reintroduction of salmon and steelhead upstream from the reservoir, depensation is a factor because at less than 100 returning individuals (and in the case of PGE’s reintroduction, many less) there are not enough spawning individuals present to easily and reliably find each other to successfully reproduce. Even without direct evidence, one can easily see that finding a mate is, if not hard, then close to impossible when you spread that few adults across the Crooked, Deschutes and Metolius rivers. 

The sad story of “Lonesome Luke” as told by Greg McMillan in DRA’s last newsletter and in song by DRA Board member John Schwartz, put a face and a name to these problems. “Luke the Steelhead” was given a tag by PGE biologists when he was captured and transported upstream thus making it possible to track his movements. From this record of Luke’s movements, we know he wandered tirelessly up and down the Crooked River looking for a mate. Based on his movements and his last known position, it is reasonable to conclude that there were so few female steelhead available that Luke never did find a mate and died without successfully reproducing, a complete waste of his life and another failure for reintroduction. 

In summary, it is impossible to successfully reintroduce a population when the numbers you are working with are so small that they can’t produce sufficient returns, when the few breeding individuals present can’t find each other to actually reproduce, and when the available number of breeding individuals is so small that the limited exchange of genetic material will not contribute to the diversity of the population.