FISHERIES
The Peril of Pink
Salmon under pressure from species
P
ink salmon are booming. That’s great for humpies, but perhaps not so great for every other species of salmon in Alaska. Indirect evidence suggests competition among species may have been compounded by the changing ocean climate, but the case is far from closed. “There's a lot of evidence that abundant pink salmon in the North Pacific are having ecosystem-scale impacts that include changes in the growth rates of different salmon populations of different species from different places,” says NOAA scientist Nate Mantua, based on a review of the scientific literature, including the work of Gregory Ruggerone. Ruggerone, former president of Natural Resources Consultants, has been researching the relationship between pink salmon and other species for about twenty years thanks to pinks' biennial life cycle. “In most regions, they're highly abundant in odd-numbered years and less abundant in even-numbered years,” Ruggerone explains. “That biennial pattern, it's an incredible tool for testing the hypothesis that salmon compete with each other in the marine environment because climate and physical oceanography typically do not vary biennially, and most marine species do not have a two-year life cycle like pink salmon.” The complicated life history and diversity of Pacific salmon species, which typically spend one or two winters in fresh water and two or three years in the ocean, can mask what Ruggerone calls the pink salmon effect. 84 | June 2022
By Isaac Stone Simonelli Pinks Versus Reds All species of salmon in the Pacific are generalist predators, leading to an overlap of diets. Pink salmon diets overlap the most with sockeye, also known as red salmon, though both also catch small fish and squid eaten by coho and chinook. Ruggerone’s work points toward competition for ocean food resources being a factor in limited salmon returns of sockeye in some regions and smaller sizes of adult sockeye returning overall. Year after year, he has documented how the biennial pattern of pink salmon has, in general, an inverse relationship with returns of other species. In even-numbered years, when pink salmon returns are lower, other salmon species tend to grow faster and survive better. “The adverse effect of numerous pink salmon on vital rates of other salmon species has the potential to be far-reaching because salmon migrate long distances,” he writes in a 2022 publication titled “Are There Too Many Salmon in the North Pacific Ocean?” The paper explores his tipping point hypothesis, which states that “an overabundance of salmon, combined with effects of recent marine heatwaves, may have been responsible for unexpectedly low returns of all five species of Pacific salmon across the North Pacific in 2020.”
Climate Plays Favorites Mantua, who is the leader of the salmon ecology team at NOAA’s Southwest Fisheries Science Center and
specializes in climate science, says that there is a lot of merit in the theory that the North Pacific's carrying capacity for salmon—and other species that depend on the same food webs—has crested, based on the evidence presented by Ruggerone and the impacts of climate extremes in the North Pacific and the Bering Sea. “We've had record-high ocean temperatures in a number of recent years that have come with widespread negative impacts on many different marine animals,” Mantua says. Warmer water increases the metabolic rates of cold-water fish, such as salmon, which means they need to eat more food during warm years than they would in cold years to grow the same amount. Compounding this issue in the Gulf of Alaska are marine heatwaves that reduce food production at a time in salmon’s lifecycle when they need to rapidly grow. “There's evidence that those bad ocean conditions just from the climate and food web productivity side have been made worse in periods where you have really abundant pink salmon,” Mantua says. The varying life cycles of salmon species mean that marine heatwaves affect them differently. Pink salmon have the most limited freshwater life, opting to quickly enter the marine environment, Mantua says. “Their way of doing business is to overwhelm things with numbers and then grow really rapidly,” Mantua says. “The ocean is a great place for very rapid growth, if you can find food, and also a
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Greg Ruggerone
competition and changing climate
