A DECLINE in the size of some species of fish in the North Sea could be due to a rise in water temperatures, according to research.

The maximum body length of haddock, whiting, herring, Norway pout, plaice and sole in the North Sea has decreased by as much as 29% over a 38-year period.

It coincides with an increase in water temperatures of between 1ÚC to 2ÚC, Aberdeen University scientists said.

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Food availability and fishing pressure may also have been factors, but the scientists believe it is less likely they could explain the change in length observed across species. Cod appears to have been immune to the change.

They say the six species studied differ in their biology and their simultaneous decrease in maximum body length suggests the common experience of increasing temperatures is contributing to "a detectable synchronous change" in individual growth rates across species.

Dr Alan Baudron, research fellow at the university's School of Biological Sciences, led the study, which was funded by Marine Scotland Science (MSS). It used data collated by the International Council for the Exploration of the Sea reporting the age and length of commercial fish in the North Sea for more than 40 years.

He said their analysis showed the majority of species examined experienced a reduction in their maximum length at the same time over the time period.

He said: "What is interesting is this was detected across a range of fish species eating different diets, living at different depths and experiencing different levels of fishing mortality."

He said the timing of the ­reduction in maximum length coincided with years when water temperature in the North Sea increased.

"Our findings are consistent with current understanding of the physiology of fish," he said. "Because fish are cold-blooded animals their metabolic rates are determined by the ambient temperature.

"In general, fish grow more rapidly during their early life when temperatures are warmer. The consequence of rapid juvenile growth is that they become mature at a smaller length and therefore don't grow as large as they would have in colder waters."

It is hoped the findings, published in Global Change Biology, will promote further research to investigate the impacts of climate change on fisheries productivity.

Dr Coby Needle, of Marine Scotland Science (MSS), Aberdeen, who collaborated in the reserach, added: "This study contributes important information to help us address the issue of how fish growth is changing through time and highlights the importance of considering environmental issues when managing fisheries."

Dr Baudron added: "The increase in temperature of the North Sea is actually quite subtle - approximately 2ÚC - yet this appears to be having a detectable impact on growth rates of fish.

"A comprehensive global ­evaluation is required in order to fully assess the impact of rising water temperatures on fish stocks."