Hotter water leaves smaller and less mobile fish

The catch with warming oceans is that there’ll be less of a catch. Smaller and less mobile fish will leave less to eat.

LONDON, 18 August, 2021 − Climate change could be about to get the world’s tastiest fish into hot water. The double jeopardy of global heating and overfishing could already be resulting in smaller and less mobile fish, turning sardines and herring, anchovies and pilchards into ever-smaller servings.

It has happened before: a new study of the evolution of fish species over the past 150 million years has found clear evidence of the ups and downs of time: as the ocean temperatures rise, fish tend to get smaller and travel less.

“Warming waters are a double whammy for fish, as they not only cause them to evolve to a smaller size, but also reduce their ability to move to more suitable environments,” said Chris Venditti, of the University of Reading, UK.

“Our research supports the theory that fish will get smaller as oceans warm under climate change, but reveals the worrying news that they will also not be able to evolve to cope as efficiently as first thought.

“This has serious implications for all fish and our food security”

“With sea temperatures rising faster than ever, fish will very quickly get left behind in evolutionary terms and struggle to survive.”

Professor Venditti and colleagues in Chile and Peru report in the journal Nature Climate Change that they applied subtle statistical techniques to evidence of fish evolution amassed in an international database called The Fish Tree of Life to learn about the link between temperature and size in one seafaring order, the Clupeiforms.

This group embraces both Atlantic and Pacific herring, the Japanese and South American pilchard, the anchovy and so on. But what is true for one order is likely to be true for almost all fish: warmer oceans mean more stress.

And stress is on the way. Over the last 150 million years, fish have had to adjust to changing temperatures, but only at rates of around 0.8°C per thousand years. Since 1981, according to the US National Oceanic and Atmospheric Administration, the seas have been warming at 0.18°C per decade.

Evolution at risk

So the finding supports what biologists already know: that animals confronted with higher temperatures tend to evolve to smaller sizes.

It’s not as if fish were not already feeling the heat. As waters warm, their capacity for dissolved oxygen dwindles. Spawning becomes more problematic. Migration becomes more urgent, but for smaller creatures with lower energy reserves also more difficult.

In the world’s traditional fishing grounds, overall catch sizes are shrinking: so too are the sizes of individual fish. And, the latest research suggests, warming waters could limit the capacity to evolve to new species that can adapt to changing conditions.

“This has serious implications for all fish and our food security,” Professor Venditti said, “as many of the species we eat could become increasingly scarce or even non-existent in decades to come.” − Climate News Network