“Security in numbers” tactic protects Pacific salmon from predators

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June 29, 2022

We see coho salmon swimming together. A new study has found that Pacific salmon, including coho, breed together in the open ocean to reduce the risk of being eaten by predators. Note: This photo was taken at the Seattle Aquarium and is used to illustrate this concept of grouping in the wild. ‘The behavior and ecology of Pacific salmon and trout’ / University of Washington Press

Animals that live in groups tend to be more protected from predators. This idea may be common sense, but some species are difficult to test, especially for wild populations of fish that live in the ocean.

A new study from the University of Washington that draws on historical data has found unique support for the ‘security in numbers’ hypothesis by showing that Pacific salmon in larger groups are less likely to die. being eaten by predators. But for some species of salmon, schooling comes at the expense of competition for food, and these fish may trade safety for a meal. The study was published June 29 in the journal Science Advances.

“With salmon, most people think they spawn in freshwater, but there’s also an awful lot of time they spend in the ocean feeding and growing,” he said. lead author Anne Polyakov, PhD student in interdisciplinary quantitative ecology and resource management at UW. Program and the School of Aquatic and Fisheries Sciences. “One of the reasons 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 draw these really strong results about how grouping affects predation risk and foraging success for each fish using this incredibly valuable data set.

Researchers examined four species of Pacific salmon – sockeye, chum, coho and pink – based on an international fisheries dataset collected for these species from 1956 to 1991. Although their individual life histories vary by species , all salmon are born in freshwater streams, then migrate to the ocean to feed and grow before returning to their home streams to spawn, spawn and die, continuing the life cycle of the next generation.

graphic showing a fishing net capturing contents in a defined area

A graphic showing the operation of a purse seine net. This style of landing net catches all fish in a discrete volume of water.Polyakov et al. Scientific Advances, June 2022

This study built on the analysis of existing historical data in new ways. For more than four decades, the UW Fisheries Research Institute, in partnership with the International North Pacific Fisheries Commission, has recorded data on salmon catches in the North Pacific Ocean as part of the management of each species. The study authors analyzed catch data from purse seine nets – a fishing gear that involves casting a net and catching all the fish in a relatively small volume of water. By examining the number of fish caught in one of these nets, the researchers were able to estimate the size of the schools in which each fish had swum.

Additionally, historical data included careful records of predator injuries to salmon, as well as stomach contents of a subset of captured fish. In this way, the researchers were able to estimate both predator encounters and salmon feeding success over 45 years, covering the entire North Pacific Ocean, making this a unique and valuable dataset.

“It was coincidental that these data were available. They suggest that salmon are social during the ocean phase of their lives and reveal the benefits and costs of this sociality,” said lead author Andrew Berdahl, assistant professor at the UW School of Aquatic and Fishery Sciences.” Clumping is very common in marine fish and we think it’s largely to help them escape predators, but there’s actually not a lot of evidence empirical evidence showing this, especially in wild populations. I think this study is a building block that many didn’t realize was missing.

Looking at the number of fish caught in the purse seine nets as an indicator of group size, the researchers then estimated predator risk by considering the fraction of fish in each set that had suffered predator injuries. Fish in large groups were much less likely to be injured, regardless of species. For example, in the case of sockeye salmon, an increase of 100 fish in a group halves the risk of predation. Additionally, injury data showed that fish with larger or smaller bodies than others in their group were more likely to be attacked by a predator. This suggests that the salmon’s safety in numbers comes from confusing its predators, as visually distinct individuals – larger or smaller – were easier for predators to track.

fish swim together in an aquarium

Coho salmon swim in groups. Note: This photo was taken at the Seattle Aquarium and is used to illustrate this concept of grouping in the wild.‘The behavior and ecology of Pacific salmon and trout’ / University of Washington Press

The researchers also found that for two species of salmon – sockeye salmon and chum salmon – fish in large groups had less food in their stomachs. These fish sometimes sacrificed a meal to stay protected in groups and avoid predators. However, the team did not notice this trend for pink salmon and coho salmon. According to the researchers, one possible reason for this is that sockeye and chum salmon spend a much longer portion of their lives in the ocean and also tend to move further away from their home streams than do salmon. other species. Spending more time and traveling further out in the ocean usually means food is harder to come by, leading to more competition and less food for fish in large groups.

The authors hope that this article will inspire eventual reflection on group size distribution and the benefits and costs of grouping in current fisheries management models, as well as dusting off other datasets to reveal results. relevant.

“A lot of these datasets have been very expensive, and I think there’s still a lot to discover,” Berdahl said. “I hope it will also motivate people to think about the ecological implications of collective behavior – in this case, how grouping affects the food web, both by changing the rate at which a species is consumed as well as the rate at which she consumes others.”

Other co-authors are Tom Quinn, professor of aquatic and fisheries science at UW, and Katherine Myers, formerly a research fellow in the UW School of Aquatic and Fisheries Sciences. This research was funded by the Fisheries Research Institute and the H. Mason Keeler Endowed Professorship at UW.

For more information, contact Polyakov at [email protected], Quinn at [email protected], and Berdahl at [email protected]

Tag(s): Andrew Berdahl • College of the Environment • Salmon • School of Aquatic and Fisheries Sciences • Tom Quinn

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