Each summer, millions of fish return to Bristol Bay and then swim on to the stream where they were born to spawn, and die. Exactly what compels them to return to the right spot is unknown, but scientists think that some hatchery-raised steelhead in Oregon might hold a clue.
Peter Westley, an assistant professor at the University of Alaska Fairbanks School of Fisheries and Ocean Sciences, and colleague Andrew Berdahl, from the Santa Fe Institute, are trying to figure out why salmon choose to swim up a given stream.
“(Some 50 million) fish last season migrated into Bristol Bay in the course of just a couple of weeks, and that’s a classic thing is that the fish all come at the same time, and it’s kind of curious to think, is there a potential social role in that?” Westley asked. “They all come as a major wave, and is it because they are in these groups and they are sort of following the leader and using social dynamics to aid their migrations.”
To try and parse that out, the researchers have been digging fake streams at an Oregon lab for steelhead to swim in.
“We actually tested this idea by giving steelhead that had migrated home to a hatchery, we brought them to the Oregon Hatchery Research Center and gave them a choice between water that smelled more like home, or all of the foreign water that came from the stream where the hatchery research center was based,” he said. “One of the things we showed with this steelhead system that indeed, fish that are moving upstream and are moving around are very social. They don’t move independently. They’re in groups moving around.”
Westley said steelhead are a good proxy for salmon because they’re pretty similar fish in terms of life history and a predictable return to their birthplace.
“They are a good model for these migratory sea-going fish that come back home,” he said. “They have subtle differences in life history but in terms of the social aspects and the migration and the orientation, I think they are a good model.”
Right now, the research is relatively small, and Westley said there are many details to work out and enhance in future experiments: this is really just a tantalizing teaser of some possible results. But it’s a little step closer to figuring out what compels salmon to come home each year.
Westley and Berdahl have been interested in this social side of fish behavior for some time, and their collaboration started with a paper that just looked at existing literature and data.
“(The paper) poses this idea of a collective social role of salmon as they are migrating home, such that salmon or other migratory fish can school together, and by being in groups, they can share information and pool their abilities to navigate and orient and by doing so the group is much more likely to get home than if the group was smaller or the individual is traveling by itself and the onus of getting to the right spot would all be on the individual,” Westley explained.
The idea behind the study is that salmon may smell their way to the right stream.
“So you can imagine, fish that are headed to the Kvichak versus the Nushagak, if it’s better to be with your Nushagak group, how do you know that you’re with the right group? Salmon have an amazing ability to use pheromones and essentially of course they’re smelling their way home once they’re in freshwater,” he said.
The role of scent is another thing the team would like to test. Westley said they’d like to repeat the experiment with a stronger scent of home-streams.
There are other changes the team is looking at in the future, too – like trying larger groups of fish, and adjusting the timing to tie-in more closely to when salmon are actually on the move.
Somewhere on the list, Westley said he’d also like to look at how wild fish fare, rather than just using hatchery steelhead – a choice made, so far, to keep things simple. And, he said, using hatchery fish can look at another interesting question: how hatchery fish impact wild fish when the two are mixing in places where they co-exist, like Alaska.
Westley said the issue of straying, or hatchery fish that go home to the wrong place, could result from those fish just following wild fish to spawning grounds.
“The challenge is always trying to scale from what you’re doing at sort of an experimental level up into the complexity of nature, and trying to assess what you’ve done at this small controlled scale, does it relate to nature as a whole,” he said. “It’s always a challenge.”