Salmon anti-freeze

With our talk about thermoregulators in class recently, it was interesting to look at the salmon's ability to produce morphological adaptations for freeze-avoidance. Freeze-avoidance traits allow certain species to use physiological and behavioral mechanisms to prevent ice crystals from forming in their tissues enabling survival in cold to extremely cold temperatures.

In this study, they questioned if the salmon's thermal tolerance (ability to optimally function in varying temperatures) in warming waters was a result of physiological limitations. Eliason, et al. (2011) hypothesized the physiological adaptations or limitations of aerobic performance, cardiac response, and thermal tolerance occur at the population level of sockeye salmon due to migratory demands, morphological and behavioral traits. With concerns that global warming and the resulting warming river temperatures could lead to increased mortality of the migrating salmon due to their morphological adaptations being better suited for colder climates. If the river waters continue to increase the salmon's ability to aerobically perform (deliver oxygen to internal systems) without over stressing and destroying the cardiac muscles will significantly decrease.

In order to determine a correlation between aerobic scope (or optimal range) above the salmon’s natural T_opt (optimal thermal tolerance) and the capacity limitations of delivering oxygen to the salmon's internal systems they used the OCULIT (oxygen-and capacity-limited thermal tolerance) hypothesis. To do this with the varying salmon populations they had to measure heat tolerance adaptations and cardiorespiratory performance capabilities of individual salmon in varying temperatures.

Generally coastal populations are better adapted to colder temperatures and possess lower optimal thermal tolerance while upriver populations are better adapted to higher temperatures with a higher optimal thermal tolerance. Meaning if the river temperatures increase at the coastal population migration paths, the salmon will not be able to handle the higher temperatures, overtaxing their hearts, and depriving their systems of oxygen.

They found the freeze-avoidance capabilities of the sockeye salmon enabled large numbers of spawning upriver sockeye to survive the colder temperatures and resulting stressors to aerobic and cardiac functions. As the water temperatures increased, the aerobic and heart optimal ranges decreased forcing the Chilko (a subspecies of sockeye salmon) to adapt and outperform the other coastal salmon populations. The upriver populations experienced a harder migration pattern, higher morphological patterns selecting for heart and aerobic morphology to better perform. They found aerobic scope, cardiac scope, and heart rate scope were positively related with changes in water temperatures suggesting the temperature dependence of heart performance was linked to aerobic performance. The more the heart works, the less oxygen they get, the sooner they die.

This means the warmer the water, selection will force salmon physiological adaptions like trait heritability, trait variability, and fitness to change with it. Aerobic performance limitations result from cardiac collapse in the warmer temperatures.

Reference:

Eliason, E. , Clark, T. , Hague, M. , Hanson, L., Gallagher, Z., Jeffries, K., Gale, M., Patterson, D., Hinch, S. & Farrell, A. (2011). Differences in thermal tolerance among sockeye salmon populations. Science, 332, 109-112. DOI: 10.1126/science.1199158