Salmon migrate upstream in a unique pulse-like pattern. Biologists have observed the pattern in streams and rivers all over the world. But why?

"The folk wisdom is that the salmon are all independently cueing off common environmental cues, and that tends to synchronize their movements," Andrew Berdahl, a research fellow at the Santa Fe Institute, explained in a news release.

Researchers have proffered tidal and temperature changes as the main driver of the pulse-like migration phases. Others suggest moon phases are responsible.

Researchers have been monitoring the movement of salmon up Alaska's Hansen Creek for the last 20 years, and have yet to find evidence of an environmental explanation for migration patterns. Hansen Creek features remarkably stable conditions -- its temperature and levels unwavering.

"Because you see this pulsed movement in places where standard explanations say you shouldn't, it leaves us searching for another mechanism," says Berdahl. "Maybe they're cueing off each other."

In a new paper published in the journal Animal Behaviour, Berdahl and his colleagues argue the movements of migrating salmon can be explained by social cues.

Researchers built a model to simulate group migrations governed by independent decisions. Each fish decides to migrate individually, but each decision is influenced by the decisions of their peers. If one fish begins to migrate, it increases the chance others will to.

The model showed a single decision to begin the upstream journey can trigger a cascade effect, creating the pulse-like pattern witnessed in Hansen Creek and elsewhere.

Salmon aren't simply copy cats. It makes sense to move in large groups. Strength in numbers improve a fish's odds of eluding predators like bears waiting upstream.

Researchers say understanding the drivers of group migration is key to protecting vulnerable species like salmon.

"Climate change is altering both the optimal timing of migrations and the cues used to stimulate those migrations," added Berdahl. "If you want to know how migratory animals are going to respond to this change, you have to pay attention to social interactions -- which is what we think determines the fine-scale movement decisions -- or you're going to miss an important part of the picture."

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