Chinook salmon have tremendous cultural, economic and ecological value in North America – plus, they're delicious. Unfortunately, at their southern range limit in central California, Chinook face several challenges that threaten their survival. One of the most concerning is the large swings in temperature they currently experience, which are expected to worsen due to climate change and human alterations to river water flow. The good news is that managers can help the fish by strategically releasing water from dams to control the temperature of their freshwater habitat. In order to know when, where and how much water to release into the rivers, managers need to know what temperatures keep the fish happy and healthy. Researcher Vanessa Lo at the University of California, Davis, USA, worked with colleagues from the University of California, Davis, the University of Amsterdam, The Netherlands, and NOAA Southwest Fisheries Science Center, USA, to inform Chinook conservation efforts in California by determining which temperatures the fish thrive in.
To accomplish this, Lo and the team measured how temperature impacts the amount of energy the Chinook need to survive and use for their digestive processes. Just like it takes money to make money, it takes energy to break down food into nutrients and fuel. The cost of digesting a meal takes up a substantial portion of a fish's energy budget. If temperature causes these digestive costs to increase, fish may have less energy available to swim and grow. This is especially concerning for young Chinook, which must grow up in their cool freshwater rearing grounds before migrating out to sea through warmer water. If the young fish are unable to make it to the ocean, the population could be lost. So, Lo and colleagues set out to find the temperatures where young Chinook's digestion was impaired to help managers set fish-friendly river temperatures. The team anticipated that as the temperature increased, the time it would take for the fish to digest a meal would decrease. At the same time, they expected that the total cost of digesting a meal would remain the same across temperatures. However, they also predicted that the fish would have to use up a larger proportion of their energy budgets while digesting a meal in warm water, to fit the same overall digestive costs into a shorter amount of time.
The researchers fed young Chinook a meal of fish pellets and then measured how much oxygen the fish were breathing to estimate their energy costs during digestion. They then repeated the procedure at temperatures ranging from 13 to 24°C and, surprisingly, their findings were the opposite of what they had expected. Instead, the time it took for the Chinook to digest a meal and their energy usage during digestion was exactly the same across the entire temperature range, up to 24°C. These results indicate that temperatures up to 24°C do not limit the young Chinook's ability to eat, swim and grow. However, when the team raised the temperature to 25°C, the fish began to struggle so much that they called off the tests.
It seems that juvenile southern Chinook are well positioned to deal with a wide range of temperatures; however, they hit a physiological wall near 25°C. And the longer a fish is exposed to a stressful temperature, the more problems it will develop. Given that the team performed all their energy use measurements after each animal had been briefly exposed to one of the temperatures, they point out that longer exposures to temperatures below 25°C could also negatively affect young Chinook. However, in the short term, Chinook could consume a meal at temperatures beneath 25°C and still have plenty of energy left over for all the other fishy activities they need to survive.
