Climate change could make salmon bigger, but may also give them less room to breed

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A chinook salmon during spawning. (Shutterstock)

The average size of the endangered chinook salmon could increase by five millimeters in the next century due to rising water temperatures, new research predicts, but the amount of suitable habitat for breeding could decrease, leading to a dramatic reduction in the fish's ability to reproduce.

While climate change is expected to have consequences for many marine species, the studies making these predictions are often done on a large scale, without considering the nuanced interactions between fish and their unique environment. This study, published May 6 in Science of the Total Environment, takes a different approach, using physics and biology to explore how Idaho's Bear Valley Creek and its chinook salmon populations will respond to climate change.

Study co-author Daniele Tonina, an associate professor in engineering at the University of Idaho, spoke to The Academic Times about the project, which was completed in collaboration with the Environmental Protection Agency. He explained that he and his team were just as interested in capturing information about the physical conditions surrounding the fish as they were in studying the fish themselves.

"We wanted to understand how the physical environment impacts the organisms that live there," he said. "And so, one of the things that was interesting to us is to understand that if the form of the river changes, how [will] their habitat will change, and how will the fish respond to that?"

The physical changes in a river, such as in temperature, water flow or water levels, could have a particularly strong effect on chinook salmon because they rely on lengthy migrations upstream as part of their life cycle. Oncorhynchus tshawytscha, the species of salmon that lives in Idaho, is also endangered, making it especially vulnerable.

And while Alaska may be the biggest producer of commercially fished salmon in the United States, the chinook salmon fisheries of Idaho contribute over $200 million to the local economy each year. Additionally, these fish are highly valued in sport fishing, which drives a further $400 million in spending by tourists and locals alike on food, transportation, hotels, campgrounds and fishing equipment.

In addition to driving tourism and providing food for humans, Tonina explained that chinook salmon are also considered key species, meaning that, "Their presence impacts the ecosystem at large, and losing them potentially starts a domino effect on the ecosystem." This makes salmon somewhat of a canary in the coal mine for the health of the overall ecosystem.

Though the researchers looked to predict the future, they first looked to data from the past to develop a baseline for the predictions. The team looked to data on fish size and temperature gathered between 1990 and 2011 as part of a long-term monitoring program conducted by the Northwest Fisheries Science Center, a government-funded research institute focusing on marine management.

By modeling the relationship between fish size and environmental parameters such as temperature, the researchers were able to develop a model that could predict how increases in temperature could affect the fish between now and the end of the century.

However, the researchers did not stop there. By applying additional modeling using detailed topographical data gathered directly from the creek itself, the researchers also predicted changes in the size of the salmon's habitat and the way water moves through it, which they used to predict how well fish could survive and reproduce.

They predicted that salmon will increase in size by an average of about 5 millimeters in the second half of the 21st century, contradicting predictions for other populations of salmon in the United States. And while being larger is often better for organisms in terms of survival, the researchers also predicted that the amount of suitable habitat for spawning will decrease significantly, reducing the fish's overall spawning suitability by 30%.

Though the size change is small, because an average salmon is around 3 feet long, this could spell trouble for the fish because of its reliance on long migrations, often through narrow channels. After migrating upstream, chinook salmon spend several years away from their freshwater birth sites before migrating home to spawn, so even slow changes could lead to increased competition and less breeding success for salmon in each successive generation.

"Even if we have a fish coming back, they will find an environment that is smaller than what it could have been," Tonina said.

The researchers are next interested in applying their analysis to different phases of the salmon's life cycle. They also hope to use their methods for larger environments and larger populations of fish.  

In the meantime, the researchers maintain that their findings demonstrate the complex nature of ecological studies, as it would be all too easy to assume that salmon survivability will improve in the coming years with the increase in size.

"And that is not the case, because that is just a response that they have that needs to be understood with the fact that their habitat will also decrease dramatically," Tonina said. "I think that is the key point."  

The study, "Some (fish might) like it hot: habitat quality and fish growth from past to future climates," published May 6 in Science of the Total Environment, was authored by William Jeff Reeder and Daniele Tonina, University of Idaho; Frank Gariglio, Idaho Power; Ryan Carnie, Geoengineers Inc.; Chunling Tang, U.S. Environmental Protection Agency; Daniel Isaak and James A. McKean, Rocky Mountain Research Station; Qiuwen Chen, Nanjing Hydraulic Research Institute; and Zhongbo Yu, Hohai University.

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