Based on large ocean harvests, some recent media reports suggest the California Chinook salmon population is robust; others paint a picture of a species in crisis and point out that hatchery contributions to the population mask the true state of affairs. Data from a recent multi-year research project offer some insights.
Dr. Jason Hassrick, now a fish biologist with the U.S. Bureau of Reclamation’s Bay-Delta Office in Sacramento, focused on the most endangered race of Sacramento River Chinook salmon, the winter-run, in his research begun as a 2011 Delta Science Fellow. The Delta Science Fellows program funds postdoctoral and graduate researchers to work with community and scientific mentors on targeted Bay-Delta research priorities. It is sponsored by the Delta Stewardship Council’s Delta Science Program in conjunction with the California Sea Grant Program.
“Most runs of California Chinook Salmon in the Central Valley are in danger or threatened by extinction,” said Hassrick as part of his initial research premise. “Efforts to recover wild stocks in the Sacramento River system are challenged by various forms of habitat degradation in the river and Delta.”
Dr. Hassrick began his research by implanting miniaturized acoustic transmitters into sub-yearling winter-run Sacramento River Chinook or “smolts.” He then tracked their migration to the Pacific Ocean using an array of 122 acoustic receivers positioned down the Sacramento River, the Delta and the San Francisco Estuary.
“The data gathered during this project on the movement and survival of juvenile winter Chinook will be combined and compared with a parallel tracking study of fall and spring-run Chinook,” said Hassrick. “We hope that our findings will allow managers to better evaluate the effects of different flow conditions and water management practices on salmon survival.”
Then the drought hit California.
As Dr. Hassrick’s research continues to evolve (especially because of the persistent drought conditions), he periodically releases updated discoveries to management agencies and public forums including last year’s Biennial State of the Estuary Conference in San Francisco and this year’s Interagency Ecological Program workshop in Folsom.
For instance, he describes winter-run Chinook salmon holding in place for a period of time upstream of the Delta.
“We had initially thought that they might rear in the Delta itself, where more food might be available to them,” said Hassrick. “It may be that they were holding upstream because they were waiting for a pulse flow of water to trigger their migration downstream.”
The absence of a “pulse flow” because of the drought may have altered winter-run movement behavior. This is prompting Dr. Hassrick to extend his study to see how fish behave in different conditions.
“We need to see what their behavior is like during a normal or wet year to determine whether the holding behavior we see is consistent or whether increased flows cause fish to spend extended time further downstream in the Delta,” said Hassrick. “It is important to understand how migration patterns are affected by different hydrologic conditions because fish are confronted with different factors that affect their chance of survival depending on where they spend their time. In the upper river, they may be more prone to mortality due to diversions whereas in the Delta they may be exposed to a greater abundance of predators.”
Dr. Hassrick points out that “within the same year, winter-run Chinook salmon exhibit markedly different movement behaviors than fall and spring-run Chinook salmon. They tend to stop moving for extended periods of time in a way that none of the other runs do.”
This finding is important he says, “Because the mortality rates are higher in the river than in the ocean, winter-run Chinook have a longer exposure to whatever factors are associated with in-river mortality than the other runs.”
Dr. Hassrick says his current research work at the Bureau is more focused on Delta Smelt. He will, however, continue to study the migration movements of winter-run Chinook salmon. “We need the context of different water year types (dry versus wet) to get a complete picture of their unique behavior in the river.”
Dr. Hassrick will present his further preliminary results at the 8th Biennial Bay-Delta Science Conference to be held October 28, 29, and 30, 2014 at the Sacramento Convention Center. To register for the conference, please click here.