Publication Abstract

So what's happening to North Sea cod?

John Casey and Benjamin Planque of the CEFAS Lowestoft Laboratory
(based on an article first published in Fishing News, 30 June 2000)

Scientists at CEFAS’ Lowestoft Laboratory have been investigating what affects the numbers of young fish in the sea. They have found that the temperature of the North Sea in spring affects the survival of young cod during their first year of life. Their findings were recently published in the scientific journal "Nature" *.

* O’Brien, Fox, Planque & Casey, 2000. Climate change and North Sea cod. Nature, Vol. 404, page 142, 9 March 2000.

Most cod in the North Sea spawn between January and April. The eggs, which are found floating in the water column, take about 2-3 weeks to hatch, depending on the temperature of the water. The newly hatched larvae live near the sea surface for several months (until about July/August), before the small codling descend to the seabed, where they spend the majority of the rest of their lives. It is during the egg and larval stage of life that the death rate of all fish, including North Sea cod, is highest. During these stages of development, the eggs and larvae are vulnerable to physical processes such as temperature, and also to predation.

The work undertaken by CEFAS scientists has shown that there is a relationship between the average sea-surface temperature during the spring, when cod spawn, and the number of cod that survive their first year of life to age 1. The number of 1-year-old cod is known as the recruitment. It appears that historically, good recruitment is associated with colder than average spring temperatures, and that if spring temperatures are warmer than average, it is unlikely that recruitment will be good.

This relationship is strongest when the spawning stock biomass (the weight of mature cod in the sea) is high (Figure 1a). It is weakest when the spawning stock biomass is low (Figure 1b). This means when the spawning stock biomass is low, as it is at present, there is less of a chance of good recruitment, even if spring temperatures are low.

Since 1988, the average spring temperature in the North Sea has been warmer than the previous 4 decades (Figure 2), and except for 1997, recruitment has been at or below-average over this period. The 1-year-old fish in 1997 were spawned in the spring of 1996, which was the only year in the last decade that the spring temperatures were colder than average. The subsequent recruitment of 1-year-olds in 1997 was the best for over a decade. This in turn gave rise to a good fishery for cod in 1998, when those fish were 2-years old. However, the numbers of 1-year-old cod recruiting to the stock in 1998 and 1999 have been the poorest on record, which partly accounts for the poor fishery for cod this year. The recruitment in 2000 is as yet unknown.

At present, the spawning stock biomass of cod is low and is below the level that is considered safe. Both the stock and the fishery are dependent on strong recruitment. In addition, the stock is dominated by young immature individuals less than 4 years of age and on average, the commercial catches of cod are mainly made up of 2- and 3-year-old fish. The present fishing mortality rate is high, with typically about 45% of the stock being killed by fishing each year. This means that only a small proportion of the stock survives to reach sexual maturity and contribute to the spawning stock.

We therefore have a stock dominated by young immature cod, and a high fishing mortality rate on these young cod. The spawning stock biomass is low and at an unsafe level, and the recent spring temperatures have recently been warmer than average. If the spawning stock remains low, there is little chance of good recruitment no-matter what happens to the spring temperatures in the North Sea. There is therefore a need to allow more cod to survive to maturity so they can boost the spawning stock, which would increase the chance of good recruitment, particularly if spring temperatures return to below-average levels.

Unfortunately, we are unable to predict the future spring temperatures in the North Sea. Historically, we have observed warm and cold periods in the North Sea on a cyclical basis. However, if colder conditions do not return, because of global warming for example, the chances of good recruitment are likely to remain low. We have no direct control over the North Sea temperatures and the only means we have to allow more cod to survive to maturity is to reduce the catches of immature fish. This means that there is an even greater need to restrict catches of North Sea cod in order to boost the spawning stock.

The arguments above apply equally to Irish Sea cod. The Irish Sea cod stock is also very low and juvenile and adult numbers need to be increased significantly in order to ensure adequate and sustainable recruitments. The only way to do this is restrict fishing until the stock recovers.