When humans interact with salmon, it’s usually in coastal areas near commercial fishing operations. Those pulling them up in nets just see a bunch of fish that look pretty much the same.
But as science revealed long ago, salmon schools in the ocean are comprised of many distinct populations of the fish, made that way by their tendency to return to their native streams for spawning. This is one of the reasons they’re so difficult to study. Another factor is their complex life cycles that move them thousands of miles from freshwater to the sea and back again.
To make things a little simpler for telling where specific salmon are from, scientists at the University of Alaska – Fairbanks led an investigation tracking the movements of Chinook salmon in Alaska’s Bristol Bay and the Nushagak River. Their work relied on otolith bones of Chinook salmon taken from the river’s watershed and the analysis of strontium isotopes. By plugging their findings into a model, they developed a map tool to predict Bristol Bay salmon origin with greater than 90 percent accuracy.
“We picked the Nushagak River because it’s a major producer of wild salmon, but it’s also geologically diverse,” said Sean Brennan, a post-doctoral researcher at the University of Washington who led the study while a PhD student at U. Alaska. “What controls strontium in the water is geology. Water essentially dissolves rocks as it passes through, and strontium is derived from those rocks.”
Different types of rocks throughout the watershed offered researchers the chance to make a robust long-term tool to help identify where salmon in Bristol Bay originate. One basis of the tool was otolith bone analysis, which involves cutting the bones with lasers and then applying mass spectrometry to measure elements that rise in the resulting dust.
In addition to collecting fish with minnow traps and stick seine nets, scientist also gathered water samples at each site. These were used to back up and confirm findings from the salmon otolith analyses. Otoliths from slimy sculpins, which live up to seven years in the same general area, were also studied to measure the variability of strontium over time.
“For the most part, the water samples lined up exactly,” said Brennan. “It’s a really good tracer that we used.”
Results of the research will come in handy for conservation efforts by providing a way to determine where salmon are from when they’re harvested, which can be done over the course of years using the approach. Knowing that could yield ecological and economical benefits. For Alaska’s Nushagak Hills, home to at least one native Alaskan village, salmon have cultural significance too.
“This is a really good way to determine production at small spatial scales,” said Brennan. “If we can tie the production in a fishery back to all the habitats, figure out the relative production of each population, then we can work to not over-exploit them.”
Scientists at the University of Utah and the U.S. Geological Survey’s Alaska Science Center also contributed to the work. Brennan was also aided by the New Stuyahok native village, whose residents served as guides and helped transport equipment.