Warm water threat to North Sea cod

A large cod landed at Lowestoft in 1991 (Photo: Greenpeace)

By environment correspondent Alex Kirby

Fisheries scientists say the health of one of the UK's main marine resources, the North Sea cod, is at risk because of rising water temperatures.

For generations of Britons, cod has been an essential part of the country's culinary highlight of fish and chips. But they have faced increasing pressure in recent years from overfishing.

Now scientists from the Centre for Environment, Fisheries and Aquaculture Science (CEFAS), at Lowestoft in eastern England, say climate variability is piling further stress on the cod stocks.

"Cut the catch"

Writing in the journal Nature, the team says it has identified "a decline in the production of young cod that has paralleled warming of the North Sea over the past 10 years".

One of the authors, Dr John Casey, told BBC News Online: "We are not saying that this warming is likely to be a permanent feature.

Old favourite: Fish and chips

"Over the past 50 years, there have been both cold and warm periods. The warm spells average about 10 or 15 years."

In order to let the cod stocks recover, the CEFAS team says, fishing mortality rates need to be reduced at least to the levels advised by the International Council for the Exploration of the Sea, which has recommended a cut of 40-60% in catches.

Caught as juveniles

In the last 40 years, the North Sea cod fishery has produced an average of 200,000 tonnes annually, though the latest catches are far below that - 100,000 tonnes in 1997, and 114,000 in 1998.

But most of the fish caught these days are less than three years old, and have not reached sexual maturity.

The cod had a good spawning year in 1996. But that year apart, the annual number of one-year-old fish in the population (called "the recruitment") has been at or below the long-term average for more than a decade.

The recruitment of cod spawned in 1997 was the lowest for 30 years.

The Ministry of Agriculture, Fisheries and Food says: "Following a long-term decline to its lowest recorded level in 1994, reduced fishing mortality has allowed the North Sea cod stock to increase.

"Nevertheless, its spawning-stock biomass in 1997 was still close to the level of 150,000 tonnes which is considered to be the safe biological level below which recruitment is reduced."

That level was established by the International Council for the Exploration of the Sea (ICES).

The CEFAS scientists say they have found evidence "that for many fish stocks there is an environmentally-driven variability in recruitment", though they are not suggesting any link with climate change.

Cold preferred

In the Northern Hemisphere, increasing temperatures favour stocks at the highest latitudes, but harm those at the southern limit.

"Cod in the North Sea are near the southern boundary of their range and, historically, strong year-classes [the fish spawned in a given year] have been associated with lower-than-average temperatures during the first half of the year.

Scientists say catches must fall

"A change in temperature patterns might prevent the stock from being able to produce recruitments as high as those that occurred during the 1960s and 1970s, even if the spawning-stock biomass were to rebuild to the abundant levels of that period.

"Since 1988, mean sea temperatures during the first half of the year in the North Sea have been higher than during the previous three decades.

"During this period, annual recruitment levels have been low, with the exception of 1996, when cold conditions prevailed.

"Since 1997, warmer conditions have returned, and the 1997 and 1998 year-classes have coincidentally been the poorest on record."