When warm-water fish species like bass, walleye and crappie that aren’t native to the Pacific Northwest but prized by some anglers, overlap with baby spring chinook salmon in the tanks from the Willamette River in Oregon, they consume more baby salmon than native fish per individual, new research has found.
Research by scientists from Oregon State University, the Oregon Department of Fish and Wildlife, and the US Forest Service Pacific Northwest Research Station, recently published in the journal Ecospheremay have implications for salmon at risk and future management strategies.
“Mixed-stock fisheries are complicated and ever-changing,” said Christina Murphy, lead author of the paper, courtesy faculty member at OSU and biologist with the US Geological Survey in Maine. “We provide the science to help managers identify trade-offs to make the best management decisions for each individual site.”
Oregon fisheries managers are increasingly identifying and addressing threats posed by illegally introduced or invasive species that overlap with native fish populations. Partly as a result of this new research, management measures now include removing harvest restrictions on non-native warm-water fish species, which are sought after by some anglers because of their white, flaky meat and ability to combat sport, where their presence can have negative impacts on sensitive species. native fish species, such as salmon.
“As the fish management agency for Oregon, it is our duty to promote and achieve native fish conservation goals and provide opportunities for anglers,” said Jeremy Romer. , co-author of the paper and assistant district fish biologist for ODFW in Springfield. “To do this effectively, we use adaptive management and base our decisions on the latest and most reliable scientific research. We always strive to pool our resources and work with our colleagues to resolve issues and inform management. This project is a good example. »
The construction of dams in the Willamette River and elsewhere has created an unnatural overlap of cold water fish, such as salmon and rainbow trout, and non-native warm water fish, such as the bass and crappie, in the tanks. Fish diet studies show that non-native species can have a significant predatory impact on salmonid populations, including spring chinook.
In the new paper, researchers from OSU, ODFW and the US Forest Service studied fish in Hills Creek and Lookout Point reservoirs on the upper Middle Fork Willamette River, just southeast of Eugene, Oregon. Spring Chinook salmon found in this river are listed as threatened under the Endangered Species Act.
They studied the diet of fish in the reservoir over two periods: March to June and August to September. Previously, it was difficult to identify whether predators had eaten baby salmon, called fry, because they are quickly digested into an indistinguishable slime. Researchers solved this problem by using two methods to identify fish that consume spring chinook salmon in the first few months of their lives: stable isotopes and stomach contents.
Isotope analysis allows researchers to determine what predators ate by tracking nitrogen through the food web. Nitrogen builds up in salmon from the food they eat in the ocean. This nitrogen is then passed from the mother to the baby salmon, and when a predator eats a salmon fry, it absorbs this nitrogen as well. The more the fry predators eat, the higher the level of nitrogen of marine origin.
This high level of fry nitrogen lasted only until May in the study. After that, isotopic analysis only told scientists that the main diet of a predator is any type of fish. The researchers then used the actual information about stomach contents to determine which species of fish were eaten.
They found that walleye were by far the most likely to have juvenile salmon in their stomachs. At the start of the period, 18.5% of walleyes had salmon in their stomach contents. In the last period, 15.8% did so. They were the only later period fish species to have salmon in their stomachs.
At the start of the period, the other predators with salmon in their stomachs were largemouth bass (5.7%), white crappie (3.1%) and then native northern pike (0.6%). ). Black crappie, cutthroat trout, rainbow trout, and yellow bullhead studied in both time periods did not have salmon in their stomachs.
Isotope signature analysis confirmed the results of the stomach contents. Walleye, largemouth bass, and white crappie also had higher levels of nitrogen isotope signatures.
Ivan Arismendi, co-author of the paper and assistant professor in the Department of Fisheries, Wildlife and Conservation Sciences at the College of Agricultural Sciences, added, “Our results support this integrated management strategy, noting that the capturing popular resources non-native warm water species such as walleye can be promoted in areas that overlap with salmon conservation priorities.