By Hanover Matz, RJD Intern

The long migrations undertaken by Atlantic and Pacific salmon to reach their spawning grounds are known by many. Salmon are anadromous; they spend their adult lives foraging at sea, and then return to the freshwater rivers of their births to spawn and reproduce. How this remarkable feat is accomplished by the salmon with such accuracy is believed to be a combination of geomagnetic and olfactory cues. However, Berdahl et al. examined evidence that salmon may also be relying on social interactions and collective behavior in order to navigate to their natal homes. The authors hypothesized that salmon migrating in large groups could more accurately navigate to their spawning grounds by reducing the effect of navigational errors made by individual salmon. In order to determine whether this was the case, Berdahl et al. compiled results from previous studies that indicated a decrease in straying with an increase in abundance, proposed potential benefits of collective navigation for salmon, and used catch size data to better understand how salmon aggregate in the wild.

The basis for collective navigation in migrating animals is that by traveling in relatively large groups, animals can reduce the effect of navigational errors made by individuals in the group by distributing those errors across the decisions of the group as a whole. A school of salmon returning to their home river can more accurately find that river by relying on the collective decisions of the group rather than an individual salmon can by reading cues on its own. Berdahl et al. synthesized results from multiple studies that showed salmon more effectively returned to their natal grounds when they were in higher abundance, indicating the possible role of collective behavior. Figure 1 shows the results taken from several studies, all of which demonstrate that as the run size or abundance of salmon increased, the straying rate of salmon migrating back to their natal rivers decreased. The more salmon present, the less likely individual salmon were to stray from the correct migration path, suggesting the possible role of collective navigation.

Results of several studies indicating a negative relationship between straying rate and run size in salmon

With this evidence for collective navigation in mind, the authors postulate several potential benefits to salmon through such behavior. Not only would collective navigation allow schools of salmon to properly orient themselves when traveling from the high seas to coastal environments, but it would also allow them to pinpoint the proper estuaries and river mouths necessary to reach their natal grounds. When presented with multiple rivers to migrate towards, collective navigation would also allow the group to effectively decide on the correct path, rather than relying only on the abilities of the individual fish. By making orientation and migration decisions based on the collective choices of the group, the salmon can reduce the effect of errors made by individual fish and the school can reach their spawning grounds based on the decisions of only a few informed individuals. Schooling offers many other potential benefits besides collective navigation, such as protection from predators, and it is likely that salmon rely on both visual and pheromone cues in order to migrate as a group. In order to further provide evidence for collective navigation in salmon, the authors collected catch size data to determine the degree to which salmon school in the high seas.  The data revealed that the catch sizes for multiple species varied from the expected Poisson distribution, indicating the salmon likely aggregate in the open sea. It was overall rare to find fish, and when they were found, they were often found in large numbers, suggesting a degree of schooling at sea. This schooling may increase with increasing migratory behavior. Figure 2 shows the results of the catch size data.

Catch size data for multiple salmon species, shown in solid lines, does not follow the expected Poisson distributions, shown in dashed lines, indicating aggregation of the salmon

What does this evidence for collective navigation indicate? The possibility that salmon may rely on collective navigation in order to reach their spawning grounds implies that populations require enough individuals to aggregate and migrate as a group. What this means for salmon fisheries is that even if a fishery is managed to allow for the future reproduction of the salmon, if enough individuals are removed from the fishery so that the remaining fish cannot effectively aggregate, they may not be able to reach their natal rivers to reproduce. The population may be numerically fished so that there are enough individuals remaining to continue reproducing, but they may be distributed in such a way that they cannot school in large enough numbers to migrate. This could potentially lead to detrimental overfishing of salmon stocks. In order to fully understand the degree that social interactions may play in salmon migration and reproduction, future studies will have to be conducted on the role of collective navigation in salmon and other organisms that migrate over long distances.

References

1. Berdahl, A., Westley, P. A., Levin, S. A., Couzin, I. D., and Quinn, T. P. 2014. A collective navigation hypothesis for homeward migration in anadromous salmonids. Fish and Fisheries.