Are wild salmon following hatchery salmon? Testing the Pied Piper hypothesis
Ever heard of the Pied Piper? What about in the context of fisheries research? Taking the concept embodied by the Pied Piper story of strong but delusive enticement, Maria Kuruvilla applied it to hatchery fish and wild salmon in three Washington State rivers during their migratory journey downriver. Maria conducted her research as a PhD student in Professor Andrew Berdahl’s lab at SAFS.
Each year, salmon species in the Skagit, Dungeness, and Puyallup Rivers begin their migration downriver, spawning in the upper reaches of the river, then making their way to the marine environment of the Puget Sound. In these same rivers, hatcheries managed by State and Tribal governments exist to supplement salmon as part of conservation efforts, especially in the face of declining populations.
Maria’s Pied Piper hypothesis is based on the idea that hatchery fish – which are released in large numbers each year and migrate downriver immediately – are playing the role of the Piper. The children? The wild salmon encouraged to follow. This could be a problematic scenario if wild fish migrate at a time that they wouldn’t normally and enter the marine environment at sub-optimal conditions. Such conditions include the size and weight of fish, or oceanographic conditions not as favorable to juveniles.
Testing this hypothesis in the three rivers with two species of salmon – coho and Chinook – Maria found results consistent with the hypothesis in four out of the six populations tested. So how do you tell the difference between wild salmon and hatchery salmon? “Hatchery salmon are usually bigger, plus they have a tag or a clip on the fin that allows tracking and distinguishes them from wild salmon,” Maria shared.
The typical migration period for wild juvenile Chinook salmon is from March to July and the typical migration period for wild juvenile coho salmon is from April to June in Washington’s Skagit, Dungeness, and Puyallup Rivers.
Using smolt traps located near the mouth of the rivers, fish present in these traps are usually at the final stages of their downriver migration, and so it’s a good location to collect data on which fish are present: hatchery, wild, or both. Using this data, provided by the Washington Department of Fish and Wildlife and the Puyallup Tribe of Indians, Maria plotted the migrations of hatchery and wild salmon, then did a deeper dive on the data to figure out if hatchery salmon are influencing wild salmon migration.
When released, hatchery salmon usually complete their migration downriver and enter the marine environment within one to two days. Wild salmon, however, are typically on a more protracted migration timeline. By collecting data on environmental stimuli, such as water temperature and flow, which is also known to affect wild salmon migration, Maria can rule this out as a possibility in the influence data.
“Looking at the whole peak and duration of migration, my data shows that as the number of hatchery salmon increases in a river, the duration of wild salmon migration decreases. This is consistent with the Pied Piper hypothesis and suggests that wild salmon are being influenced by the migrating hatchery salmon”, Maria said. “Generally, a population all migrating over the space of a day or two is not beneficial, as ocean conditions may be unfavorable or there could be a lot of hungry seals waiting, things like that.” There is also an important possibility to consider in Maria’s work, which she shared: “There might also be advantages of hatchery salmon influence on wild salmon migration. Moving with a big group of hatchery salmon could provide an increased level of protection from predation when entering marine environments.”
Hatcheries have a lot of control on how and when they release fish into rivers, and this is where Maria hopes her research will be particularly useful. “Managers of hatcheries can take the results of my paper and edit their release timing if needed, to more closely align with wild salmon migration patterns, or even marine conditions when it’s most favorable for fish to transition from freshwater to marine environments,” Maria said.
There were two salmon populations part of Maria’s study that didn’t correspond to the Pied Piper hypothesis, which Maria explained could be a result of hatchery release decisions. “The theory is that the Skagit River hatchery released their Chinook salmon quite late in the migration season, and a lot of wild Chinook had already left the river, or this particular part of the river, by that point in time.” For the Dungeness River coho, the wild salmon are usually in the headwaters, whereas the release of hatchery salmon is more downstream. “And so, it’s possible that the wild coho are not being influenced as much in this situation,” Maria shared.
This study not only looks at how hatchery salmon affect the migration timing of wild salmon, but also supports the idea that salmon rely on social cues to decide when to migrate. Just like in other animal populations, social behavior can influence these timing decisions and it’s becoming more recognized that considering social behavior is important for understanding migration timing. If salmon and other species use social information to make better decisions during critical events like migration, a decrease in population size could impair their decision-making. This, in turn, could lead to further population declines.
Recently defending her PhD at SAFS, Maria is now a post-doctoral fellow at the University of Victoria. The manuscript is currently under peer review by the Movement Ecology Journal; however, the preprint is available for reading below.