pink salmon invasive danger north pacific pink salmon

A ICES Journal of Marine Science study shows that the North Pacific Pink Salmon as one of nature’s true competitor. Photo: Eva Thorstad/NINA

A new study published today in ICES Journal of Marine Science involved careful examination of nearly 25,000 scales collected from Alaskan sockeye salmon and reveals a clear pattern of reduced growth in odd years in the North Pacific since 1977, and the culprit appears to be pink salmon, who consume many of the same prey as sockeye.

The study relies on a “natural experiment” that occurs at the scale of the North Pacific – odd years tend to have much higher abundance of pink salmon owing to their two-year fixed life cycle. “When you plot their abundance over time, you see this very regular, sawtooth pattern” describes Pete Rand, the study’s lead author and an ecologist at the Prince William Sound Science Center. Odd year lineages of pink salmon (the generations that started as fertilized eggs in river gravel in odd years) have been roughly twice as abundant as even year lineages, and this pattern has persisted in the North Pacific for decades.

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Past studies have shown how pink salmon compete with sockeye salmon for prey at sea and affect their growth, but this study reveals the effect extends to many populations that originate in Alaska. The study’s authors reveal that eight populations of sockeye salmon in Alaska grew less (on average up to 17% less) in odd ocean years when competition at sea is more intense. For sockeye that enter the ocean in an odd year and return two years later, this translates into smaller body size when they return to spawn (a reduction of 2% in their length or a 6% drop in mass). The authors also noted that there has been a long-term decline in sockeye salmon growth with increasing abundance of pink salmon. This has important implications for sockeye fisheries and the fish’s own reproductive fitness.

north pacific pink salmon

“To understand this phenomenon, we had to look through both the microscope and the macroscope”, says Rand. The study linked the changes in growth measured in millimeters on thousands of fish scales to the abundance of pink salmon that occupy around 15 million square kilometers or nearly 6 million square miles of ocean. The effect is driven by population cycles of pink salmon originating from North America and as far away as the Kamchatka peninsula in Russia. The cycles are a trait of wild pink salmon populations, but pink salmon released from production hatcheries contributes to the overall effect. The authors liken it to a “zero-sum game”, where any additional production of hatchery pink salmon in the future can contribute to declines in growth of sockeye salmon across a broad swath of coastal Alaska.

Both species are important to a variety of fisheries in Alaska (including commercial, sport, and subsistence fisheries), and the authors encourage fishery and hatchery managers to consider this trade-off when making decisions that affect overall salmon abundance in the ocean.

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