Why are fish getting smaller due to climate change?

The experiment used American brook trout (University of Massachusetts Amherst)
The experiment used American brook trout. (University of Massachusetts Amherst)

Fish around the world are shrinking due to climate change, and no-one is entirely sure why.

A new study by scientists at the University of Massachusetts Amherst found that there is no physiological evidence supporting a leading theory (which involves the surface area of fish gills) as to why many fish species are "shrinking" as waters grow warmer due to climate change.

The researchers did long-term experiments on brook trout, and found that rising temperatures are leading the animals to be smaller, but that the gill surface area theory does not explain why.

"We know that global climate change is happening and our oceans and rivers are getting warmer," said Joshua Lonthair, lecturer in biology at UMass Amherst and the paper's lead author. "We know that many animals, not just fish, are growing to smaller adult body sizes under warmer temperatures.

"We even have a name for this, the temperature size rule. But despite decades of research, we still don't understand why size decreases as temperature increases."

How much smaller could fish get?

King Salmon fishing at a pristine green river in the wilderness of Canada
Yukon salmon are already 10% smaller, according to researchers. (Getty)

In other studies, it’s been suggested that as climate change takes hold, the size of fish could shrink by 20-30%, according to researchers from the University of British Columbia.

A 2020 study in Alaska found that Yukon region chinooks are 10% smaller than they were in 1990.

What did the new study find?

The new study ruled out the leading theory for why fish are getting smaller – that fish growth is limited by how much oxygen gills can pull from the water.

Known as the gill oxygen limitation (GOL) theory, it has been proposed as the universal mechanism explaining fish size and has been used in some predictions of future global fisheries yields.

As the water warms, the biochemical processes of the fish speed up and require more oxygen. The GOL theory contends that gills have limited surface area that constrains the amount of oxygen they can supply, and thus, fish cannot grow as large under warm water conditions. Therefore, fish are "shrinking" to fit the limited oxygen that their gills can supply.

The GOL theory underlies widely cited model projections of drastic reductions in future global fisheries yields.

This includes projections used by the International Union for Conservation of Nature, but it has never been directly tested.

Lead author Lisa Komoroske, assistant professor in environmental conservation at UMass Amherst, said: "We designed a series of long-term experiments which collectively are the first effort to empirically test GOL."

What did the experiment show?

Once they had their test subjects – brook trout small fry that initially weighed between one and two grams each – they were placed into tanks, some of which contained normal, 15C water, and some of which contained water warmed to 20C.

The fish were weighed and measured at the start of the experiment, and then monthly thereafter. Their oxygen consumption was also measured at two weeks, three months and six months as a way of ascertaining metabolic rate.

Finally, the researchers collected gill samples from the same fish to measure changes in their gill surface area. The brook trout in the warmer tanks were smaller, as expected, and in line with the temperature size rule.

However, the gill surface area was more than enough to meet the fishes' energetic demands, which means that their growth was not limited by the surface area of the gills, as GOL predicts.

Furthermore, the team found that while the warm-tank fishes' metabolic rates did increase at the three-month mark, by six months their oxygen rate returned to normal, suggesting that the fish could adjust their physiology over time to account for the increased water temperatures.

What does this mean?

The researchers say that more interdisciplinary research is required to understand why fish are getting smaller.

The researchers say that understanding this will be key to predicting the impact of climate change on fish in the future.

Komoroske said: "Our work highlights the importance of interdisciplinarity. Fisheries and macroecology scientists tend to work on the population and species level, while the physiologists tend to work on the individual and cellular levels.

"But these are academic distinctions, not natural ones, and if we're going to help fish survive warming waters, we need to work across biological scales and join the insights of all these fields."

Lonthair said that the scientists ‘don’t know’ what is causing fish to get smaller, adding: “It may not be a single mechanism—it may be a host of factors, including oxygen use. We need more interdisciplinary long-term studies so that we can understand how best to adjust to our warming world."

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