Why are fish getting smaller as waters warm? It’s not their gills, finds study

A joint team of scientists led by the University of Massachusetts Amherst recently found that there is no physical evidence to support an important theory—involving the surface area of ​​fish gills—as to why many fish The species is “shrinking” because the waters are warming due to climate change. Known as the gill oxygen limitation (GOL) theory, it has been proposed as a universal mechanism for explaining fish size and has been used in some predictions of future global fisheries production.

However, researchers, representing the National Oceanic and Atmospheric Administration, the US Geological Survey, the University of California Davis, as well as UMass Amherst, conducted a series of long-term experiments on brook trout and found that although increasing temperatures significantly Decrease occurs. Body size, Gill surface area change not explained. The results of the study were recently published Journal of Experimental Biology.

“We know that global Climate change happening and our oceans and rivers are warming,” says Joshua Lonthier, a biology lecturer at UMass Amherst and lead author of the paper. Hot temperature. We even have a name for this, the temperature-size principle. But despite decades of research, we still don’t understand why size decreases as temperature increases.”

In both marine and freshwater Fish speciesincreasing water temperature has an important effect on metabolism, reproduction and other life functions, but an important factor in which most models Fisheries management Rely on the size of the fish. Commercial fishing is often regulated by tonnage, and when fish shrink, it takes longer to fill a ton. Low weight is also associated with low fertility. Overall, this means that managers need to adjust their models to our changing world.

But how?

A leading theory, GOL, holds that fish growth is limited by how much oxygen the gills can draw from the water. As the water warms, the fish’s biochemical processes accelerate and require more oxygen. GOL claims that gills have a limited surface area which limits the amount of oxygen they can provide, and thus, fish cannot grow as large in warmer water conditions. Therefore, fish are “shrinking” to fit the limited oxygen their gills can provide.

The GOL theory contains widely presented model projections of drastic future declines in global fisheries production, some of which have been used by the International Union for Conservation of Nature—but its direct Not directly tested.

“We found that previous studies on GOL relied on data replicated from other, unrelated research projects that specifically looked at the theory,” says Lisa Komorowski, assistant professor of environmental conservation at UMass Amherst and senior author of the paper. were not designed to be tested.” “We designed a series of long-term experiments that are collectively the first attempt to test GOL experimentally.”

Specifically, Lonthair, Komoroske and their colleagues wanted to see how three key components of GOL—growth, energetic demands and gill surface area of ​​fish—changed with increasing water temperature. To do so, they turned to brook trout, ideal test subjects: Scientists already know a lot about the species, it’s fast-growing, economically and ecologically important to the northeastern United States. are and are relatively easy to work with.

Once they had their test subjects — small fry that initially weighed between one and two grams — they were placed in tanks, some with normal, 15º Celsius water, and some with water as hot as 20º Celsius. was Fish were weighed and measured at the start of the experiment, and monthly thereafter. Their oxygen consumption was also measured at two weeks, three months and six months, which is a way of determining metabolic rate. Finally, the researchers collected gill samples from the same fish to measure changes in their gill surface area.

Once they started analyzing their data, a couple of things became clear: Brook trout in the warm tanks were smaller, as expected, and the temperature was consistent with the size rule. However, the surface area of ​​the gills was more than sufficient to meet the energy needs of the fish, meaning that their growth was not limited. Surface areas of gulls, as predicted by GOL.

Additionally, the team found that the metabolic rate of fish in the warm tank increased at the three-month mark, with their oxygen levels returning to normal by six months, suggesting that the fish can adjust their physiology over time. are so that the water can increase. temperature

“Oxygen consumption may still be an important limiting factor in fish size,” says Lonthier, “but, taken together, our results suggest that GOL cannot predict what we are seeing. are, and this has implications for predicting future fisheries and climate impacts on ecosystems.”

“Our work highlights the importance of interdisciplinarity,” says Komorowski. “Fisheries and macroecologists tend to work at the population and species levels, while physiologists work at the individual and cellular levels. But these are cognitive distinctions, not natural ones, and if we keep fish in warm waters I’m going to help survive. We need to work at the biological scale and get involved in the insights of all these fields.”

So what is the mechanism for controlling fish size and temperature?

“We don’t know yet,” says Lonthair. “And it may not be one mechanism—it may be a number of factors, including oxygen consumption. We need more interdisciplinary long-term studies to understand how best to cope with our warming world. be adjusted.”