Something unusual is happening in the oceans. Fish are shrinking globally, and the cause is still a mystery.
This phenomenon is observed in salmon near the Arctic Circle and skate in the Atlantic. According to a recent analysis, almost 75% of fish are shrinking globally, and marine fish populations sampled worldwide experienced a reduction in their average body size between 1960 and 2020.
A team of scientists, spearheaded by the University of Massachusetts Amherst, recently discovered no physiological evidence to support a prominent theory regarding the shrinking of fish species as water temperatures rise due to climate change. This theory, known as the Gill Oxygen Limitation (GOL) theory, suggests that the surface area of fish gills is the universal factor determining fish size and has been used in predictions of future global fishery yields.
One undeniable reason for the reduction in marine life size is fishing. Recreational anglers and commercial fishers prefer to catch large fish, leaving behind the smaller ones.
Rick Stuart-Smith, a marine biologist at the University of Tasmania, Australia, has surveyed coral reefs globally. During his dives, he mentioned that small and sometimes even shy fish indicate illegal fishing in supposedly protected areas.
“In genuinely protected areas, you’ll even find big fish in less than ideal habitats,” he said.
Rising water temperatures critically impact marine and freshwater fish species metabolism, reproduction, and other life functions. However, a crucial factor in fisheries management models is fish size. Commercial fisheries are often regulated by tonnage, so when fish shrink, they need more to meet the tonnage requirements. Additionally, lower weight is associated with reduced reproduction. Consequently, managers must adjust their models to account for these changes in our evolving world.
Testing the Gill Oxygen Limitation (GOL) Theory: New Insights and Experimental Evidence
The Gill Oxygen Limitation (GOL) theory posits that fish growth is restricted by how much oxygen their gills can extract from the water. As water temperatures rise, the fish’s biochemical processes accelerate, increasing their oxygen needs. According to GOL, fish are shrinking globally and the limited surface area of the gills restricts oxygen supply, preventing fish from growing as large in warmer waters.
The GOL theory forms the basis of widely cited projections predicting significant reductions in future global fisheries yields, including some used by the International Union for Conservation of Nature. However, this theory has never been directly tested.
“We observed that previous studies on GOL used data from other research projects that were not specifically designed to test the theory,” says Lisa Komoroske, assistant professor in environmental conservation at UMass Amherst and the paper’s senior author. “We designed a series of long-term experiments that collectively represent the first empirical test of GOL.”
Putting the Gill Oxygen Limitation Theory to the Test
For years, the prevailing explanation for why fish reach smaller adult sizes in warmer waters centered on their gills. Unlike humans, fish cannot effectively regulate their body temperature. As water temperatures rise, so does their need for oxygen. Scientists proposed that the limited surface area of fish gills couldn’t meet the increased oxygen demand of a larger body, causing fish to limit their growth.
“That’s the crucial point,” said Daniel Pauly, a marine biologist at the University of British Columbia who developed the Gill Oxygen Limitation (GOL) theory. “Gills don’t grow as quickly as volume.” When Pauly began developing this theory in the 1970s while studying tiny tropical fish, climate change was not yet a consideration.
However, in a recent study published earlier this year, Komoroske and Lonthair discovered that the gills of brook trout raised in warm water were adequately sized to fulfil the animals’ energy requirements. This finding challenges the prevailing theory regarding why fish were becoming smaller.
“We still lack a definitive answer,” states Lonthair. It’s possible that there isn’t just one mechanism at play—it could involve multiple factors, possibly including oxygen utilization. What’s clear is that we require more interdisciplinary, long-term studies to gain insight into how to adapt to our planet’s increasing temperatures.”
