As we near the end of the week, many of Brits will be looking forward to ‘fish and chip Friday’.
But it’s bad news for those who’ve got their heart set on a large portion.
While cod used to be giants – over a metre in length and weighing up to 40kg – today a fully-grown cod can fit neatly on a dinner plate.
In fact, the body length of the tasty fish has decreased by 48 per cent – nearly half – since 1996, experts have warned.
The shrinking population, in terms of both number and in size, is the result of human influence, they found.
In their new study, scientists have demonstrated for the first time that decades of intense fishing, combined with environmental change, have profoundly affected the genetic make-up of a fully marine species.
Their analysis involved 152 cod caught in the Bornholm Basin in the Baltic Sea, between 1996 and 2019.
They worked out how the size of cod has changed over the years, and found it had decreased significantly.
There’s nothing quite like a fresh portion of fish and chips after a long day at the beach – but cod portions could be shrinking, experts warn (stock image)
A few decades ago, Baltic cod grew to lengths of over one metre. This photo was taken during a research expedition in 1987 with Finnish fisheries biologist Eero Kalevi Aro
Dr Kwi Young Han holding a cod. Once a giant, today a full-grown specimen can be held in two hands. The ‘shrinking’ of the cod, as well as the decline in its population, is due to decades of intensive fishing combined with environmental changes
Analysis showed that while the largest fish caught in 1996 measured 115cm long, the largest in 2019 measured just 54cm long.
They also found that the length at which 50 per cent of the population reached maturity has declined from 40cm to 20cm.
Cod that grow slowly but reach reproductive maturity at a smaller size have had a survival advantage under high fishing pressure, the researchers explained.
‘When the largest individuals are consistently removed from the population over many years, smaller, faster-maturing fish gain an evolutionary advantage,’ Professor Thorsten Reusch, from the Helmholtz Centre for Ocean Research Kiel, said.
‘What we are observing is evolution in action, driven by human activity. This is scientifically fascinating, but ecologically deeply concerning.’
The researchers also identified genetic changes that confirm the ‘shrinking’ of cod has a genetic basis and that human activities have left a measurable mark on their DNA.
‘Selective overexploitation has altered the genome of Eastern Baltic cod,’ Dr Kwi Young Han, first author of the study, said.
‘We see this in the significant decline in average size, which we could link to reduced growth rates.
Analysis showed that while the largest individual fish caught in 1996 measured 115cm long, the largest in 2019 measured just 54cm long (stock image)
‘For the first time in a fully marine species, we have provided evidence of evolutionary changes in the genomes of a fish population subjected to intense exploitation, which has pushed the population to the brink of collapse.’
The team warned that the evolutionary consequences of this could be severe.
It could mean that the fish are less able to adapt under future environmental changes, they explained.
‘Evolutionary change unfolds over many generations,’ Professor Reusch said.
‘Recovery takes far longer than decline, and it may not even be possible.’
There has been a ban on targeted cod fishing in certain parts of the Baltic Sea since 2019, but ‘there’s no sign of a rebound in body size’, the researchers said.
While the UK gets most of its cod from the Barents Sea and the waters around Iceland, overfishing in these areas could potentially lead to similar effects.
Writing in the journal Science Advances the team said their findings ‘underscore implications for conservation policy’.
Experts have previously urged Brits to ditch white, flaky fish like cod in favour of more local varieties such as herring and mackerel.
Dr Anna Sturrock, from the University of Essex, said we should opt for species more common to our own waters instead of importing the likes of cod and haddock from other countries.
Separately, a report from the WWF has called for ‘urgent’ efforts to strengthen regulation of the seafood sector amid concerns our love for seafood is killing off dozens of species that rely on it for food.
The report, titled ‘Risky Seafood Business’, quantified the total volume of seafood eaten by Britons for the first time.
It claimed that in 2019, 887,000 tonnes of seafood was eaten by people in the UK – the equivalent of 5.2 billion portions of fish and chips.
Whitefish, including fish and chip favourites haddock and cod, accounted for almost a third of the fish consumed (29 per cent).
The vast majority (81 per cent) of this seafood was fished or farmed outside of UK waters, according to WWF.
The likes of whales, dolphins, seabirds and sharks have been directly impacted by fisheries supplying UK markets, the report warned, as one of their main food sources is being depleted.
