by Lauren Frisch
Spawning salmon may pool their abilities and memories of what home smells like to navigate back to the waters in which they were born.
“One of the patterns we see is that when the number of fish returning in a given year is high, the rate of fish that end up straying is lower,” said Peter Westley, a professor at the University of Alaska Fairbanks School of Fisheries and Ocean Sciences. Westley and Santa Fe Institute Postdoctoral Fellow Andrew Berdahl hypothesize this is because groups of salmon are better at navigating collectively than as individuals.
In February 2016 the researchers conducted a pilot experiment to test if populations of steelhead salmon migrate in groups, and if these groups can correctly choose to migrate to the branch of a 100-foot long artificial stream that smells of home. The next step is to test if social behavior is influencing the ability of the fish to make the correct choice by manipulating the group sizes of fish in the experiment.
This project could have important implications for salmon population sizes. “If salmon are working together to navigate, reductions in population size due to overharvesting or changes in the environment could limit the ability of the salmon to migrate home,” said Berdahl.
In 2015, Alaska’s commercial salmon landings were valued at approximately $400 million. This research is important because it adds to our fundamental understanding of the biology of these economically important species.
It is well understood that salmon attempt to return to their natal rivers—the rivers where they hatched and developed—when it is time to spawn. After spending time in the ocean, a salmon will try to find its way home using its memory of what home smells like. However, not every salmon is able to successfully return to its natal river.
“Most fish appear to try to head home, but some of them stray and end up somewhere new,” Westley said. “For a variety of reasons, maybe a purposeful choice to not go somewhere or an inability to recall what home smells like, there can be a failure for them to remember how to get home.”
Berdahl explained that most salmon are well adapted to the particular river or stream that they hatched in. Environmental conditions in rivers and streams can vary considerably. If salmon stray to water they are not adapted to, it may be harder for them or their offspring to survive.
Biologists think about the process of swimming home as an individual activity. Each fish reaches a branch in the river and must choose right or left. However, “there is a growing body of studies suggesting that animals traveling in groups make collective decisions, and that these group decisions are often more accurate than decisions made by individuals,” Berdahl said. Salmon often do migrate in groups, so they have the potential to benefit from the wisdom of crowds while heading upstream.
Westley explained that people tend to be more successful navigators in groups as well. “If you’re totally by yourself in an unfamiliar town, all of the pressure to navigate is on you. If you’re with a crowd of friends and each has some different ability to remember things, you can pool your information to make a decision.”
Westley was studying patterns of homing and straying in salmon when he met Berdahl, who was working on his PhD researching how fish pool their information to track resources. By combining research expertise, the pair made a perfect team for this project. Funding comes from the Santa Fe Institute and UAF.
The Oregon Hatchery Research Center housed the pilot experiment and supported the logistics. Fish used in the experiment were taken from the Alsea River Hatchery after having completed a successful journey home from the ocean. The researchers chose to test steelhead salmon because they are a close relative of other Pacific salmon that also spend a portion of their lives in the ocean before migrating home for spawning.
The researchers dug a Y-shaped channel with two branches at the end. The channel and both of its branches were filled with water from Fall Creek, which the experimental fish had no prior exposure to. In one branch, they added water from the Alsea River Hatchery, with the hope that the branch would smell more like home.
“The first part of the experiment clearly shows that the fish are behaving socially,” Westley said. “They were not making decisions totally at random, but were indeed moving as social groups, which was very cool.”
“However, the next part was very tantalizing but not convincing that they were making the right choice,” Westley said. The fish chose the correct path in five of the seven trials. Westley explained that this is not a high enough percentage to be confident about a particular trend, so the results were considered inconclusive.
The pilot project was an important first step to understanding the mechanism of group navigation, but failed to give clear results. However, it hints at the possibility that salmon could be using social behavior to aid in navigation during their spawning migrations. “New experiments take a lot of preparation and trial and error,” Westley said. “Having things fail—or not work as well as we would like—is a fundamental aspect of science.”
Westley and Berdahl are making two major modifications to the process before doing a full experiment in 2016.
First, Westley thinks it is possible that the water didn’t smell enough like home to convince the fish to choose one channel over another. In the next experiment, Westley and Berdahl are going to test different ways to get a stronger odor signal in the water. One option is to replicate the amino acid profile of the water in the Alsea River Hatchery to intensify that signal.
They are also going to conduct the experiment earlier in the year, ideally in December 2016 or early January 2017, in case the fish used later in the season had lost the urge to return home. Because the pilot experiment was late in the season, the focus of the fish may have shifted from navigation to spawning.
Westley and Berdahl plan to test groups of one, two, four, eight and sixteen fish to learn more about how different sized groups navigate.
“This failure doesn’t mean that we messed up and did something wrong,” Westley said. “It reveals things we really don’t know about these critters. It reveals our fundamental ignorance about how some of these things work. But this failure also makes it all the more exciting to tackle this question again.”