How Can We Protect Salmon from Climate Warming? It Depends on the River

Source Article: Andrew S, Swart S, McKenna S, Morissette J, Gillis C-A, Linnansaari T, Currie S, Morash AJ (2024) The impacts of diel thermal variability on growth, development and performance of wild Atlantic salmon (Salmo salar) from two thermally distinct rivers. Conserv Physiol 12(1): coae007; doi:10.1093/conphys/coae007.

Featured Image Caption: The researchers collected and conducted tests on salmon parr, or young salmon that live in freshwater. Image Source: Peter Steenstra at the Green Lake National Fish Hatchery, Public domain, via Wikimedia Commons

Some Like it Hot, but Salmon Like it Cold

Like all fish, Atlantic salmon (Salmo salar) are sensitive to changes in temperature. Extreme temperatures can cause stress, poor nutrition, slow development, and altered migration patterns. Yet freshwater bodies, where they spend their early life, are always changing temperature. Temperatures in Canadian rivers where Atlantic salmon live can climb up to 30 degrees Celsius (86 degrees Fahrenheit) during the warmest days and can sink down to 17 degrees (63 degrees Fahrenheit) at night. That’s a huge change in temperature over just a few hours! To survive, salmon must be adaptable, particularly in a warming and increasingly unpredictable climate. However, little is known about how salmon respond to these dramatic changes in temperature.

Atlantic salmon populations are on the mend after overfishing resulted in their status as an endangered species. Today, they are considered “near threatened,” a major improvement! Image source: Flickr user NYS DEC, CC BY-NC-ND 2.0 DEED 
Gone Fishing

Researchers in New Brunswick, Canada designed an experiment to fill this gap in our knowledge. Using electrofishing, a technique which uses electricity to stun fish temporarily, the scientists collected juvenile salmon (or paar, to use the technical term) and brought them back to the lab. They caught 100 wild Atlantic salmon from each of two small streams, one connected to the Miramichi River and one connected to the Restigouche River. The Miramichi River reaches warmer temperatures than the Restigouche River, and experiences more dramatic changes in temperature.

The Miramichi River in New Brunswick. Image Source: Tourisme Nouveau-Brunswick, CC BY 2.0 via Wikimedia Commons
Putting Salmon to the Test

In the lab, the scientists exposed the salmon to temperatures ranging from 16 to 21 degrees Celsius (61-70 degrees Fahrenheit) every day for a month, after which they were considered acclimated to the cooler environment. Researchers then moved half of the fish to temperatures ranging from 19 to 24 degrees Celsius (66-75 degrees Fahrenheit) every day for a month, and these fish were considered acclimated to the warmer environment.

Once a month for several years, the scientists measured the salmons’ mass, length, and life stage. They then calculated their growth rate and condition, based on size and weight. They also measured their anaerobic capacity, or ability to perform intense, short-term exercise. To do this, the scientists chased individuals around the tank, pinching their tails until the fish were too tired to continue! Thankfully, after this exercise, the fish were allowed to rest and were returned to their acclimation tank. Finally, the scientists measured thermal tolerance by exposing fish from each acclimation tank to warmer and warmer temperatures until the fish lost the ability to stay upright in the water. The fish were then cooled off and returned to the appropriate acclimation tank.

Different River, Different Salmon

With these data on growth rate, condition, anaerobic capacity, and thermal tolerance, the scientists used statistics to test how the fish from the two different rivers in the two different acclimation tanks differed from each other. 

Salmon from the two rivers had different performance in different temperatures. The Restigouche River can be found near the northern border of New Brunswick near Quebec. The warmer Miramichi River lies south of the cooler Restigouche. Image Source: © Sémhur CC-BY-SA-4.0 via Wikipedia Commons

They found that the acclimation temperatures did not affect early life growth or anaerobic capacity, but the source of the fish did. Fish from the cooler river grew more slowly and had higher anaerobic capacity, regardless of their acclimation temperature. However, more fish from the cooler river grew more slowly and died when exposed to the warmer acclimation temperature. Fish from the warmer acclimation temperature had a higher thermal tolerance, though thermal tolerance was also influenced by the river the fish came from.

Overall, the researchers concluded that Atlantic salmon from different rivers respond differently to changes in temperature. Fish from the warmer river perform better in warmer temperatures than those from the cooler river. This difference could have been caused by either  evolutionary adaptation (genetic) and/or by phenotypic plasticity (non-genetic). Phenotypic plasticity is the ability of organisms to change their body structures or chemistry within their lifetime to better tolerate new environmental conditions; in this case, acclimation to new, warmer temperatures.

Using this Knowledge to Protect Salmon

The results of this study show that Atlantic salmon are fairly resilient to changes in temperature. The different acclimation temperatures the fish experienced in the lab did not particularly affect their health and performance. However, the river that the fish came from seriously affected their responses to changes in temperature.

Atlantic salmon continue to have immense cultural value to many groups of people in Europe and North America. Image source: Flickr user US Fish and Wildlife Service, Public Domain

For conservationists, this means that salmon from different rivers may be at different levels of risk in a warming climate. The risk level in individual rivers can inform which populations require immediate conservation efforts. In addition to climate change, rivers can also be warmed by hydroelectric dams. These structures should be carefully studied to ensure that they do not greatly warm the water or require fish to migrate through small passages and holding tanks that can warm up quickly. If we humans are thoughtful about our impacts on Atlantic salmon and consider the different thermal tolerance of fish from different rivers, then we can better protect these valuable fish!

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Julia Bebout

Julia Bebout

I am a PhD student at the University of California San Diego studying how the timing of germination and flowering shapes plant communities. I'm fascinated by how past environments can affect present and future ecosystems, especially faced with climate change. My favorite things to write about are community ecology, wetland and alpine ecosystems, and regenerative agriculture. I also love hiking, climbing, baking, and dancing! Twitter: @BeboutJulia

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