Local Chinook - smaller & less fit

BLUE RIVER: Hatchery raised male spring Chinook are far less fit than their natural counterparts. That’s the conclusion of a report on the trap and haul operation at the 518-foot tall Cougar Dam on the South Fork of the McKenzie River. In addition, researchers found that overall, natural origin females return as larger adults, giving them a spawning advantage in the wild.

The trap and haul operation, which began in 2010, traps both hatchery and wild spring Chinook salmon. They are then trucked above the dam and released into the river. The work aims to reintroduce salmon to habitat lost during the dam’s construction 50 years ago.

Spring Chinook salmon in the upper Willamette River are listed as threatened under the federal Endangered Species Act. Overall, they’ve lost 32 percent of their historical habitat in the Willamette River system due to dams. About 25 miles of their range in the McKenzie River was lost, according to the report published in the Canadian Journal of Fisheries and Aquatic Sciences.

What’s more, natural production of juveniles in the basin is estimated to have declined by 95 percent since the dams were built.

The study found that, on average, hatchery origin salmon produced fewer offspring compared to salmon that spent their entire lives in the natural environment and that the effect was stronger between hatchery and natural origin males, according to lead researcher Nick Sard, a PhD student with Oregon State University, Department of Fisheries and Wildlife, Hatfield Marine Science Center.

“Our findings regarding the number of offspring produced by hatchery and natural origin salmon corroborate results from several other studies,” Sard said. “We also found that hatchery origin salmon are smaller in length than natural origin salmon in our study system. Interestingly, this size difference may partially explain why hatchery origin salmon produce fewer offspring compared to their natural origin counterparts, at least in our study system.”

According to the study, reintroduction programs above dams will likely be important to recover some migratory fish: a successful program could increase the spawning distributions and the natural production of fish.

An important ingredient of a reintroduction program is having an adaptive management framework, so that new information can be acted upon quickly, Sard said, adding that it’s “also important to create short and long term monitoring plans to evaluate the demographic viability of the population.”

Reproductive success is “the number of juvenile offspring (less than a year old) produced by a reintroduced adult. We estimated reproductive success through genetic parentage assignments to roughly 2000 juveniles sampled in each of four years evaluated,” according to Sard.

In the project, juveniles are hauled downstream from a screwtrap at the upper end of the Cougar Dam reservoir and 731 to 1,386 adult spring Chinook salmon are trapped and hauled each year in the fall from below the dam.

Juvenile salmon passage is important, Sard said. “If downstream passage is poor, managers may need to consider either structural or operational changes that favor fish passage improvements.”

Every reintroduction project is different: species and stocks are different, barriers to migration are different and so is habitat within rivers, Sard said.

“Nevertheless, I would expect and am familiar with similar results from at least some other systems, particularly with respect to effects from release date and hatchery and natural origin on reproductive success,” he said.

The study finds that spawner length is positively related to greater fitness. For example, female salmon will delay spawning with smaller males. In addition, larger females dig deeper redds, are less susceptible to superimposition and are more fecund than smaller females.

Based on these findings, the report concludes, it may be prudent to limit the number of hatchery origin males in reintroduction programs because hatchery males “may be maladapted to the ‘wild’ environment.”

Image above: The new facility at Cougar Reservoir captures salmon for inspection before they are trucked above the dam.

McKenzie River Reflections