Warmer oceans mean less oxygen, and that’s why fish are shrinking

Scientists already have a fairly solid explanation for why marine animals shrink in warming oceans today. Warmer water simply holds less dissolved oxygen, and at the same time, a warmer body burns through oxygen faster just to keep functioning.

For a cold-blooded animal that keeps growing throughout its life, this creates a genuine bottleneck, since its gills eventually cannot pull in enough oxygen to support a larger body. Staying smaller becomes a simple way to ease that pressure, which is why many marine species stop growing sooner and settle at a smaller adult size once waters heat up.

A massive new study, drawing on almost 9,000 recorded changes and over 1.6 million measurements from fossils, historical records and living animals, found this pattern has repeated for nearly 450 million years. Sea creatures shrank far more sharply during periods of intense global warming compared to crises caused by cooling or falling oxygen levels alone.

Wolfgang Kießling, who leads a paleoenvironmental research group at Friedrich Alexander Universität Erlangen Nürnberg, believes ancient warming events like these can act almost like a preview of what warmer oceans may bring in the coming decades. One existing study of thousands of reef fish surveys around Australia found that average fish length dropped by roughly 5% for every 2-degree Fahrenheit rise in ocean temperature.

Image: Wikimedia Commons/by Xplore Dive