New research examines how Chinook salmon from West Coast rivers travel through the ocean. It shows that endangered Southern Resident killer whales do not have access to as many salmon prey as previously thought. That does not mean the number dropped, but that it has always been lower than estimated.
Calculating the number as accurately as possible is important because it determines whether to shut down West Coast salmon fisheries. These limits are imposed at certain places and times when the number of salmon falls to a certain level, or what fisheries managers call a “threshold.” The Chinook salmon threshold is a measure of where the endangered whales have a markedly harder time finding prey.
The Pacific Fishery Management Council recently asked NOAA Fisheries to update the threshold to incorporate new science showing fewer salmon in parts of the ocean accessible to the whales. The new science illustrates the way technology is revealing the Pacific salmon life cycle and its intersection with other species in ever-greater detail.
The numerical threshold for fishing limits was first established under Amendment 21 of the Pacific Coast Fishery Management Plan. The plan drew thousands of supportive comments in 2021. The threshold was determined as the average of 7 years of low salmon abundance that included years when the Southern Residents were in varied health. It considered the prey needs of pregnant and nursing whales.
Revision Improves Accuracy
The new science reset the threshold to 623,000 adult Chinook salmon, lower than the original 966,000 first estimated under the Fishery Management Plan 2 years ago. The lower number does not take fish away from the whales or the fishing fleet. Rather, it reflects a more precise and accurate count of how many salmon were present in areas the endangered whales could access off the West Coast all along.
The revision is retroactive to past estimates of salmon numbers. It it resets the entire playing field at once—so, years of low salmon abundance are still low. Future salmon numbers would be expected to hit the new threshold and trigger fishing limits just as often as under the original threshold.
Salmon abundance has never hit the threshold. The last time numbers would have fallen low enough under the new threshold to trigger the fishing limits was in 2007.
The numerical threshold was never a “set it and forget it” number. Instead, it is determined by a formula based on our understanding of where salmon travel off the coast. The Council expected to refine the number as scientists better understand how many salmon leave West Coast rivers and spread through the ocean. The updates reflect improved technology, more data, and a more detailed understanding of salmon biology.
Salmon Follow Different Paths
Improved tracking has refined our understanding of how different salmon stocks move through the ocean. They travel along what some scientists have termed “ancestral feeding routes,” each following its own patterns and cues.
“We are gaining a more sophisticated understanding of how salmon use the ocean,” said Andrew “Ole” Shelton, a research scientist at NOAA Fisheries’ Northwest Fisheries Science Center in Seattle who studies salmon distributions. “People may think, ‘Salmon go out and come back.’ It turns out to be much more interesting than that, and that affects what is available to predators such as killer whales.”
One of the best sources for where salmon go in the ocean is coded wire tags—tracking tags embedded in the snouts of mainly hatchery fish. Fishing crews who catch salmon recover and report the tags and the location. Scientists feed that location data into a model that uses it to map the fish distribution.
Since 2021, researchers have incorporated nearly 20 years worth of additional coded-wire tag collections for fall-run Chinook salmon into the model. The additional data includes about four times as many salmon catch locations. This increases the detail of salmon distribution like additional pixels on a higher resolution television.
Scientists also recently incorporated new coded-wire tag data from salmon inadvertently caught by boats fishing for hake, or whiting, off the West Coast. The hake season extends beyond salmon seasons, so the reports help provide data for other parts of the year. The hake fleet has greatly reduced inadvertent bycatch of salmon, but even the locations of only a few salmon helps fill out the picture, Shelton said.
Salmon Stocks Separate in Ocean
Scientists also now better understand salmon that return to different rivers and tributaries. Scientists had long treated fall-run Chinook salmon from the upper Columbia River and the Snake River as a single group in the ocean. They presumed the fish largely traveled the same way on a similar schedule. That would have made them equally available to the Southern Resident killer whales.
The reality turned out to be not so simple.
Increasing reports indicated that the Chinook salmon from the upper Columbia swim north into the distant North Pacific Ocean. That’s far from the coastal waters where the endangered killer whales forage in winter. The threatened fall-run Chinook salmon from the Snake River, in contrast, remain closer to the coast where they are more accessible to the whales, Shelton said.
That further sharpened the picture of how many Chinook salmon can be expected off the Washington and northern Oregon Coast. That data further reduced the Amendment 21 threshold for fishing limits.
Shelton is now analyzing tag data from spring-run Chinook salmon. He expects to fold that into the picture of salmon distribution later in 2023. That will lead to another revision of the models that calculate the threshold.
The revisions are less about the specific numbers than they are about accurately capturing the conditions likely to stress the whales in their search for prey, noted Jeromy Jording, a fisheries biologist in NOAA Fisheries West Coast Region.
“It’s all about using the best science and best information we have to recognize when the whales have a greater need,” he said. “The more accurately we can reflect that, the better.”