Publication Abstract

The state of North Sea fish stocks

Dr Joe Horwood of the Centre for Environment, Fisheries and Aquaculture Science (CEFAS), Lowestoft
(Based on an article first published in Fishing News, 4 December 1998)

Fishing News (20 November 1998) reported the advice for next year’s TACs from ICES’ Advisory Committee on Fishery Management (ACFM). For most stocks fisheries ministers will agree the level of 1999 TACs at the fisheries council in Brussels on 17-18 December.

Some TACs will be increased and others decreased. There are two main reasons for the change. First, there is the need to ensure that fishing rates are not so high, and stock sizes are not so low, as to threaten the sustainability of stocks. Concerns over sustainability have been expressed for both North Sea herring and cod in recent years.

Second, the numbers of young fish born each year are very variable. This means that the stock and the catches are also very variable. If fishing effort and practices were left totally unchanged from year to year, then catches of North Sea cod would change on average by 10% each year and catches of North Sea haddock by 30% each year - all due to natural fluctuations in the numbers of young fish.

This article describes the current state of some of the important North Sea stocks, which forms the basis of the advice for next years TACs. The time series of the weight of the mature part of the stock, called the spawning stock biomass (SSB), are shown in Figure 1 for North Sea cod, haddock, plaice, sole and herring.

Figure 1. The weight (in thousands of tonnes) of the mature component of the North Sea stocks of cod, haddock, plaice, sole and herring.

North Sea cod

Figure 1 shows the trends in mature biomass from 1963. The increase in SSB in the late 1960s was due to increased recruitments of young fish over that period, known as the ‘gadoid outburst’. Stock, catches and fishing effort all increased. But with high fishing the SSB then began a steady decline. In the 1990s the stock reached an historically low level and the fishery was severely restricted.

Figure 1 also shows a recent upturn in the fortunes of the cod. This is due to two main features. The 1993 year class was near to the long-term average and better than for many years. Further, and of greater importance in the long-term, it appears that fishing is now only taking about 45% of the fish each year compared with 55-60% a few years ago.

The numerous cod from the 1996 year class, which are now an important part of the catch as two year olds, still have not entered the mature part of the stock. Unfortunately, the 1997 year-class is the lowest on record, less than 20% of the average, and the two year-classes together will result in an about average contribution to the mature stock.

North Sea haddock

Haddock is the most naturally variable of our stocks, and Figure 1 shows this variability reflected in the trends in mature biomass.

Along with cod, good recruitments in the late 1960s boosted the SSB. The 1967 year class was nine times the average abundance.

The haddock experienced good average sized recruitments in 1992 and 1994 which has helped to improve the status of the haddock. As with cod, the percentage killed by fishing each year has also decreased, significantly improving the prospects for the haddock fishery in the long-term.

The immediate future is somewhat less rosy, as the 1995 to 1997 year classes are below average, and the 1998 year class appears to be very low. But further sampling of the 1998 year class will be required to confirm its size.

North Sea plaice

Recent plaice quotas and landings have been very low for the reasons evident in Figure 1. The abundance of plaice has slowly declined as the fishing pressure has increased. Over the past 40 years the percentage of plaice killed each year by fishing has doubled.

The plaice stock was boosted by very good recruitments born in 1963, 1981 and in 1985. But from 1990 the stock declined as the 1985 year class was fished down, and recruitments born in 1992 and 1993 were particularly poor. The recent small upturn is due to better recruitments in more recent years.

The fishing rate on the plaice is still high, but a recruitment of nearly twice the average in 1996 will allow catches to be maintained at the same time as reducing the percentage of plaice killed.

North Sea sole

The North Sea sole is at the northern extremity of its range, restricted by temperature. Like the haddock, its recruitments are particularly variable, and this can be seen in the large ups and downs of the mature biomass with time on Figure 1.

Very big recruitments were born in 1958, in 1963 (as for plaice and following the very cold 1962/63 winter), and 1987. The SSB was boosted two to three years later as the youngsters matured.

The fishing pressure on the sole has increased even more than for the plaice, and these good year classes do not last long. In recent years the stock has been declining rapidly. Nevertheless, immediate catches will be maintained by the above average 1996 recruitment even if fishing is restrained.

North Sea herring

The movements of the mature stock of North Sea herring are shown in Figure 1. Prior to 1950 the herring stock was near to 5 m tonnes. By 1977 it was at 50,000 tonnes. Overfishing had reduced the stock to levels that could not produce sufficient young, and the fishery was closed.

A slow recovery started in 1980. By 1990 the stock was much improved and was over 1 m tonnes. But the fishing rates on both adults, and juveniles caught in the sprat fisheries, were high.

The stock size began to fall rapidly to below 500,000 tonnes. In 1996 the TAC was halved mid-year, and herring by-catch restrictions were put on the industrial fisheries. These very restrictive measures, which are still in place, have helped the stock, and in 1999 we expect that the mature stock will be secure at above 1.5 m tonnes for the first time in over 30 years.