Warmer seas 'making fish smaller'
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.
Scientists found that the maximum length of haddock, whiting, herring, Norway pout, plaice and sole decreased by as much as 29% over a 38-year period when temperatures in the North Sea increased by between 1C (1.8F) and 2C (3.6F).
The availability of food and an increase in fishing could also be factors in the reduction in length but the "synchronised" fall in size across a range of species led the fisheries scientists at the University of Aberdeen to identify climate change, and particularly higher water temperatures, as a common theme.
Between 1970 and 2008 the length of haddock in the north part of the sea and whiting in the south decreased most at 29%.
The reduction did not apply to all species, with the length of cod remaining steady and sole decreasing by only 1% over the study period.
Fish are maturing earlier in the warmer waters and not continuing to grow, according to the researchers.
The project was funded by Marine Scotland Science and analysed more than three decades of annual data from the International Council for the Exploration of the Sea on the age and length of commercial fish.
Alan Baudron, research fellow at the University of Aberdeen's school of biological sciences, led the study.
He said: "Our analysis showed that the majority of species examined - specifically haddock, whiting, herring, Norway pout, plaice and sole - experienced a synchronous reduction in their maximum length over the time period.
"What is interesting is that this was detected across a range of fish species eating different diets, living at different depths and experiencing different levels of fishing mortality.
"The synchronicity suggests that the one common factor they all experienced - increasing water temperatures - could have been at least partly responsible for the observed reductions in length.
"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. 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."
The research has been published in the Global Change Biology journal and the Aberdeen-based team hope it will promote further research into the impacts of climate change on fisheries productivity.
Dr Baudron added: "The increase in temperature of the North Sea is actually quite subtle - approximately 2C - yet this appears to be having a detectable impact on growth rates of fish.
"It's known that sea temperatures are increasing at different rates globally.
"We would anticipate that synchronous reductions in length across species could be occurring in other regional seas experiencing a strong degree of warming."