SOURCE: Nolan N. Bett, Scott G. Hinch, Nicholas J. Burnett, Michael R. Donaldson & Sean M. Naman (2017) Causes and Consequences of Straying into Small Populations of Pacific Salmon, Fisheries, 42:4, 220-230, DOI: 10.1080/03632415.2017.1276356
All salmon spawn in freshwater, and most are “anadromous”. This means that to complete their life cycle, juvenile salmon must migrate out to the ocean and return as adults to reproduce, or “spawn”, in the river where they were born. The process of adult salmon finding their way back to their natal river after years at sea is called “homing”. Homing back to the same spawning site allows salmon to pass down advantageous traits to their offspring.
Like many people, I love eating salmon, and I was surprised to learn that many populations of several different salmon species are listed as threatened, endangered, or even extinct from their native ranges. This is mostly due to overfishing during the 19th and 20th century, as well as the continued human impact on the environment, including pollution, warming temperatures, and dams that block key spawning habitat. Because of their strategy of “homing”, salmon could be listed as threatened or endangered as specific populations under the Endangered Species Act. Additionally, scientists can track populations of salmon that always return to the same stream and try to figure out why that particular population is struggling.
Due to their high value as a commercial and cultural resource throughout their range, salmon have been intensively studied along both coasts of the United States for years. It is difficult to track a species like salmon in the ocean, so researchers primarily study them in freshwater streams, both after they are born and when they return as adults. As they have tracked more and more of these fish, scientists are discovering that some adult salmon do not always return to their home stream. If an adult salmon does not successfully find their way back to spawn, they are referred to as a “stray”. Strays are of high interest to fisheries biologists because we are beginning to realize the impacts they can have on vulnerable populations of salmon.
Up the Creek: A Case Study
This study involved a relatively small population of wild Sockeye Salmon (approximately 26,000 fish) in in the Seton River of British Columbia. Researchers collected tissue samples from 152 fish caught at the Seton Dam fishway (Figure 2) and analyzed their DNA. They determined that 55 of the 152 fish were strays, and likely would have attempted to spawn outside of their own spawning area.
Why is this important?
There are several consequences of salmon straying into small populations. For example, imagine that you are throwing a party on a small island. You’ve carefully decided on a small guest list and given your friends important information like the weather forecast and the lack of fresh water sources, so they can arrive ready to party. Now imagine that a huge boat shows up full of people who did not have that information and are not prepared to stay on the island, but they join anyways. These newcomers disrupt your evening with their complaints and make it harder for your original guests to enjoy the party, even though they came prepared.
In a similar way, populations that receive strays may suffer by losing traits that have allowed them to adapt to their environment by mating with the newcomers, which could reduce overall survival. However, straying may also provide the benefit of a “rescue effect” and save declining populations from extinction by adding diversity and higher numbers of fish. As we struggle to manage dwindling salmon populations throughout the West Coast by battling centuries of negative human impacts like overfishing, it is important to continue to learn all we can about the life cycle patterns of these fish.