Herring Stocks In The North Sea

Introduction

Spawning grounds

During the 1970s there was a massive decrease in the spawning stock biomass of herring, largely caused by over exploitation, followed by periods of poor recruitment. In 1977, the fishery was closed to safeguard the future of the stock. In response to management measures and a succession of strong year classes, the stock eventually recovered and the North Sea fishery re-opened in 1983.

By 1996, exploitation had increased to such a level that stricter measures were introduced to reduce fishing mortality. Each year, the number of adult herring in the sea depends on the numbers of young fish which survive from spawning to enter the fishery as recruits. Variation from year to year in the numbers of herring recruits, combined with high fishing mortality, means that management control is essential for the survival of this stock.

Biology

2008 distribution of landings By Scottish vessels (Tonnes)

During daytime, herring shoals remain close to the sea bottom or in deep water. At dusk they move towards the surface and disperse over a wide area. Although most fish species have a single spawning season, herring is one of the exceptions. The sub-populations of North Sea herring spawn at different times and localised groups of herring can be found spawning in almost any month. The North Sea stock is dominated by autumn spawners.

Currently, three major populations can be identified by differences in spawning time and area. These are: Buchan / Shetland herring, spawning off the Scottish and Shetland coasts during August and September; Banks or dogger herring, spawning in the Central North Sea and off the english coast from August until october; and the Southern Bight or downs herring, spawning in the english Channel from November until January. For most of the year the different populations mix, but during the spawning season they migrate to their separate areas.

Some herring mature and spawn at two years of age, but most are three or four before they spawn. The number of eggs produced by an average-sized female varies between populations. A 28cm female from the downs stock produces around 42,000 eggs per year whereas a similarly sized female from the Buchan stock can produce 67,000 eggs. herring are demersal spawners, depositing their sticky eggs on coarse sand, gravel, small stones and rock. Shoals of herring gather on the spawning grounds and spawn more or less simultaneously. Females release eggs in a single batch and the resulting egg carpet may be several layers thick and cover a considerable area. The eggs take about three weeks to hatch depending on the sea temperature.

Many autumn hatched larvae spend their first winter drifting towards nursery areas on the eastern side of the North Sea, around the Moray firth, the firth of forth and in the western North Sea. Their drift rate is variable and in some years many do not reach the nursery areas. As they grow, the juvenile herring move offshore, eventually joining the adult populations in the feeding and spawning migrations in the western areas of the North Sea. Some of the herring in the North Sea nurseries originate from the spawning grounds on the north and west coasts of Scotland.

ICES Advice on Management

Summary

• Fishing mortality for adult herring is just below F_pa
• Spawning stock biomass is expected to remain below B_pa
• EU and Norway management plan adjusted to account for recent poor recruitment

Precautionary approach reference points

For North Sea herring, ICES has proposed that the precautionary level for stock biomass (Bpa) be set at 1.3 million tonnes, and that the precautionary level for fishing mortality (Fpa) be set at 0.12 for juvenile fish and 0.25 for adults. The limit reference point for spawning stock biomass (B_lim) is 800,000 tonnes, and the limit reference point for fishing mortality (F_lim) has not been defined.

State of stock

Based on the most recent estimates of spawning stock biomass and fishing mortality, ICES classifies the stock as being at risk of having reduced reproductive capacity and harvested sustainably. The SSB in autumn 2008 was estimated at 1.0 million t, and is expected to remain below B_pa (1.3 million t) in 2009. Fishing mortality of adult fish (F_2-6) in 2008 was estimated at 0.24, above the management target F_2-6 (for this state of the stock = 0.14). The year classes since 2002 are estimated to be among the weakest since the late 1970s.

Management advice

In November 2008 the EU and Norway agreed on an adjusted management plan taking in to account the recent poor recruitment. ICES evaluated this plan and concluded that it is consistent with the precautionary approach.

The main elements of the plan are to maintain SSB above 800,000 tonnes (B_lim); to limit TAC by ± 15 per cent from the TAC of the preceding year; a fishing mortality of 0.25 for adult fish and 0.05 for juvenile fish if the SSB is above 1.5 million tonnes (the trigger biomass B_trig); if SSB is between Btrig and B_lim then the fishing mortality will be set between 0.25 and 0.10 for adult fish depending on SSB, and no more than 0.05 for juvenile fish; if SSB falls below B_lim then a fishing mortality of no more than 0.10 for adult fish and 0.04 for juvenile fish will be set.

A reduction in fishing mortality close to the target fishing mortality is expected to be achieved in 2009. The SSB is expected to increase slightly both in 2010 and further in 2011, indicating that the current management has the potential to reverse the decline in the stock and stabilise it above the present level. The 2008 year class is estimated to be within the range of recent low recruitments. ICES assumes that the recruitment will remain around the current low level. Delay in implementing substantial reductions in catch by not following the management plan has resulted in the SSB being at greater risk of being below B_lim and lower catches.

Seven catch forecasts were presented to show a range of options for differing overall exploitation rates, based on an interpretation of the harvest control rule or other options:

1. No fishing
2. Catches that are estimated to lead to SSB> Bpa in 2011
3. A 15 per cent decrease in main North Sea fleet TAC between 2009 and 2010
4. The EU-Norway management plan with 40 per cent catch increase for herring fleets in the Skagerrak and kattegat
5. The EU-Norway management plan
6. A roll-over TAC from 2009-2010 of 171,000 tonnes for main North Sea fleet
7. A 15 per cent increase in main North Sea fleet TAC between 2009 and 2010.

ICES advises that exploitation for 2010 is based on option e) The agreed EU–Norway management plan. following this plan implies catches of 164,300 tonnes for the directed herring fisheries in the North Sea.

Management action

At the December 2009 meeting in Brussels, the Council of Ministers decided that the EU Total Allowable Catch for North Sea herring in 2010 should be 93,773 tonnes. The UK quota has been set at 24,223 tonnes.