Every year, millions of adult salmon return from the ocean to their home streams, where they lay eggs and produce the next generation of fish. But far fewer sockeye salmon are making it back to their freshwater mating grounds compared to a few decades ago, and that’s seriously affecting population sizes of the species throughout the Northwest, from Alaska to Washington State.
The discovery suggests that changing ocean conditions may be making life harder for some groups of wild salmon -- possibly by reducing their food supply or increasing populations of predators.
By zeroing in on what, exactly, is causing the widespread decline, researchers hope to help managers figure out what to do about the problem.
“We found that substantial reductions in productivity of over 50 percent have occurred in the last two decades in a wide geographical area,” said Randall Peterman, a fisheries scientist at Simon Fraser University in Burnaby, Canada. “We were quite stunned to see these results.”
“We’re telling researchers that in order to really explain the decline in a give population, you should be looking at what is causing the decline in productivity in other populations,” he added. “Depending on which mechanism is the cause of the decline, we may or may not be able to do anything about it.”
On the West Coast, salmon are an icon of clean water, clean air and healthy ecosystems. Sockeye are particularly valuable. At the highest cost per kilogram of any salmon species, they are important to commercial fisheries. For First Nations people, the fish are also intricately tied to a trifecta of food, social and ceremonial needs.
In 2009, scientists recorded the lowest number of sockeye salmon in 50 years in the massive Fraser River system in British Columbia. The extreme dip, which came despite a reduction in harvesting rates, followed a long series of years of declining numbers that reduced the number of fish available to native people and sparked conservation concerns.
To find out how widespread the problem might be, Peterman and colleagues gathered data on 64 sockeye populations in Alaska, British Columbia, and Washington, dating back to the 1950s. They focused on productivity, which describes the number of adult offspring that return to a home stream per single spawning fish.
In 24 of 37 stocks living from Washington to southeastern Alaska, the researchers reported in the Canadian Journal of Fisheries and Aquatic Sciences, productivity has dropped substantially. In healthier times, Peterman said, as many as six to 20 fish returned for every one original spawner. Today, the ratio is more like three returns for every spawner in many places – as much as an 85 percent drop in some places.
In the worst-off locations, more fish are swimming off to sea than are coming back to spawn. Most of the decline has happened in the last 20 years. The trend has also spread north over time, though stocks in central and western Alaska remain stable, and some are even increasing.
The patterns echo a rise in sea surface temperatures along with other changing climatic and oceanographic conditions, Peterman said, though the ocean is not yet warm enough to harm salmon directly. Instead, he suspects that warmer waters reduce the supply of zooplankton, the main source of food for salmon.
Warmer conditions have also given rise to greater populations of tuna, mackerel and stellar sea lions, which eat young salmon. Another possibility is that as-yet-unidentified viruses, bacteria or parasites may be spreading from one salmon population to another.
Overall, the results emphasize the importance of the ocean environment, which can affect large numbers of salmon over a wide region, said Ritchie Graves, a biologist with the NOAA Fisheries Service in Portland, Oregon.
But even as many wild sockeye stocks decline, he added, others are increasing, including in the Columbia River system, which stretches from Oregon to B.C. Scientists can’t explain those trends, either.
By getting to the root of why some salmon are suffering while other thrive, it may become possible to soften the blow of a changing environment for the fish.
“We’ve observed a pattern, and the question is why that pattern is occurring,” Graves said. “To me, what’s interesting is why most stocks are going down but some are going up. What are some stocks doing differently than others?”
“Salmon are a pretty good bellwether of the overall health of the environment,” he added. “They should be important to the rest of us that live in the environment.”