Results of University of Alaska Fairbanks DNA analysis of a massive database on pink salmon shows a remarkable ability of these fish to spawn at nearly the same spot within streams as their parents spawned.
The project, which reviewed genetic data from over 30,000 pink salmon, taps into an ongoing study in Alaska’s Prince William Sound that has collected DNA samples from pink salmon carcasses since 2011.
The Alaska Hatchery Research Program (AHRP) samples pink salmon in 30 streams, including five where researchers try to obtain samples from every salmon returning to spawn.
The AHRP is a collaboration of the Alaska Department of Fish and Game (ADF&G), nonprofit hatcheries, the fishing industry and academia, largely focused on the impact of hatchery fish on wild salmon populations.
The database also provides much information for researchers looking at other issues.
“We can connect the parents and offspring from multiple populations, along with body size, when they were sampled and where they were sampled,” explained Samuel May, the postdoctoral fellow at the University of Alaska Fairbanks College of Fisheries and Ocean Sciences (UAF CFOS) who led the study. “There’s all this fine-scale diversity that we didn’t really appreciate before.”
The study found that once these salmon return to a home stream or tideland, after a journey of traveling thousands of kilometers, they generally spawn within 100 meters (1,076 feet) of the spot where their parents spawned.
“Although we did not specifically test for population of origin for all fish, we know that most of these fish originated in Prince William Sound because we sampled returning adult parents and their returning adult offspring, two years later, in the same streams,” May said.
“We excluded all hatchery-origin fish from our study because we could not have sampled their parents and therefore they weren’t relevant to the questions we were investigating about site fidelity. That said, we know that hatchery fish can constitute a substantial proportion of the run in these steams, ranging generally from about 10% to 75%,” he said.
May noted that there have been several other studies, mostly on sockeye, showing site fidelity on similar scales.
“Sockeye are generally presumed to home with the greatest fidelity, while pink salmon are assumed to be the least accurate,” he remarked. “Our results suggest maybe it is more complicated than we previously thought.”
As part of his doctoral dissertation work, May showed similar results with a much larger sample size: of sockeye salmon returning to sites where they were born in small creeks in Bristol Bay.
May also confirmed evidence of hatchery fish mating with wild pink salmon. A study released in 2022, led by Kyle Shedd of ADF&G, showed that these hatchery-wild matings tend to generate about half as many offspring as wild-wild mating pairs.
“The big questions we are currently investigating have to do with what kinds of long-term impacts these hatchery fish have for wild population productivity and resilience,” he said.
Genetic analysis also highlighted distinctions between areas within the streams where pinks spawn. About 75% of the salmon spawn in intertidal areas, a zone affected by regular saltwater intrusion and apparently preferred by wild pinks.
Hatchery-origin pink salmon tend to travel into freshwater areas of those streams to reproduce. There, they may be more likely to mate with other fish that also were stocked by hatcheries in those locations. Their origins are also associated with a variety of biological differences, including body size and reproductive success.
May noted that pink salmon haven’t received the same attention from researchers as other salmon species because they’re often viewed as homogenous—spawning pinks are all two years old and roughly the same size. Through DNA analysis, researchers are revealing a more diverse species, which should help its ability to adapt as the climate changes, May said.
“All this variation impacts the ability of this species to persist in the face of change,” he stated.
Peter Westley, an associate professor at the College of Fisheries and Ocean Sciences, who contributed to the study, and is a member of the AHRP’s Science Panel, said these findings are just the beginning and that within a few years, there will be over 200,000 pinks in their genetic database.
“We’re leaning on a dataset that’s had a ton of work put into it,” he said. “This is just the tip of the iceberg.”