Fish are growing smaller as sea temperatures rise, with adults now reaching a smaller maximum size than in 1970.

Idongesit Ikpewe at the University of Aberdeen in the UK and his colleagues have found that warmer seas are linked to changes in fish size. Their analysis looked at trends in four commercially fished species – of cod, haddock, whiting and saithe – in the North Sea and in waters west of Scotland.

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

The four species are demersal fish, meaning that they spend most of their time near the sea floor, so the team compared the length-at-age data with annual water temperatures at the seabed in the two areas over the four decades, and found that adult length was inversely correlated with temperature.

Only adult cod off the west of Scotland didn’t reduce in size – all other species in both areas showed length-at-age declines.

The team also found that the mean length‐at‐age of juvenile fish – those aged younger than four years – had increased and was correlated with rising temperature.

Previous laboratory studies have found that ectotherms – animals, including fish, that rely on heat from their environments – develop faster at warmer temperatures and reach smaller maximum body sizes, a phenomenon known as the temperature-size rule, so the findings are in line with expectations.

“When the temperature increases, the metabolic rates of the species seem to increase,” says Ikpewe. The increase in metabolic rate means younger fish grow faster and reach maturity 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,” says Ikpewe.

The decrease in adult body size is likely to reduce yields for commercial fisheries and may have ecological effects on predator-prey interactions, he says.

Next, the researchers plan to investigate whether the faster-growing and larger juveniles may compensate for this, and what effect different scenarios of warming may have in future.

Journal reference: Journal of Applied Ecology, DOI: 10.1111/1365-2664.13807