Publication Abstract

Saving North Sea herring

John Nichols of CEFAS Lowestoft describes the nature and history of the North Sea herring stock, examines recent trends and developments and sets out future prospects for this fishery.
(Based on an article first published in Fishing News, 12 February 1999 - with updated data for 1998)

We are not dealing with a single stock in the North Sea but a complex of three separate stocks, each with separate spawning grounds, migration routes and nursery areas (Figure 1).


Figure 1. The spawning areas and periods for the three stocks of North Sea autumn spawning herring, showing the direction of larval drift to the nursery grounds. Former spawning grounds around the Dogger Bank are also shown.

The North Sea autumn spawning herring stock is distinct from the inshore stocks such as the Thames, and Wash herring, which spawn in the spring.

Herring spawning areas are limited by the need for a gravel substrate to which the eggs are attached. The larvae hatch on the seabed after two to three weeks and then drift passively as plankton for the following four to six months. The abundance of juveniles in the different nursery areas is dictated by annual variations in the strength and direction of the drift of the larvae and their variable mortality on route.

In spite of the separate spawning areas, the adults and juveniles mix and are caught together in the central North Sea fisheries. Therefore, North Sea autumn spawning herring have to be managed as a single unit because the catches cannot be apportioned to the separate stocks. Individuals from each stock cannot be identified either genetically or by visual characters, such as measurements or vertebral counts.

Stock assessment and management is further complicated by the fact that five separate types of fisheries exploit North Sea autumn spawning herring. Only two of these fisheries are in the North Sea - the others are in the nursery areas in Division IIIa (Skagerrak and Kattegat).

 

The five fisheries are:

• Directed fisheries with purse seiners and trawlers in the North Sea.
• All other vessels where herring is taken as a by-catch in the North Sea (including the small meshed fishery for sprat).
• Directed fisheries with purse seiners and trawlers in Division IIIa.
• Vessels fishing for herring and sprat in Division IIIa.
• Other vessels in fisheries where herring is taken as a bycatch in Division IIIa.

Only two of the fisheries exploit mainly adult fish for human consumption - the others exploit the juvenile part of the stock as by-catch in small meshed industrial fisheries.

 

Annual landings from 1947 through to the early 1960s were high but stable, averaging around 650,000 tonnes (Figure 2). Over the same timeframe, the high fishing mortality resulted in a rapid decline in the spawning stock biomass (SSB) from around 5m tonnes to 1.5m tonnes (Figure 3). Recruitment over this period was reasonable (Figure 4), but there were fewer and fewer year classes present in the adult stock - a clear indication that the stocks were being overfished.

Figure 2. Annual landings of North Sea autumn spawning herring from 1947 to 1998 with the trend in fishing mortality shown as a percentage of the stock caught each year.

Figure 3. Annual trends in the size of the spawning stock from 1947 to 1997 with the predicted level shown for 1998.

Figure 4. The number of herring less than one year old coming into the stock each year, showing the high variability over the period 1946 to 1998.

This period witnessed the complete collapse of the old East Anglian autumn drift net fishery, which was based entirely on the Downs stock moving south to the Southern Bight and eastern English Channel to spawn.

The reasons for that failure have been attributed both to high mortality of the juveniles in the North Sea industrial fisheries, and to heavy fishing by bottom trawlers on the spawning concentrations in the English Channel during the 1950s. Bottom trawling on the spawning grounds not only disturbs spawning fish but also destroys the spawn and damages the substrate on which successful spawning depends.

Fishing mortality on the herring in the central and northern North Sea began to increase rapidly in the late 1960s. Landings peaked at over one million tonnes in 1965 (Figure 2), around 80% of which were juvenile fish. This was followed by a very rapid decline in the SSB and the total landings.

By 1975 the SSB had fallen to 83,500 tonnes, although landings were still over 300,000 tonnes. At the same time, spawning in the central North Sea contracted to the east coast of England, while spawning grounds around the edge of the Dogger Bank were no longer used. This serious collapse of the North Sea autumn spawning herring stock led to a moratorium on directed herring fishing in the North Sea from 1977 to 1980. Until that time there had been no control, other than market forces, on the catches of North Sea herring.

International larvae surveys and acoustic surveys were used to monitor the state of the stocks during the moratorium. By 1980, these surveys were indicating a modest recovery in the SSB from its 1977 low point of 52,000 tonnes. By 1981, the SSB had increased to over 200,000 tonnes and the fishery was reopened. Since that time, the directed fisheries for North Sea autumn spawning herring have been managed by a total allowable catch (TAC).

Recent developments
 

Until 1996, the only control on the by-catch of herring in industrial, small meshed fisheries was a sprat TAC, with a 10% bycatch limit of herring applying to individual trips. Over the period 1987 to 1995 there was a very high mortality on immature herring, mainly in the small meshed fisheries. That mortality averaged 76% of the total catch in numbers of North Sea autumn spawning fish.

Early in 1995, ICES scientists found that North Sea herring catches in 1994 had continued to exceed the TAC and that the SSB had fallen below the minimum acceptable level of 800,000 tonnes.

In October 1995, acting on this information, ICES Advisory Committee for Fisheries Management (ACFM) recommended a severe reduction in the TAC for 1996, equivalent to about half the 1995 level of 440,000 tonnes. It also recommended that fishing mortality on herring be reduced by at least 50% in the small meshed fishery for sprat in the North Sea, and in other fisheries which take North Sea herring in Division IIIa.

In 1996 a herring by-catch ceiling of 44,000 tonnes was set for the industrial fisheries in the North Sea

The TAC eventually agreed between the EU and Norway for 1996 was 313,000 tonnes. However, a new assessment early in 1996 showed that the SSB had further declined to less than 500,000 tonnes. Concern about this decline, and the continuing high fishing mortality rate on juveniles, led to ACFM recommending in May 1996 that urgent action be taken to avoid a repeat of the stock collapse of the 1970s.

It recommended that the total North Sea herring catch should not exceed 298,000 tonnes in 1996 and that the catches by all fleets exploiting this stock should count against this figure. This advice implied an immediate 50% reduction in the 1996 TAC for the North Sea directed fishery to 156,000 tonnes, and a herring by-catch ceiling for the small meshed fisheries in the North Sea of 44,000 tonnes.

Unless drastic steps were taken, fishermen faced the prospect of the North Sea herring fishery being closed in 1997. The emergency measures were therefore quickly approved by the EU and Norway and came into force midway through the 1996 season. This rapid action was in stark contrast with the years that it took to protect the herring in the 1970s.

In spite of these measures, the total catch in the North Sea directed fishery in 1996 again exceeded the TAC by 70,000 tonnes. It was known that some of this excess herring had been taken in the North Sea, but misreported into other areas.

Such misreporting has traditionally been attributable to a number of countries and fishing areas, with the North Sea herring variously reported as west of Scotland herring, Atlanto-Scandian herring and herring from Division IIIa.

 
Emergency measures in 1996 were quickly approved by the EU and Norway and came into force midway through the 1996 season. A purse seiner takes a catch of herring on board.

The UK has introduced progressively more rigorous licensing arrangements to curb the UK share of misreporting. From 1996 onwards, a single licence policy has been applied to the UK fishery for Atlanto-Scandian herring. Those wishing to pursue this fishery must do so north of latitude 62°N, and must further surrender their licences and entitlements to fish on other stocks (such as North Sea herring) south of 62°N.

For 1997 and 1998, similar licensing restrictions have been applied to the UK fishery for North Sea and west of Scotland herring. Skippers are required to choose between one area and the other, but may hold concurrent licences to fish in both areas.

The ACFM advice in 1998 notes that these measures appear to have led to a significant reduction in misreporting.

In line with ACFM advice, the EU and Norway agreed to maintain the emergency arrangements introduced midway though 1996 in 1997,1998 and 1999. This has resulted in a progressive and gradual increase in the TAC for the directed fishery, which has now been raised to 265,000 tonnes in 1999.

Although the SSB was only just above the minimum acceptable level of 800,000 tonnes in 1998, there is clear evidence that the current management measures are having a dramatic impact in rebuilding the stock, so that it is expected to increase to above 1.3m tonnes at spawning time in 1999.

Future prospects

The EU and Norway have agreed a new management system for North Sea herring which will come into operation when the stock has recovered sufficiently to allow the current emergency measures to end, probably from 1 January 2000. The key elements of this new regime, which is based on the precautionary approach, are:

• every effort will be made to keep the SSB above the minimum acceptable level of 800,000 tonnes;
• a medium term strategy under which annual TACs should be set for the directed fishery and by-catches based on fixed fishing mortality rates for adult and juvenile fish;
• should the SSB fall below the precautionary reference point of 1.3m tonnes, a recovery plan will be adopted to ensure the rapid rebuilding of the stock to above this level.

It is fair to conclude that the stringent management measures introduced mid-way through 1996 were essential and are proving increasingly effective as the continuing growth in SSB shows. There is also an above average year class from 1995, over half of which should have spawned for the first time in 1998 and an excellent year class from the 1998 spawning. The immediate prospects for this stock, and future TAC levels, are therefore encouraging.

In addition to the TAC, the other regulations in force to protect herring will continue. These include a ban on targeted fishing for herring at certain times of the year along parts of the UK east coast where herring spawn.

There are also bans on small meshed fisheries targeted at sprat off the NE coast of England and off the Danish west coast, where juvenile herring are abundant.

In spite of the improved picture for North Sea herring generally, there has still been no recovery of spawning on the former grounds around the Dogger Bank. We also need to be vigilant over the state of the southern North Sea (Downs) stock.

The fishery on the Downs stock is concentrated on spawning aggregations and they are particularly vulnerable to excessive fishing pressure. Although TACs for this component – which are set separately – have been reduced by 50% since 1996, they have been significantly exceeded each year since then.

Larvae surveys, which have been considerably reduced in recent years, are the only indication of the state of this component. In 1995 they indicated a very low level of SSB, comparable to that in 1980 when the fishery was closed. More recent surveys indicate a small improvement in the state of this stock.


English and Scottish steam and sailing drifters in Lowestoft at the height of the old East Anglian autumn drift net fishery. The 1950s and 1960s saw the complete collapse of the fishery - 1963, the poorest season for 100 years was the last to see a major Scottish presence.

A trawl catch of herrring being brought aboard off the Yorkshire coast.