One might wonder why fish and other marine ectotherms aren’t just taking in ever more oxygen to coincide with this natural growth due to maturation and the rise of ocean temperatures. They don’t because at a certain point they cannot keep up.
The researchers point out that the surface area of an animal’s gills — where oxygen is obtained — does not grow at the same pace as the rest of its body.
“This is because gills, in order to work, must function as a two-dimensional surface — width by height — and thus cannot grow as fast as the three-dimensional volume — width by height by depth — they have to supply with oxygen,” Pauly said.
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He and Cheung liken how a fish gill works to a car radiator. Both are made up of numerous thin layers that allow for the transfer of heat, which permits cooling. But both can only work in two dimensions because air or water pass through only once.
“There is not much that fish can do to solve this problem,” Pauly said. “They can have bigger gills — just as sports cars have bigger radiators — but ultimately, the weight always catches up, and the ratio of gill surface to body weight becomes too low.”
The researchers believe this set of principles, which they have named the Gill-Oxygen Limitation Theory helps to explain why so many populations of marine species are shrinking. They and others predict that the reductions will be in the range of 20–30 percent if ocean temperatures continue to climb due to climate change.
At the higher end of that range is one of the world’s most important commercial fish: tuna.
“Tunas are active, mobile, and fast-swimming animals that need a lot of oxygen to maintain their lifestyle,” Cheung said. “In fact, they have to keep swimming non-stop in order to get more water through their gills to obtain sufficient oxygen. Thus, when temperature increases, they are particularly susceptible to not having sufficient oxygen to support their body growth.”
He added that for a 2 degree Celsius (3.6 degree Fahrenheit) increase in water temperature, which is approximately what is expected to occur in oceans around the world by the mid-21st century, tunas such as the Atlantic bluefin tuna will potentially decrease in body size by 30 percent.
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Sharks, many of which are already threatened with extinction, are also predicted to decrease in size, especially larger species.
In the case of tuna, haddock, cod, and other fish consumed by humans, shrinkage is predicted to decrease potential fisheries production. Since marine ecosystems are structured in part by the body size of organisms — basically larger fish eat smaller fish — the projected changes to body sizes will likely affect predator and prey interactions, as well as ecosystem structure and functions, Cheung said.
He said the most effective way to prevent these problems from occurring is to mitigate carbon dioxide emissions.