RESEARCH: Fish are getting smaller as sea temperatures rise

Scientists think that adult fish are getting smaller as sea temperatures rise. 

Research scientists have been studying the size of fish in the ocean over the past 50 years, since 1970, and they think that they are shrinking in size due to warmer oceans.

At the University of Aberdeen in the United Kingdom, Idongesit Ikpewe and his colleagues have found that warmer seas are linked to changes in fish size. They looked at trends in four commercially fished species: cod, haddock, whiting, and saithe. They researched these fish in two locations: (1) the North Sea and (2) in the waters of Scotland. 

The researchers examined existing data between 1970 and 2017, specifically at the average length-at-age, which is a measure of the mean length of a species for each year between the age of one and seven.

The four species are demersal fish. Demersal fish spend most of their time near the sea floor. 

The scientists compared the length-at-age information with the annual water temperatures at the seabed in the two locations over almost 50 years. They found that the adult length of fiish was inversely correlated with temperature. This means that as the sea temperatures rise, the size of fish shrinks. 

Only adult cod off the west coast of Scotland did not shrink in size. The other three species in both locations showed that the length-at-age was decreasing. 

The research team also found that the length-at-age of juvenile fish had increased as sea temperatures rise. Juvenile fish are fish younger than four years of age.

Why is this happening?

Laboratory studies in the past have found that ectotherms develop faster at warmer temperatures and reach smaller maximum body sizes. Ectotherms are animals, including fish, that rely on heat from their environments to keep their body temperatures regulated. The phenomenon of ectotherms growing faster at warmer temperatures is known as the temperature-size rule. So, the findings of the researchers at the University of Aberdeen are the same as the temperature-size rule – their findings are as expected. 

Idongesit Ikpewe said, “When the temperature increases, the metabolic rate of the species seem to increase.” The increase in metabolic rate means that juvenile fish grow faster and reach adulthood earlier. They mature quicker and earlier. Most of their energy is then channelled from growth into reproduction. 

“We would expect to see continued changes in fish size and growth changes with further rises in sea temperature,” he said.

The decrease in adult fish body size is likely to reduce the size of the catch (the yields) for commercial fisheries and may have ecologicial effects on predator-prey interactions, said the researchers.

In further studies, the researchers plan to investigate whether the faster-growing and larger juvenile fish may compensate for rising sea temperatures, and what effect different temperatures have in the future for fish sizes. 

Readers can see the information in the New Scientist magazine. The study was published in the Journal of Applied Ecology. DOI: 10.1111/1365-2664.13807

Photographer: Martina Nicolls

Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM

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