Standard HCR: The first set of simulations applied the basic harvest rule agreed by Norway and EU from 2004:

At SSB > 1.3 million tonnes: F0-1 = 0.12 and F2-6 = 0.25

At SSB < 1.3 million tonnes and SSB > 800 000 tonnes:

F0-1 = 0.12 – (0.08*(1300 000 – SSB)/500 000)

F2-6 = 0.25 – (0.15*(1300 000 – SSB)/500 000) 800 000 tonnes:

For SSB < 800 000 tonnes: F0-1 = 0.04 and F2-6 = 0.10

The agreement does not state the year which the SSB refers to. The SSB considered by STPR3 is the SSB in the quota year.

Run 2 – 15% rule: The second set applied the rule to not change the TAC by more than 15%

per year. The other parameters were as in the first set.

Run 3 - Overfishing: The third set assumed an overfishing of the derived quotas by 15% . The other parameters were as in run 1, i.e. the rule constraining catch variation was not applied.

For comparison, 3 similar runs were also carried out assuming that the recruitment was normal as used in previous years.

2.8.3 Results

Figure 8.3.1a shows the risk of SSB being below Bpa of 1300 000 tonnes in each of the simulation years. In this figure, the effect of recruitment reverting to the normal pattern from 2007 onwards is also shown for comparison. The deterministic short term prediction indicates an SSB slightly above Bpa. Figure 2.8.1b shows the risk of SSB being below Blim of 800 000 tonnes in each of the simulation years. This risk is considerable if the 15% rule for constraining catch variation is applied, but small in the other cases.

Figures 2.8.2 a-c shows the SSB as median and upper and lower percentiles for the three cases. The median for 2006 is slightly above the deterministic estimate, and the variation due to the assessment uncertainty is small. This leads to a low probability of being below Bpa in the intermediate year 2006. These figures show that if the current harvest rule with F01 = 0.12 and F2-6 = 0.25 is implemented exactly, the SSB can be expected to be in equilibrium in the range of 1.0 to 1.6 million tonnes, with a median slightly below1.2 million tonnes, which is somewhat below the current Bpa of 1.3 million tonnes. If the constraint on year to year variation in annual quotas is applied, the SSB will fall for several years ahead, but recover gradually after that. As noted above, there is a considerable risk that it will even be well below Blim in some of the years. Assuming an overfishing of 15% on average leads towards an equilibrium between 0.9 and 1.5 million tonnes. In all cases, the rule to reduce fishing mortality when the SSB is below 1.3 million tonnes leads to an average F2-6 at about 0.25, in accordance with the equilibrium of 1.3 million tonnes with F2-6 at 0.25.

Figure 2.8.3 shows the risk for SSB < 1.3 million tonnes for a range of F2-6, with a low (0.04) and high (0.12) option for F0-1. In order to avoid the present Bpa with a high probability, the F0-1 will have to be kept low and F2-6 will have to be reduced to about 0.17.

2.8.4 In conclusion:

There is high probability (>60%) of the stock going below Bpa in 2007 and 2008 in all situations

There is a high probability (>60%) of staying below Bpa under the current HCR if recruitment continues at the low level.

There is a significant probability (~20%) of the stock going below Blim if the 15% rule continues to be used after SSB falls below Bpa.

Reducing the fishery on either juveniles or adults can reduce the probability of SSB being below Bpa in 10 years (2017)

Under the current regime only if recruitment returns to normal does the risk to SSB reduce substantially.

2.9 Precautionary Reference Points

In 2003, SGPRP (ICES 2003 ACFM:04) suggested to reduce Blim from the current 800 000 tonnes to about 560 000 tonnes, based on the results of the segmented regression analysis of the stock and recruitment data. Fitting an “Ockham Razor” stock-recruit function with non-linear minimisation of the SSQ of log residuals suggests a break point at 537 000 tonnes.

Although it is apparent that the recruitment historically has been at about the same level when the SSB was somewhat below 800 000 tonnes as above, HAWG decided not to propose any revision of the Blim reference points at present for the following reasons:

- There is some doubt as to the validity of the calculation procedure used by the SGPRP - Currently there is concern that the stock dynamics are changing

- HAWG would prefer to consider all reference points together, rather than revising just Blim.

Most importantly, a downward revision of reference points now would not be helpful in precautionary management of the stock. While the harvest control rule in place for this stock has worked well in the recent past, and apart from Blim, the current reference points are derived from this HCR. The target F in the HCR was adopted by ACFM as Fpa, while the trigger point at which F should be reduced below the target is adopted as Bpa.

2.10 Quality of the Assessment

The details of the assessment have been discussed in great detail in the benchmark analysis presented in Section 2.6. The important issues on sensitivity of the assessment to sampling variability in the data, options for weighting of indices and model settings for ICA have been examined in section 2.6.8. A small range of model settings do not influence the assessment very much. Sampling errors important but the dominate effect is the different signals in the tuning indices, which is discussed below

2.10.1Sensitivity to measured maturity

In previous years the measured maturity of the 2000 year class has been a source of variability in the estimates. This year class is now thought to be almost fully mature at age 4wr (95%) and the precision of this measure is quite good with 95% intervals at about ±2.5%. Other maturing year classes show typical levels of maturity. The assessment this year is therefore not particularly sensitive to measurement of maturity.

2.10.2Use of tuning indices in the 2006 assessment

In this year’s surveys, the MLAI surveys display an upward trend in SSB, in contrast to the Acoustic index that shows a decline. In single fleet tuning of the ICA assessment these translate into Acoustic Index: 14% decline, IBTS: 1% increase and MLAI: 7% increase in SSB from 2004 to 2005. The MIK can also be used to tune the assessment but as this only provides a recruitment index the results are not that informative as a tuning index for the older parts of the population. The final assessment shows a decline of 6% which compares with a 4% decline if the 2005 procedure had been used. While the change in the last year terminal F and SSB is consistent among indices and with the precision of the combined assessment, the recent longer term trajectories are less so. ICA provides a variance/covariance method to bootstrap parameters estimated in the assessment. The scatter plot from 1000 bootstrap estimates using a the ICA variance covariance resampling method on parameter estimates is shown in Figure 2.10.1. Along with this are plotted the results of assessments with the indices

alone. The Acoustic survey suggests a lower SSB and higher F, the MLAI the highest SSB and lowest F. In particular the MLAI gives a much higher overall perception that is outside the range suggested by the combined assessment.

In conclusion the WG recognises that the different signals in the tuning data are currently the major cause of concern in the assessment. The WG has examined the indices through correlation to show they contain useful consistent data, reconciled the different tuning indices through the use of inverse variance weighting in a single assessment. Then checked that errors indicated by the retrospective patterns are small enough to support the utility of the assessment.

2.10.3Comparison with the 2005 assessment and projection The 2006 assessment is in good agreement with last years assessment see table below.

Assessment year SSB in 2004 F2-6 in 2004 SSB in 2005 F in 2005

2006 1.81 M t 0.27 Assessed 1.69 Mt Assessed 0.35

2005 1.89 M t 0.25 Projected 1.82 Mt Projected 0.30

SSB in 2004 is revised down by 5%. SSB projected for 2005 was only a 7% overestimate of the current assessment with a fishing mortality at 0.35 instead of the projected 0.3.

2.10.4Uncertainty in the 2005 assessment

The ICA plot of historic uncertainty is given in Figure 2.10.2. This shows 10% to 90% spread from -15% to +21%. Figure 2.10.1 provides a scatter plot of 100 terminal values in F against SSB, the spread of SSB is slightly narrower than in 2004 suggesting a slightly more precise assessment than last year. However, this ignores the disparity between the potentially different outcomes obtained when using the indices are used alone.

2.10.5Comparison with earlier assessments

Cohort retrospectives are shown in Figure 2.10.3. The earliest cohorts shown have some revision over the early years. Latterly with the exception of the 2001 cohort retrospective evaluations suggest the WG is providing a fairly consistent evaluation of each year class. In particular the dominant 2000 year class has been estimated consistently since it was first seen in 2001. The early revisions occurred during the period following management changes in 1996/1997 and are thought to be mostly due to this but also the more equitable index weighting that was used before 2001.

2.10.6Predictions

The short-term prediction method was substantially modified in 2002. Following the review by SGEHAP (ICES 2001/ACFM:22), which recommended that a simple multi-fleet method would be preferable, the complex split-factor method used for a number of years prior to 2002 has not been used since. The multi-fleet, multi-option, deterministic short-term prediction programme (MFSP) was accepted by ACFM and was developed further in 2004. It is intended to continue to use this programme in the future for as long as multifleet advice in the current form is requested. Last year’s short-term prediction suggested that the North Sea autumn-spawning herring stock SSB in 2006 would be around 1.62 Mt at an F of 0.30. This does not compare so well with this year’s projection of the 2006 SSB which is 1.3 Mt, a reduction of 19%. However, the currently estimated F was higher at F=0.35. This demonstrates that the

current prediction procedure may be underestimating the fishing mortality, which for a declining stock may be an additional problem for management.

2.11 Herring in Division IVc and VIId (Downs Herring).

Over many years the working group has attempted to assess the contribution of winter spawning Downs herring to the overall population of North Sea herring. There is a separate TAC for herring in Divisions IVc and VIId as part of the total North Sea TAC.

Historically, the TAC for herring in IVc and VIId has been set as a proportion of the total North Sea TAC and this has varied between 6 and 16% since 1986 and as a proportion has been relatively high in recent years. ACFM in 2005 expressed a range of concerns regarding Downs herring and recommended that the proportion used to determine the TAC should be set to the long term average of the proportions used since 1986 (11%). This resulted in an agreed TAC in 2006 of 50,023 tonnes, a reduction of 33% compared with 2005 (Figure 2.11.1). The TAC in 2006 is 1.18 times the long term mean TAC for Downs (compared with 1.75 in 2005).

ACFM has in the past expressed concern that there is a persistent tendency to overfish the Downs TAC. However, this tendency has been markedly reduced in recent years (Figure 2.11.2), possibly because TACs have been much higher.

A further concern is that recent high catch levels in IVc and VIId have been driven largely by the strong 2000 year class. This year class accounted for 67% and 51% of the catch in numbers in 2004 and 2005, respectively. As has been noted previously these fish are smaller than average for their age and also have a lower proportion mature than average.

Historically, the Downs herring has been considered highly sensitive to overexploitation (Burd, 1985; Cushing 1968; 1992). It is less fecund and expresses different growth dynamics and recruitment patterns to the more northern spawning components. Furthermore, the directed fishery in Q4 and Q1 targets aggregations of spawning herring.

Preliminary studies suggest that the population profile of herring caught in Divisions IVc and VIId is slightly steeper than that for Divisions IVa and IV, particularly on the older ages (Figure 2.11.3)..The profiles were derived taking the log of the average catch numbers at age for Q4 over the period 1996 to 2004. Moreover, time-series estimates of total mortalities from within cohort catch-curves suggest that Z has been significantly higher on the 1998 and 1999 year classes of Downs herring (Figure 2.11.4).

Downs herring is also taken in other herring fisheries in the North Sea. Downs herring mixes with other components of North Sea herring in the summer whilst feeding, but it has not been possible to quantify the complete Downs component in catches. There is also a summer industrial fishery in the eastern North Sea exploiting Downs and North Sea autumn spawning herring juveniles. Tagging experiments (Aasen et al, 1962) estimated that around 15% of catches comprised Downs recruits. Recent otolith microstructure studies of Dutch catches in the summer of 2004 and 2005 from the northern North Sea suggest that the proportion of Downs herring may vary considerably from year to year. The percentage contribution was estimated as 60% in 2004 and 26% in 2005 (Dickey-Collas et al., 2005).

The proportion of the autumn and winter spawning components in recruiting year classes of North Sea herring has been traditionally monitored through the abundance of different sized fish in the IBTS. 1 wr fish from Downs spawning sites (winter) are thought to be smaller than those from the more northern, autumn spawning sites (<13 cm and >13 cm respectively).

Both the total abundance and the proportion of Downs herring have, on average, been higher since the early 1990s, although there is considerable variation between year classes (Figure 2.11.5). These data (Table 2.3.3.3) suggest that around 35% of the strong 2000 year class came from Downs production and that approximately 70% of the 2002 year class originated

from Downs production. In contrast, only 15% of the very weak 2004 year class was derived from Downs spawners.

Contribution by recent year classes

2000 year class- The Downs component continues to contribute significantly to the total fishery on the North Sea herring stock. Separate catches of this component are not available for all areas in the North Sea, however where separate data are available this year class in catches in IVc and VIId alone accounted for 8% and 5% of the total North Sea catches by weight in 2004 and 2005, respectively.

2003 and 2004 year classes- It appears probable that the recruitment for the 2003 and 2004 year classes of Downs herring is poor (Figure 2.11.6), based on the MIK index for the southern North Sea and the IBTS 1wr estimates (<13cm) (Table 2.3.3.3).

2005 year class- The preliminary results from the MIK survey (Figure 2.11.6) suggest that the 2005 year class may be the second highest since these data became available (1995).

However, this estimate should be treated with extreme caution because it is based on a single large catch.

There remains an expectation of a low recruitment in recent years despite high levels (but with high variance) of larval abundances (Fig. 2.11.6). Hence it is probable that the productivity of the Downs component will reduce over the next few years, when the 2000 and 2002 year classes are fished out.

The Downs herring has returned to its pre-collapse state of being a major component of the stock but is currently dominated by one year class. Hence the management of the fishery on the spawning aggregations of Downs herring should continue to be cautious. More evidence about the dynamics and catches of Downs herring is required. Hence, HAWG continues to recommend that existing surveys of herring in the southern North Sea and English Channel be maintained and that the microincrement analysis of otoliths (to determine spawning type) is expanded to other fleets in the North Sea and also carried out on samples collected during the annual acoustic survey.

Last year the EC set a TAC for herring in IVc and VIId in concordance with the ICES advice.

The TAC is specific to the conservation of the spawning aggregation of Downs herring.

Evidence from catch-curve studies indicates a higher total mortality (Z) on this component and Downs herring is also caught in large numbers in other areas during the rest of the year.

The TAC in 2006 remains 18% above the long-term mean TAC and low recruitment to the component is probable in the next few years. Thus, in the absence of other information HAWG recommends that the IVc-VIId TAC should be maintained in 2007 at 11% of the total North Sea TAC (as recommended by ACFM). This recommendation should be seen as an interim measure prior to the development of a more robust harvest control for setting the TAC of Downs herring, supported by increased research effort into the dynamics of this component in fisheries in the central and northern North Sea. Any new approach should provide an appropriate balance of F across stock components and be similarly conservative until the uncertainty in the Downs contribution to the catch in all fisheries in the North Sea is reduced.

Towards a Harvest Rule to determine IVc and VIId herring TAC allocation

The larval index although noisy seems to be a reliable index of North Sea herring SSB.

Comparison between the Downs and the North Sea indices of SSB based on larval production (Dickey-Collas WG D3) suggests differences in the trends corresponding to both stocks (Fig.

2.11.7), particularly in recent years where estimated SSB for the Downs has declined compared with an increasing trend for total North Sea SSB. Given the lack of key information to provide a well-substantiated southern North Sea TAC allocation, an indicator of Downs abundance relative to the total North Sea would be useful. The ratio of the IVc and VIId to the

North Sea index of SSB based on larval production is noisy so a three-year running average was applied to smooth the series. The historic series of the proportion of the catch in IVc – VIId compared to the whole of the North Sea is plotted along with the smoothed larval SSBs ratios in Fig. 2.11.8. The smoothed larval ratios suggest an increasing trend in the proportion of Downs for the period although a persistent decline in the annual ratio was seen from 2000.

However, the series of total mortality estimates by cohort presented early in this section (Fig.

2.11.4) suggested that fishing mortality for the component could have been high in recent years compared to the rest of the North Sea. Therefore, although the proportion of the Downs component relative to the total North Sea may on average have increased in the period 1986 to 2004 it is possible that the effective catch exerted on the component is not sustainable particularly if the in-coming year-classes continue to be weak. In recent years, the actual proportion of the southern North Sea component in the catch has substantially exceeded the 11% of the TAC long-term average reaching 22% in 1997 (Fig. 2.11.8). There is no scientific basis for the proposed 11% and given the indications of high mortality it becomes increasingly important that the 11% proposed as a conservative allocation is revised.

A number of rules to set the percentage TAC allocation could be proposed but they will have to be tested on a simulation framework. The SSB ratios should be investigated as a potential index to derive a percentage southern North Sea allocation and indices of in-coming year-class strength such as the proportion of small fish (<13cm) in the IBTS survey and the MIK index for the Channel should also be taken into account when developing rules. Flexibility and

A number of rules to set the percentage TAC allocation could be proposed but they will have to be tested on a simulation framework. The SSB ratios should be investigated as a potential index to derive a percentage southern North Sea allocation and indices of in-coming year-class strength such as the proportion of small fish (<13cm) in the IBTS survey and the MIK index for the Channel should also be taken into account when developing rules. Flexibility and

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