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Over the past four decades, cod in the North Sea has been a valuable fishery, yielding an average of 200,000 tonnes per year. Nowadays, the catch is mainly constituted of fish that are younger than three years old, most of which are immature. This resource is currently managed by imposing landing quotas based on annual stock assessment: the latest advice from the International Council for the Exploration of the Sea (ICES) is to reduce catches by 40–60%.

For North Sea cod, apart from fish spawned in 1996 (termed the 1996 year-class), the annual number of one-year-old cod in the population (known as the recruitment) has been at or below the long-term average for over a decade1. The recruitment of cod spawned in 1997 was the lowest for 30 years. Taking account of the dependence of recruitment on stock size alone, it has been proposed2 that the collapse of this stock may be imminent, as the spawning-stock biomass is currently at a low level.

There is evidence, however, that for many fish stocks there is an environmentally driven variability in recruitment3. The recruitment of cod in the North Atlantic appears to be related to sea temperature for stocks located at the latitudinal limits of the species' distribution4. In the Northern Hemisphere, increasing temperatures are favourable for stocks at the highest latitudes but detrimental for those at the southern limit5. Cod in the North Sea are near the southern boundary of their range and, historically, strong year-classes have been associated with lower-than-average temperatures during the first half of the year (Fig. 1a).

Figure 1: Cod recruitment and temperature in the North Sea.
figure 1

a, Year-class strength (millions of one-year-old fish) plotted against temperature in the year spawned. The three lines show the relationship between recruitment and temperature at three levels of spawning-stock biomass (SSB) (low: 80,000 tonnes; medium: 160,000 tonnes; high: 240,000 tonnes) based upon a three-dimensional spline fit. Recruitment and SSB are from ref. 1. b, Seasonal sea-surface temperature averaged over February to June (dashed line). Horizontal lines indicate decadal average. Temperatures are derived from the Comprehensive Ocean Atmosphere Dataset and provided by the National Center for Atmospheric Research (Boulder, Colorado).

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Weak year-classes have also occurred during cold years (such as 1986 and 1987), but only when the spawning-stock biomass was low. So when the spawning-stock biomass is low, as now, the likelihood of good recruitment is low and the effect of temperature is less pronounced (Fig. 1a). 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 (Fig. 1b). During this period, annual recruitment levels have been low, with the exception of 1996, when cold conditions prevailed. Since 1997, warmer conditions have returned to the North Sea and the 1997 and 1998 year-classes have coincidentally been the poorest on record1.

The North Sea cod stock is currently dominated by immature fish of less than five years old, and both the stock and fishery are dependent upon years of strong recruitment. The 1996 year-class was the strongest for over a decade but, at the present high exploitation rate of young cod, few individuals have survived to reach sexual maturity.

The combination of this exploitation with the recent changes in North Sea temperature, a low spawning-stock biomass and a stock dominated by young immature individuals, means that fishery managers must take precautionary measures6. In order to give the mature stock a chance to rebuild, fishing mortality rates need to be reduced to at least the precautionary levels advised by ICES1.