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

A collaborative team of scientists led by the University of Massachusetts Amherst recently discovered that there is no physiological evidence supporting an advanced theory – which involves the surface area of ​​fish gills – explaining why many fish species “shrink” as waters warm due to climate change. Known as the gill oxygen limitation (GOL) theory, it has been proposed as a universal mechanism explaining fish size and has been used in some predictions of future global fisheries yields.

However, the 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 discovered that although increasing temperatures caused a significant decrease, body size and gill surface area did not explain the change. The results of the study were recently published in the Journal of Experimental Biology.

“We know that global climate change is happening and that our oceans and rivers are warming,” says Joshua Lonthair, assistant professor of biology at UMass Amherst and lead author of the paper. “And we know that many animals, not just fish, reach smaller adult sizes under warmer temperatures. We even have a name for it, the temperature size rule. But despite decades of research, we don’t We still don’t understand why the size decreases as the temperature increases.

In marine and freshwater fish species, increasing water temperature has a critical effect on metabolism, reproduction, and other vital functions, but a critical factor upon which most rely One of the models underlying fisheries management is fish size. Commercial fishing is often regulated by tonnage, and when fish decline, more are needed to fill a ton. Lower weight is also linked to reduced reproduction. All in all, this means that managers must adapt their models to our changing world.

But how?

A major theory, GOL, holds that fish growth is limited by the amount of oxygen the gills can extract from the water. As the water warms, the fish’s biochemical processes speed up and require more oxygen. The GOL argues that gills have a limited surface area which limits the amount of oxygen they can provide and therefore fish cannot grow as large in warm water conditions. As a result, fish “shrink” to accommodate the limited oxygen their gills can provide.

GOL theory underpins widely cited model projections of drastic reductions in future global fisheries yields, including some used by the International Union for Conservation of Nature, but it has never been directly tested.

“We noticed that previous GOL studies relied on reused data from other, unrelated research projects that were not designed specifically to test the theory,” says Lisa Komoroske, assistant professor of environmental conservation at UMass Amherst and lead author of the article. “We designed a series of long-term experiments that collectively constitute the first effort to empirically test GOL.”

Specifically, Lonthair, Komoroske and their colleagues wanted to see how the three key ingredients of GOL – growth, energy demand and fish gill surface area – changed as water temperature increased. To do this, they turned to brook trout, which constitutes an ideal test subject: scientists already know a lot about this species, it is growing rapidly, is economically and ecologically important for the northeastern United States. United States and is relatively easy to work with.

Once they had their test subjects – small fry that initially weighed between one and two grams each – they were placed in tanks, some of which contained normal water at 15°C, and others contained water heated to 20°C. 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 allows us to know the metabolic rate. Finally, the researchers collected gill samples from the same fish to measure changes in the surface area of ​​their gills.

Once they started analyzing their data, a few things became clear: Brook trout in the warmer reservoirs were smaller, as expected, and consistent with the temperature size rule. However, the gill surface area was more than sufficient to meet the fish’s energy needs, meaning their growth was not limited by gill surface area, as predicted by GOL.

Additionally, the team found that although the metabolic rates of the warm tank fish actually increased after three months, after six months their oxygen levels returned to normal, suggesting that the Fish could adjust their physiology over time to account for increased water. temperatures.

“Oxygen use may still be an important limiting factor in fish size,” says Lonthair, “but, taken together, our results show that GOL cannot predict what we observe, which has implications for predicting climate impacts on future fisheries and ecosystems.”

“Our work highlights the importance of interdisciplinarity,” adds Komoroske. “Fisheries and macroecology scientists tend to work at the population and species level, while physiologists tend to work at the individual and cellular levels. But these are academic distinctions, not natural ones, and if we want to help fish to survive in warmer waters, we must work at several biological scales and combine knowledge from all these areas.

So, what is the mechanism that governs fish size and temperature?

“We don’t know yet,” Lonthair says. “And it may not be a single mechanism, but a multitude of factors, including oxygen consumption. We need more long-term, interdisciplinary studies to understand how best to to adapt to global warming.”