A team of scientists from the University of Massachusetts Amherst discovered that there’s no physical proof backing the prominent theory, which connects the surface area of fish gills to the shrinking size of many fish species in warmer waters due to climate change. This theory, called the Gill Oxygen Limitation (GOL) theory, has been suggested as the primary explanation for fish size and has been utilised in forecasts for future global fisheries yields.
Nonetheless, 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 extended experiments on brook trout. They discovered that while higher temperatures do indeed result in significantly smaller body sizes, the surface area of the gills did not account for this change. The findings of the study were recently published in the Journal of Experimental Biology.
Joshua Lonthair, a biology lecturer at UMass Amherst and the lead author of the paper, commented, “We know that global climate change is happening and our oceans and rivers are getting warmer. And we know that many animals- not just fish- are growing to smaller adult body sizes under warmer temperatures. We even have a name for this, the Temperature Size Rule. But despite decades of research, we still don’t understand why size decreases as temperature increases.”
Increasing water temperatures have significant impacts on the metabolism, reproduction and various life functions of both marine and freshwater fish species. However, a crucial element that many models guiding fisheries management hinge on is fish size. Commercial fisheries often operate under regulations based on tonnage, and when fish sizes diminish, it requires a greater quantity of them to meet the same tonnage. Additionally, smaller fish are associated with decreased reproductive capacity.
The prominent theory, GOL, posits that fish growth is restricted by the capacity of gills to extract oxygen from water. As water temperatures rise, the biochemical processes within fish accelerate, demanding more oxygen. According to GOL, gills have a finite surface area that limits the oxygen they can deliver, resulting in fish being unable to attain large sizes in warm water conditions. Hence, fish are “shrinking” to match the restricted oxygen supply from their gills.
The GOL theory forms the basis for widely referenced model forecasts indicating significant declines in future global fisheries yields, including those utilised by the International Union for Conservation of Nature. However, it has never undergone direct testing until now.
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