Single-stock exploitation boundaries


Exploitation boundaries in relation to existing management plans


The revised management plan implies a fishing mortality (F) for ages 2–6 of 0.17, and an F for ages 1–2 of 0.08. Corresponding TACs and allocations among fleets are indicated in the catch options tables below. Changes in TAC between years would be limited to 15% unless paragraph 6 of the management plan is invoked. The management plan is not specific as to when paragraph 6 should be invoked.

Due to the current circumstances of a sequence of five poor recruiting year classes of North Sea herring, it is particularly important that the decline of future spawning stock biomass is addressed with sufficient action to ensure the safety of the spawning stock in the next few years. If the 15% constraint is applied, the SSB in 2008 will remain well below B_pa in 2008 and decline further in 2009 (option 5 in the table below). Accordingly, ICES recommends that paragraph 6 should be invoked. This would result in a catch for Fleet A of 175 kt (option 2 in the table below).

Exploitation boundaries in relation to high long-term yield, low risk of depletion of production potential, and considering ecosystem effects

Evaluations have shown that candidates for reference points which are consistent with high long-term yields and a low risk of depleting the productive potential of the stock are those found in the HCR. Environmentally-induced poor recruitment was included in the evaluations.

Exploitation boundaries in relation to precautionary limits

In order to bring the stock above B_pa = 1.3 million tonnes by 2009, there should be no fishing in 2008.

Conclusion on exploitation boundaries


The current management plan has been evaluated, and the agreed plan is considered precautionary because the risk of SSB falling below B_lim in the medium term is less than 5%, when:

there are no year-to-year restrictions on change in TAC;
current low levels of recruitment continue;
implementation is constrained to give a less than 10% TAC overshoot.

ICES therefore advises in relation to the agreed management plan. This implies a 50% reduction in TAC relative to that agreed upon for 2007, corresponding to a catch of about 200 000 tonnes for all fleets, and 175 000 for fleet A (North Sea human consumption).

Management considerations


The dynamics of North Sea herring have changed. For five years (2002–2006) herring recruitment has been unusually low due to environmental effects. This poor recruitment has reduced catch potential to 40% of the average and will reduce it further. Management must account for the reduced catch potential. In addition, due to a combination of various enforcement and management issues, exploitation is well above the recommended harvest rate for the last thee years (2005–2007). Failure to comply with the harvest rule for a fourth year combined with the poor recruitment will bring the stock to B_lim by 2009. Potentially this could lead to further reduced recruitment and stock collapse. Given the current likely levels of future recruitment, recovery from such a depleted state is likely to be uncertain or slow, reducing catch opportunities even more in the future. If management wishes to avoid these risks it should be regarded as very high priority to get exploitation back to the precautionary exploitation rate recommended by ICES immediately, which would also be in concordance with the agreed management plan.

Management of the autumn-spawning herring must be considered together with the western Baltic herring. The options for TACs for the C- and D-fleets have been selected to be compatible with the exploitation of Western Baltic Spring Spawners (see Section 6.4.7).

Medium-term simulations

Medium-term simulations show that the risk of SSB falling below B_lim is small provided the harvest rule is implemented with low error and that the constraint on interannual variability in TACs is not used (Figure 6.4.18.2). The median SSB settles slightly above 1 million tonnes, and the catches are in the order of 2–3 hundred thousand tonnes. If, on the other hand, the constraint on interannual variability would be used, an additional risk to the stock occurs because the year-on-year restriction was first implemented at the highest level of TAC for 18 years.

The simulations have been done with weights and maturities representing the whole historical time-series. The mean weights in recent years have tended to be lower than previously, which may explain why the simulations for 2007 give a somewhat larger biomass than the short-term prediction. There is, however, no firm basis for assuming lower growth in the future. The simulations have used the recent low recruitment regime.

Downs herring

The sub-TAC for Divisions IVc and VIId was established for the conservation of the spawning aggregation of Downs herring. The Downs herring has returned to its pre-collapsed state and is now again a major component of the stock, but it is currently dominated by one year class. Hence, the management of the fishery on the spawning aggregations of Downs herring should be more cautious than for the North Sea herring as a whole.

For the last three years, ICES proposed that a share of 11% on the total North Sea TAC (average share 1989–2002) would be appropriate for distributing the harvesting among Downs herring and other stock components. In accordance with ICES advice the sub-TAC was reduced from 74 300 t in 2005 to 50 000 t in 2006. The Divisions IVc and VIId sub-TAC is specific to the conservation of the spawning aggregation of Downs herring. Downs herring is caught in large numbers in other areas during the rest of the year. The basis for this exact 11% figure is weak; however, there are strong indications that the total mortality on the Downs component, of which fishing is the major component, has recently been significantly higher than for other North Sea components. This implies that exploitation of this group has been relatively high, so any increased proportional allocations to Divisions IVc and VIId should be avoided. In the absence of data to the contrary ICES proposes that a share of 11% on the total North Sea TAC (average share 1989–2002) would still be appropriate for distributing the harvesting among Downs herring and other stock components.

Management plan evaluations


Given the current sustained low level of recruitment, considering the Btrig in the management plan as Bpa may be unrealistic and it is preferable to evaluate the precautionary nature of the management plan as a whole rather than referring to a biomass reference point that may not be achievable. With the current low recruitment, medium-term simulations show that the SSB will fluctuate around 1.1 million tonnes if the realized fishing mortalities are at the level intended in the agreed HCR.

The performance of the HCR may easily break down if assessment and/or implementation, enforcement, and compliance are sufficiently biased. The TACs in the consumption fishery have been regularly overshot by 10% or more. In this situation, a HCR is required that is robust to errors in the assessment and implementation; the current HCR is thought to be sufficiently robust provided these errors do not exceed 10%.

ICES advice for 2008 is to implement the current HCR. Medium-term simulations suggest that at the current low recruitment, future catches of between 200–240 kt could probably be taken. Managers might consider it helpful to evaluate optimal harvest strategies under these conditions when further developing the HCR as required by the end of 2007.

Ecosystem considerations


Herring is considered to have a major impact on most other fish stocks as prey and predator and is itself prey for seabirds and sea mammals in that area. Herring spawning and nursery areas, being near the coasts, are particularly sensitive and vulnerable to anthropogenic influences. The most serious of these is the ever-increasing extraction of marine sand and gravel. This activity has the potential to seriously damage and destroy the spawning habitat, to disturb spawning shoals, and to destroy spawn if carried out during the spawning season. Similarly, trawling at or close to the bottom in known spawning areas can have the same detrimental effects.

This stock has recently produced five poor year classes in a row, which has never been observed before. Larval surveys show a large abundance of larvae in recent years. However, survival of these larvae seems to be very poor. The specific reasons for this are not known. An ICES study group has reviewed the hypotheses for the serial poor recruitment in North Sea herring (SGRECVAP 2007) and commented that trends in herring recruitment are similar to the warming of the water on the spawning grounds and changes in the hydrography. These hydrographic changes are also associated with changes in the zooplankton community. Further investigation of the causes of the poor recruitment will require targeted research projects.

The management objective is described in the agreed harvest control rules presented at the end of this document.

ICES examined the performance of the harvest control rule in 2006. ICES considers that the strict application of the TAC change limit of 15% (rule number 5) is not consistent with the precautionary approach. Assuming that paragraph 6 would be invoked when SSB falls below B_pa, the revised HCR is in accordance with the precautionary approach.

Catch and TACs


Herring in Subarea IV, Division VIId, and Division IIIa (autumn spawners): advice and catchesCatch and TACsWeights in ‘000 t.
¹Catch in directed fishery in IV and VIId.
²Revision of advice given in 1995.
³Revised in June 1996, down from 263.
⁴ Landings are provided by the working group and do not in all cases correspond to official statistics.
⁵ACFM catch includes unallocated and misreported landings, discards, and slipping.