Science

Rising sea temperatures are SHRINKING our favourite fish, including cod, haddock and whiting

Rising sea temperatures are shrinking our favourite commercial fish — including cod and haddock — in the North Sea and West of Scotland, researchers have found.

Experts from Aberdeen analysed 30 years of trawl survey data on cod, haddock, whiting and saith from the International Council for the Exploration of the Sea.

They found that while juvenile fish in the North Sea and the West of Scotland have been getting bigger faster, the size of adults has been decreasing.

Furthermore, these changes in size are correlated with the increases in bottom sea  temperatures in both areas, the analysis concluded. 

According to the researchers, the data predicts a reduction in commercial fishery yields in the short term — with the long-term forecast presently unclear. 

Fisheries will need to factor temperature changes into their forecasts, the team added, as to mitigate the effects of global warming and maximise sustainable yields.

The fishing industry in the UK presently employs some 24,000 thousand people and is worth around £1.4 billion to the economy.

Rising sea temperatures are shrinking our favourite commercial fish — including cod (pictured) and haddock — in the North Sea and West of Scotland, researchers have found

Rising sea temperatures are shrinking our favourite commercial fish — including cod (pictured) and haddock — in the North Sea and West of Scotland, researchers have found

‘Both the changes in juvenile and adult size coincided with increasing sea temperature,’ said paper author and biologist Idongesit Ikpewe, of the University of Aberdeen.

‘Importantly, we observed this pattern in both the North Sea, which has warmed rapidly, and the west of Scotland, which has only experienced moderate warming.’ 

‘These findings suggest that even a moderate rise in sea temperature may have an impact on commercial fish species’ body sizes.’ 

Warming waters limit fish body size because it both contains less oxygen but also increases metabolic rates and, therefore the demand for oxygen.

This means that fish more quickly reach the size at which growing larger would prevent them acquiring enough oxygen to meet their metabolic demands. 

Data used in the study was collected as part of the fisheries-independent Bottom Trawl Surveys, and covered the period from 1970–2017 for the North Sea and 1986–2016 for the West of Scotland.

‘Our findings have crucial and immediate implications for the fisheries sector,’ Mr Ikpewe added.

‘The decrease in adult body size is likely to reduce commercial fisheries’ yields.’ 

‘However, in the long-term, the faster growing and larger juveniles may compensate, to some extent, for the latter yield loss, as although the increase in length (and, therefore, weight) per individual may be small, younger fish are far more numerous.’

‘It is this trade-off that we now need to investigate,’ he continued.

Experts from Aberdeen analysed 30 years of trawl survey data. They found that while juvenile fish (blue) in the North Sea (right column) and on the West of Scotland (left column) have been getting bigger, the size of the corresponding adults has been decreasing. Furthermore, these changes in size are correlated with the increases in sea temperatures in both areas

Experts from Aberdeen analysed 30 years of trawl survey data. They found that while juvenile fish (blue) in the North Sea (right column) and on the West of Scotland (left column) have been getting bigger, the size of the corresponding adults has been decreasing. Furthermore, these changes in size are correlated with the increases in sea temperatures in both areas

Alongside the impacts on the fishing industry, changes in the size of these species will also likely affect marine ecosystems, the researchers warned. 

‘Of the four species we looked at, three — cod, whiting and saithe — are fish eating predators towards the top end of the food chain and therefore have an important ecological role in the ecosystems they inhabit,’ said Mr Ikpewe.

‘Since predator size dictates what prey they can target, a change in the body size of these fish species may have implications or predator-prey relationships.’

This, he added, could have ‘consequences [for] the structure of the food web.’ 

Data used in the study was collected as part of the fisheries-independent Bottom Trawl Surveys, and covered the period from 1970–2017 for the North Sea and 1986–2016 for the West of Scotland. Pictured: Saithe (Pollachius virens), one of the fish examined in the study

Data used in the study was collected as part of the fisheries-independent Bottom Trawl Surveys, and covered the period from 1970–2017 for the North Sea and 1986–2016 for the West of Scotland. Pictured: Saithe (Pollachius virens), one of the fish examined in the study

Although the findings do provide strong empirical evidence for changes in fish size and growth rates in warming seas, the team noted that the study had its limitations.

The team, for example, only considered so-called ‘demersal’ species — that is, those that live close to the seabed — and also did not examine some of the UK’s other commercially important species, such herring and mackerel.

‘The next stage of our work is to consider the management implications based on modelling these populations,’ said Mr Ikpewe.

‘The idea is to work out what the size changes we observed may mean for future fish productivity and yield under different scenarios of warming.’ 

The full findings of the study were published in the Journal of Applied Ecology. 

THE THREATS FACING COD AND HADDOCK 

Overfishing has decimated cod and haddock stocks a number of times. 

Most recently, haddock from three North Sea and west of Scotland fisheries were removed from sustainable seafood lists because stocks had fallen below acceptable levels in 2017.

During the 1990s, Newfoundland in Canada was forced to ban cod fishing because stocks were nearly wiped out.

While rising sea temperatures reduces the size of cod and haddock, it also forces the fish further north in search of cooler waters.

Other fish not traditionally found in UK waters will appear more frequently too — with mixed consequences.

For example, cuttlefish and sardines are being caught in increasing numbers, but researchers have warned slipper limpets could ruin oyster and mussel beds.


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