While it is assumed that living in a large group offers protection for prey species, it is a challenge for scientists to test this hypothesis with many animals. Ocean-dwelling fish like salmon, for example, are hard to keep track of – except during their brief visit to fresh water.
“With salmon, most people think of them spawning in freshwater streams, but there’s also this huge amount of time they spend in the ocean feeding and growing,” said lead author Anne Polyakov, a doctoral student in the interdisciplinary Quantitative Ecology and Resource Management Program and the School of Aquatic and Fishery Sciences at the University of Washington.
“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.”
Since monitoring salmon directly through the salt water portion of their lives is highly impractical, the researchers used fishing data instead. The study drew on historical purse seine net data for sockeye, coho, chum and pink salmon from 1956 to 1991.
The scientists estimated the populations of different salmon groups based on the amount of fish caught in each net. They then looked at the number of salmon with wounds and found that a smaller number of fish were wounded within large groups compared to smaller groups. Lastly, the experts found that in some larger groups of salmon, many of the fish had empty stomachs.
“It was serendipitous that these data were available. They suggest that salmon are social during the ocean stage of their life and reveal the benefits and costs of this sociality,” said senior author Andrew Berdahl, an assistant professor in the UW School of Aquatic and Fishery Sciences. “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. I think this study is a piece of the foundation that many didn’t realize was missing.”
The scientists hope that this study may better inform fisheries management in the future as well as show the importance of fisheries data we might already have.
“Many of these data sets came at great cost and I think there’s a lot in them still ready to be uncovered,” said Berdahl. “I would hope it also motivates people to think about the ecological implications of collective behavior – in this case, how grouping impacts the food web, both by changing the rate a species is being eaten as well as the rate at which it is consuming others.”
The study is published in the journal Science Advances.