UW Study Supports ‘Safety in Numbers’ Hypothesis for Pacific Salmon

A pair of sockeye salmon. File photo.

A University of Washington study published in late June found that Pacific salmon in large groups face a lower risk of being consumed by predators, although for some salmon species that tradeoff means more competition for food.

While most people think of salmon spawning in freshwater streams, they also spend a huge amount of time in the ocean feeding and growing, said Anne Polyakov, lead author of the study, which was published in the journal Science Advances.

“One of the reasons why this study is so unique is that we essentially can’t observe these fish at all in their natural ocean environment, and yet we’re able to pull out these really strong results on how grouping affects predation risk and foraging success for individual fish using this incredibly valuable dataset,” said Polyakov, who’s a doctoral student in the UW Interdisciplinary Quantitative Ecology and Resource Management Program and the School of Aquatic and Fishery Sciences.

Researchers drew on information from an international fisheries dataset collected from sockeye, chum, coho and pink salmon from 1956 to 1991. These salmon are born in freshwater streams, then migrate to the ocean for food and growth before returning to their natal streams to spawn and die.

The study relied on analyzing historical data collected over more than four decades by UW’s Fisheries Research Institute in partnership with the International North Pacific Fisheries Commission, recording salmon catch data across the North Pacific Ocean as part of their management program.

By looking at numbers of fish caught in purse seine nets, researchers were able to estimate the size of schools of fish in which each fish had been swimming.

Additional historical data analyzed included records of predator wounds on salmon and stomach contacts for a subset of fish caught. This way, researchers could estimate both predator interaction and feeding success of salmon aver a 45-year period in the North Pacific Ocean.

“It was serendipitous that these data were available,” said Andrew Berdahl, a senior author of the study and assistant professor at the UW School of Aquatic and Fisheries Science. “They suggest that salmon are social during the ocean stage of their life and reveal the benefits and costs of this sociality. Grouping is very common in marine fish and we think this is largely to help them evade predators, yet there’s actually not much empirical support showing this, especially from wild populations.”

The study “is a piece of the foundation that many didn’t realize is missing,” Berdahl said.

Researchers observed numbers of fish caught in purse seine nets as a proxy for group size, then estimated predator risk by considering the fraction of fish in each set that had predator wounds.

They found that, across species, fish in larger groups were less likely to be wounded. The data also showed that larger or smaller fish were more likely to be attacked by predators.

This suggests that salmon safety in numbers comes from confusing predators because visually distinct larger or smaller individual salmon were easier for predators to keep track of.

Researchers also found that sockeye and chum salmon in larger groups had less food in their stomachs, which they interpreted to show that fish sometimes sacrifice food to stay with the group and avoid predators.

The same pattern was not found with pink and coho salmon; one possible reason for this may be that sockeye and chum salmon spend more time in the ocean and also tend to travel further from their natal steams than other species, so it is harder for them to find food.

The research was funded by the Fisheries Research Institute and the H. Mason Keeler Endowed Professorship at UW. Other co-authors were UW Professor of aquatic and fisheries science Tom Quinn and Katherine Myers, a former research scientist at the UW School of Aquatic and Fishery Sciences.