The state of North Sea fish stocks
Based on an article first published in Fishing News, 4 December
1998
by Dr Joe Horwood of the Centre for
Environment, Fisheries and Aquaculture Science (Cefas),
Lowestoft
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 bycatch 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.