Which types of fish suffer from oxygen starvation?

Declining oxygen levels – due to rising water temperatures caused by climate change and pollution – represent significant problems for fish and other aquatic organisms. A new study led by Radboud University has found that larger fishes are more likely to experience oxygen deficiency in warming water than smaller fishes. Moreover, marine fishes appear to be less tolerant of oxygen-depleted water than freshwater fishes. These findings will help scientists predict which aquatic species are at risk due to habitat changes caused by human activities and global warming.

Biologists have long debated the role of oxygen in the sensitivity of fish to warming water. “Many oxygen hypotheses are being fiercely debated. The problem is that the various effects are lumped together. For example, some studies look at how fish respond to oxygen levels in the water but do not account for the temperature of the water or the size of the fish. As a result, the reported patterns are variable,” said study lead author Wilco Verberk, an expert in Aquatic Ecology at Radboud University.

By systematically separating the various effects and compiling data on tolerance to oxygen deficiency from 195 fishes, Professor Verberk and his colleagues discovered that larger fishes are more sensitive to oxygen stress, but only in warmer water. Surprisingly, when the water is cold, the effect seems to be reversed. Moreover, a similar effect was observed in fishes with relatively large cells.

“Many people think that all animal species have the same cell size, but some animals have large cells, and some have small cells, even within the same species. There are many advantages to having small cells, especially in warm water. For example, small cells have relatively more membrane area, which is needed to absorb oxygen from their surrounding environment,” Professor Verberk explained.

“The explanation probably involves different selection pressures on freshwater fish during their evolutionary history,” Verberk said. “In the ocean, the temperature is relatively stable, but in fresh water the fish are more often confronted with higher temperatures. Fluctuations in oxygen levels are also larger in rivers and especially in lakes, for example, due to the presence of algae.”

Further research is needed to clarify whether these findings apply to different fish species in order to reliably map the effect of climate change on marine and freshwater ecosystems.

The study is published in the journal Global Change Biology.   

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By Andrei Ionescu, Earth.com Staff Writer