For decades, notebooks with fish facts and scale samples sat in a basement. The government of British Columbia had commissioned the collection of scales from commercially caught sockeye salmon in the Skeena River watershed from 1912 to 1947. Conservation concerns were already cropping up at that time, and the effort sought to study the animals’ life histories.
“We’ve been using fish scales as our window into the past.”
Those samples, rediscovered around 2000, show patterns of population decline in the watershed, according to a 2019 study. Now, the same team has pried yet more information from the scales—revealing that juvenile salmon in the area have gotten about 20% larger over the past century as the climate has warmed.
“We’ve been using fish scales as our window into the past,” said Michael Price, a biologist at Simon Fraser University in Burnaby, B.C., Canada. The scale collection had gone missing for 50 years. Skip McKinnell, one of the study’s coauthors, knew about the scale catalog and went looking for it. After a year of “intensive searching,” McKinnell found it at the Pacific Salmon Commission’s building in Vancouver, Price said.
People collected the scales by running a butter knife along the side of a fish. “They’d get a clump with all its slime, mucus, everything that contains the genetic information,” Price said. Genetic variability among the sampled fish meant Price and his colleagues could analyze DNA to connect individuals to their nursery lakes. In their 2019 study, that analysis and other records allowed the team to reconstruct historic trends in the Skeena’s sockeye populations: The area’s salmon lost about 75% of their abundance across the different populations. Some of the populations declined by more than 90%.
In the new work, published in Global Change Biology, Price and his colleagues looked to the Skeena sockeye scales and another set of scales to study how populations responded to warming waters and how climate change may influence the fishes’ futures. The team collected some of the recent scales, but most were collected between 1960 and 2015 by First Nations’ fisheries and Fisheries and Oceans Canada.
History on the Scales
Sockeye salmon spend the first 1 to 3 years of their lives in a lake. When they reach a certain size, they go out to sea. Their scales bear the marks of that life transition and their growth.
Fish scales develop concentric rings over time that can be read like tree rings. The distance between bands on a scale records the animal’s growth during a season. Because sockeye grow much slower in fresh water than they do in the ocean, researchers can tell when a fish migrated to the ocean.
Growth in juveniles across all populations was around 20% higher in recent years than it was prior to 1960, according to the scales. That extra growth in early life doesn’t necessarily translate to better survival. But it does seem to be increasing the proportion of salmon that leave their lakes after only 1 year.
With their genetic analysis, the researchers learned that how much a fish plumped up in its juvenile years depended on which population it belonged to. In some lakes, recent juveniles weren’t heftier than their historic counterparts. But in others, fish were 35% bigger. The team explored the relationship between enhanced growth and factors such as each lake’s temperature, depth, area, elevation, glaciation, and competition from other fish.
Temperature was a key driver of growth, the researchers found. As water gets warmer, fishes’ metabolism increases, leading to faster growth. But that didn’t explain juvenile size entirely. With declining numbers of fish in some lakes, competition for food decreased. That allowed the remaining fish to eat more and grow larger.
“This body of work is really excellent,” said Jack Stanford, a river ecologist at the University of Montana who was not involved with the study. “A very powerful set of data for a very important watershed or catchment basin and an array of populations.”
“We’re starting to see salmon moving into these new habitats, so that’s also another hopeful story.”
The research also hints at what may happen to sockeye as waters warm. Fish in shallow lakes that are already at elevated temperatures are growing faster than those in big, deep, cold lakes. So those shallow-lake populations may be more vulnerable to warmer conditions in the years to come, Stanford said.
The tools and approach could be useful in any lake where salmon are rearing, according to Stanford. Other researchers could compare the Skeena sockeye’s growth trends to that of salmon in other spots, he said.
Price is working with several First Nations groups to develop rebuilding plans for diminished populations, and his research may help inform conservation goals. The historical data reveal the past diversity of Skeena salmon, and forecasts can suggest which populations need the most help.
But they also show where Skeena salmon may be able to weather warming waters in at least the near term. Some populations have nursery habitat that will provide improved growing conditions in the future, Price said. And some cold, glacier-fed systems are becoming promising areas for salmon. “We’re starting to see salmon moving into these new habitats, so that’s also another hopeful story.”
—Carolyn Wilke (@CarolynMWilke), Science Writer