A collaborative team of scientists led by the University of Massachusetts Amherst recently found that there is no physiological evidence to support a leading theory—involving the surface area of fish gills—as to why many fish species are “shrinking” as temperatures warm water due to climate change.
Known as the Gill Oxygen Limitation (GOL) theory, it has been proposed as the universal mechanism explaining fish size and has been used in several predictions of future global fishing outcomes. However, researchers, representing the National Oceanic and Atmospheric Administration, 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, even the increased temperature leads to significantly reduced body size, gill surface area does not explain the change. The results of the study were recently published in Journal of Experimental Biology.
“We know that global climate change is happening and that our oceans and rivers are getting warmer,” said Joshua Lonthair, lecturer in biology at UMass Amherst and the paper’s lead author. “And we know that many animals — not just fish — grow to a smaller adult body size under warmer temperatures. We even have a name for it, the Temperature Size Rule. But despite of decades of research, we still don’t understand why size decreases as temperature increases.
In both marine and freshwater fish species, increasing water temperature has a critical impact on metabolism, reproduction and other life functions, but is a critical factor that most models underlie management. of the fishery is the size of the fish. Commercial fisheries are often regulated by tonnage, and as fish get smaller, more of them are needed to fill a ton. Lower weight is also associated with reduced reproduction. In sum, this means that managers need to adjust their models for our changing world.
But how?
A leading theory, GOL, holds that fish growth is limited by how much oxygen the gills can pull from the water. As the water warms, the fish’s biochemical processes speed up and require more oxygen. GOL argues that the gills have a limited surface area that limits the amount of oxygen they can provide, and thus, the fish cannot grow as large under warm water conditions. Therefore, fish “shrink” to fit the limited oxygen their gills can provide.
The GOL theory underlies widely cited model projections of severe reductions in future global fishing 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 data repurposed from other, unrelated research projects that were not designed to specifically test the theory,” said Lisa Komoroske, assistant professor in environmental conservation. at UMass Amherst and is the senior author of the paper. “We designed a series of long-term experiments that together are the first effort to empirically test GOL.”
Specifically, Lonthair, Komoroske and their colleagues wanted to see how the three main components of the GOL—growth, energetic requirements and the surface of the fish’s gills—changed as water temperatures increased. To do this, they turned to brook trout, which are ideal test subjects: scientists already know a lot about the species, they grow quickly, are economically and ecologically important in the Northeastern US and are 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, 15º Celsius water, and some contain water heated to 20º Celsius. The fish were weighed and measured at the beginning of the experiment, and then 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 both fish to measure changes in their gill surface area.
Once they started analyzing their data, a few things became clear: brook trout in warmer tanks is smaller, as expected, and consistent with the Temperature Scale Rule. However, the surface area of the gills is more than enough to meet the energetic needs of the fish, which means that their growth is no limited to the surface of the gill area, as predicted by GOL.
Furthermore, the team found that as the fish warmed the tank their metabolic rate increased made increased at the three-month mark, by six months their oxygen rate had returned to normal, suggesting that the fish may adjust their physiology over time to account for the increased water temperature.
“Oxygen use may still be an important factor in limiting fish size,” said Lonthair, “but, taken together, our findings show that GOL cannot predict what we see, and this has implications for predicting climate impacts on future fisheries and ecosystems.”
“Our work highlights the importance of interdisciplinarity,” added Komoroske. “Scientists of fisheries and macroecology tend to work at the population and species level, while physiologists tend to work at the individual and cellular levels. But these are academic differences, not natural ones, and if we’re going to help fish survive warming waters, we need to work across biological scales and join the perspectives of all these fields.
So what is the mechanism that governs fish size and temperature? “We don’t know yet,” said Lonthair. “And it may not be a single mechanism—there may be multiple factors, including oxygen consumption. We need more interdisciplinary long-term studies so we can understand how best to adjust to our warming world.”
