The Reasons Behind the Decline of Many Fish Species

A group of researchers from the University of Massachusetts Amherst found that the widely accepted notion linking the surface area of fish gills to the reduction in size of many fish species in warmer waters as a result of climate change is not supported by any physical evidence. This notion, known as the Gill Oxygen Limitation (GOL) theory, has been used in projections for future world fisheries yields and has been proposed as the main reason for fish size.

Still, the scientists from the US Geological Survey, the University of California Davis, UMass Amherst, and the National Oceanic and Atmospheric Administration carried out a number of lengthy studies on brook trout. They found that whereas higher temperatures indeed cause noticeably reduced body sizes, this difference is not explained by the gills’ surface area. The Journal of Experimental Biology just published the study’s results.

Lead author of the article Joshua Lonthair, a biology instructor at UMass Amherst, said, “We know that global climate change is happening and our oceans and rivers are getting warmer.” Furthermore, we are aware that in warmer climates, a wide variety of animals—not only fish—are developing to lower adult body sizes. The Temperature Size Rule is the term we even have for this. But we still don’t know why size reduces as temperature rises, even after decades of study.

Both freshwater and marine fish species’ metabolism, reproduction, and other life processes are significantly impacted by rising water temperatures. Fish size, however, is a critical component that many models directing fisheries management rely on. Tonnage-based restrictions govern commercial fisheries, and when fish sizes decline, more fish must be caught to reach the same tonnage. Smaller fish are also linked to a lower potential for reproduction.

According to the widely accepted idea known as GOL, a fish’s ability to develop is limited by its gills’ ability to draw oxygen from the water. Fish’s metabolic activities quicken with rising water temperatures, necessitating more oxygen. According to GOL, fish cannot grow to great proportions in warm water because their gills have a limited surface area that restricts the amount of oxygen they can carry. Fish are thus “shrinking” to fit the limited oxygen flow from their gills.

The International Union for Conservation of Nature and other widely cited model estimates suggesting severe losses in future global fisheries yields are based on the GOL hypothesis. But up until today, it has never been put through direct testing.