How Rapid Evolutionary Change has Made Landlocked Salmon Less Seaworthy
About 14,000 years ago, the expansive Laurentide ice sheet that covered much of Canada and the northern United States receded. What remained were not only the Great Lakes, but also a number of small, isolated lakes scattered throughout the region that trapped once migratory Atlantic salmon. For biologists like Prof. Nick Bernier of the Department of Integrative Biology these salmon offer a fascinating opportunity to examine evolutionary change in action – including how being landlocked has affected the ability of salmon to acclimate to seawater.
A migratory, “anadromous” Atlantic salmon makes a round trip journey between fresh and saltwater. The first few years of life are spent as a juvenile fry in freshwater before developing into a smolt which heads to the ocean. At this stage, critical physiological changes driven by hormones help it acclimate to saltwater, including the ability to actively regulate water and salt concentrations in the body.
In contrast, while landlocked salmon also imprint on their natal stream, they do not migrate to saltwater, instead developing in nearby lakes. Because these fish no longer make the migration to saltwater, Bernier and Dr. Stephen McCormick, a colleague with the U.S. Geological Survey (USGS), were curious if the relaxed selection pressure on landlocked salmon has affected their tolerance for seawater. If so, what are the physiological mechanisms underlying this change?
The coordinated efforts of four fish facilities were required to investigate these questions. On the day of fertilization, eggs from landlocked and migratory salmon were transported from the hatchery to a common facility for rearing under the same conditions. The fish were later transferred to the USGS Conte Anadromous Fish Research Center where development and seawater tolerance of both types of fish was determined. Known as a common garden experiment, the shared rearing conditions ensured consistency between the fish populations so that the scientists could better detect physiological differences between them.
Bernier and McCormick found that the fishes’ salt tolerance, survival, and initial growth in seawater were indeed greater in the migratory, anadromous salmon than in their landlocked counterparts. In addition, the migratory, anadromous salmon produced more of the enzyme known as gill Na+/K+-ATPase, which is required to pump salts out of the body.
The team also examined the suite of hormones involved in seawater acclimation, including cortisol and growth hormone, and found they were expressed in lower amount in landlocked salmon. In contrast, there were no differences in the hormones associated with imprinting or migration. This is likely because landlocked salmon still make local migrations, spending their adult life in lakes before heading back to their natal streams.
These results demonstrate that there are “genetic differences in the hormonal control during development between landlocked and migratory, anadromous populations of Atlantic salmon that are likely to have evolved over the last 14,000 years,” says Bernier. “Since the hormones that regulate saltwater tolerance also play a role in the metabolic and behavioral changes that occur during smolting, we don’t know if the reduced hormonal responses observed in the landlocked salmon will continue to decrease over evolutionary time, or if the fish are currently at a point where they have reached maximum changes physiologically.”
Bernier has enlisted a PhD student, Brett Culbert, to uncover other hormonal mechanisms in adjusting to seawater. No matter the outcome, they are confident of one thing: such studies are making important advances in our understanding of how complex physiological changes can evolve over a relatively short period of time.
Funding for this study was provided by the Natural Sciences and Engineering Research Council and the U.S. Fish and Wildlife Service.
Read the full study in the journal Scientific Reports.
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