CPUE - second half year
5 Norway Pout in ICES Subarea IV and Division IIIa
5.2 Short-term prognoses
Deterministic short-term prognoses were performed for the Norway pout stock. The forecast was calculated as a stock projection up to 1st of January 2009 using full assessment information for 2006 and 1st half year 2007, i.e. is based on the SXSA assessment estimate of stock numbers at age at the middle of 2007.
The purpose of the forecast is to calculate possible catch of Norway pout in 2008 leaving a SSB at or above Bpa 1st of January 2009 (Bpa = 150 000 t). The forecast is based on an escapement management strategy for Norway pout (see section 5.3 and ICES AGNOP Report ICES CM 2007/ACFM:39) where the fishery has been closed in 2007. The forecast is using the 25 percentile of the geometric mean for the stock-recruitment relationship (see below).
Input to the forecast is given in Table 5.2.1. The forecast assumes 2007 (the assessment year) closed for directed fishery (F=0) and a 2008 (the forecast year) fishing pattern scaled to long term seasonal exploitation pattern for 1991-2004 (standardized with yearly Fbar to F(1,2)=1) which has been used in the ICES AGNOP Report as well (ICES CM 2007/ACFM:39).
Recruitment in the forecast year is assumed to the 25 percentile = 72 558 millions (of the long term geometric mean = 101525 millions).
The weight at age in the catch per quarter is based on estimated mean weight at age in catches during 2003-2006. The constant weight at age in stock by year and quarter of year used in the SXSA assessment has also been used in the forecast for 2007 and 2008.
Ten percent of age 1 is mature and is included in SSB. Therefore, the recruitment in 2007 does influence the SSB in 2008.
The results of the forecasts are presented in Table 5.2.2. It can be seen that if the objective is to maintain the spawning stock biomass above Bpa by 1st of January 2009 then a catch around 175 000 t can be taken in 2008 according to the escapement strategy. Under a fixed F-management-strategy with F around 0.35 a catch around 95 000 t can be taken. Under a fixed TAC strategy a TAC of 50 000 t can be taken in 2008 according to what is presented in section 5.3 below.
5.3 Management
5.3.1 Management up to 2007
There is no specific management objective set for this stock. With present fishing mortality levels the status of the stock is more determined by natural processes and less by the fishery.
The European Community has decided to apply the precautionary approach in taking measures to protect and conserve living aquatic resources, to provide for their sustainable exploitation and to minimise the impact of fishing on marine ecosystems.
Previous to 2005 a precautionary TAC was set to 198 000 t in the EC zone and 50 000 t in the Norwegian zone. On basis of the ICES advice for 2005 from ICES, EU and Norway agreed to close the directed Norway pout fishery in 2005 and in the first part of 2006. Accordingly, the TAC was in 2005 and for the first part of 2006 set to 0 in the EC zone and 5 000 t in the Norwegian zone – the latter to allow for by-catches of Norway pout in the directed Norwegian
blue whiting fishery. On basis of the real time management advice provided by ICES in spring 2006 EU set a quota on 95.000 t for 2006 (intended for the whole year). The fishery for this TAC was by the EU Commission not opened before the 4th of August 2006 for the second half year 2006. Norway did in the beginning of September 2006 open a directed Norway pout fishery without quota limitations in Norwegian EEZ. However, the area (Egersund Bank) was closed for this fishery from 1st of October 2006. Based on the management advice from ICES in autumn 2006 taking the low recruitment in 2006 into consideration the fishery was closed again by 1st of January 2007. This advice was by ICES confirmed in May 2007, and has resulted in a management where the directed Norway pout fishery has continued to be closed in all of 2007.
In managing this fishery by-catches of other species have been taken into account. Existing technical measures such as the closed Norway pout box, minimum mesh size in the fishery, and by-catch regulations to protect other species have been maintained.
An overview of recent relevant management measures and regulations for the Norway pout fishery and the stock can be found in the Stock Quality Handbook (Q5).
5.3.2 Long term management strategies 5.3.2.1 Background
On basis of an joint EU and Norwegian Requests in autumn 2006 with respect to Norway pout management strategies and by-catches in the Norway pout fishery as well as on basis of the work by ICES WGNSSK in autumn 2006 and spring 2007 during the ICES AGNOP 2007 (ICES CM 2007/ACFM:39) ACFM has already by May 2007 evaluated detailed output from management plans and harvest control rules evaluations considering two different management strategies for Norway pout, i.e. the real time escapement management strategy and the long term fixed F or E management strategy. This has been based on use of advanced stochastic simulation models and results from here supplied by DIFRES. The fixed TAC long term management strategy were not evaluated in depth by the ICES AGNOP as it was not considered realistic at that time because of substantial loss in yield, but have later in early summer 2007 been evaluated in depth by DIFRES using the same advanced stochastic simulation models.
Based on the above as well as on basis of a Danish Government request to ICES by September 2007 also to evaluate the long term fixed TAC management strategy for Norway pout we here in this report include the combined management strategy presentation and evaluation for the 3 strategies (through the ICES WGNSSK report to the ACFM by September/October 2007).
5.3.2.2 Long Term Harvest Control Rules for Norway pout in the North Sea and Skagerak
In 2006 EC and Norway jointly requested the International Council for Exploration of the Sea (ICES) for advice on management of Norway pout. ICES responded to the request partly in autumn 2006 and finally in spring 2007.
ICES in its response addresses three types of management strategies:
− Fixed TAC strategy,
− Fixed fishing effort strategy and
− Escapement strategy
ICES considered the fixed TAC strategy to be less interesting as such strategies most likely would result in a substantial loss in long-term yield compared to other strategies if the risk of SSB falling below Blim is to remain reasonably low. ICES, therefore, did not fully explore this option.
To give a full picture of all three strategy options DIFRES has, following the model and methods used by ICES, performed a full evaluation of the fixed TAC strategy.
This section gives a short description of the evaluation methods used for the three strategies and present the combined results of the evaluations.
Escapement strategy
ICES evaluated an escapement strategy defined as follows: 1) an initial TAC that would be set for the first half of the TAC year, based on a recruitment index, and 2) a TAC for the second half of the year which would be based on a survey assessment conducted in the first half of the TAC year and the setting TAC for the second half of the year based on an SSB escapement rule.
The time line would be as follows:
This escapement strategy that generally will assure an SSB above is formulated as
follows: pa
I
is the geometric average index of age 0 in the 3rd quarter IBTS survey*
TACy is the TAC in year y in the first half of the year
B
is the maximum TAC for the first half of the year (50 kt)A
is the slope that converts the relative IBTS index to a TAC for the first half of the year is determined such that a recruitment three times higher than average will give a TAC at 50 kt ) A(
=B/3)
C is the survey threshold (equivalent to the index for the long-term geometric mean, 70 billions)
2. The TAC for the second half of the year,TAC*y*, is based on a full assessment in April–May which includes the results from the 1st quarter IBTS for the present year:
( )
y is the SSB at the start of the year after the TAC year as projected from a survey assessment based on 1st Q IBTS results
SSB
B
paF
is the PA biomass reference point
y
F
is the fishing mortality for the TAC year y
capis an upper limit on the fishing mortality in year y
The target of obtaining an SSB that is truly above Blim with a high probability (95%) appears feasible when realistic values of uncertainties in the assessment (CV=0.3 on y+1 ) and survey (CV=0.42 on y ) are used and when a cap on fishing mortality of 0.8 is used. In practice this Harvest Control Rule (HCR) becomes an escapement strategy with an additional maximum effort.
SSB
I −1The equilibrium median yield is around 110 kt (Fig 3.3.1). There is a 50 % risk for a closure of the fishery in the first half-year and a 20–25% risk of a closure in the second half-year. The distribution of F (Fig 3.3.2) shows that the fishery will mostly alternate between a low and a high effort situation. When the fishery has been closed in the second half-year, there is around 20 % probability for another closure in the following year.
The robustness of the HCR to uncertainties in stock size indicates that annual assessment might not be necessary for this stock; an annual survey index could be sufficient.
Caveats to the evaluation of the escapement strategy:
• The sensitivity of the parameters in the HCR used for TAC in the first half-year has not been fully evaluated;
• Non-random distribution of residuals in the surveys may give biased perceptions and need to be included in the evaluation.
Effort control strategy
The effort control scenario with a fixed F indicates that an F of around 0.35 is expected to give a low (5 %) probability of the stock going below Blim (Fig 3.3.3). The scenario appears robust to implementation uncertainties (Fig 3.3.4). A target F below 0.35 and an implementation noise CV around 25 % (Fig 3.3.5) is expected to give a long-term yield below 90 kt and no closures of the fishery would be needed. This management strategy is independent of an assessment because it assumes a direct link between fishing effort and fishing mortality which is also apparent from the historical assessment of this stock (Fig 3.3.6).
Caveats to the evaluation of the effort control strategy:
• A regime shift towards a lower recruitment level will not be detected by this approach and there is a risk of over-fishing in such a situation with a fixed effort approach;
• Implementation of a fixed standardized effort (which is not measurable) can be difficult;
• Effort management in by-catch fisheries (e.g. by-catch of Norway pout in blue whiting fishery) is difficult to regulate;
• Effort – F relationships are known to suffer from technological creep and this aspect needs to be tested in the evaluation.
Fixed TAC strategy
The scenario with fixed TAC indicates that a long term TAC on around 50.000 t will be sustainable with a low (5 %) probability of the stock going below Blim (Figs 3.3.7 and 3.3.8).
The evaluations indicate that if a target TAC below 50 kt is implemented no closures of the fishery would be needed.
Caveats to the evaluation of the fixed TAC strategy:
• A regime shift towards a lower recruitment level will not be detected by this approach and there is a risk of overfishing in such a situation with a fixed TAC approach;
• For a short-lived species with highly variable recruitment such as Norway pout, a catch-stabilizing strategy (fixed TAC) is likely to imply a substantial loss in long-term yield compared to other strategies if the risk of SSB falling below Blim is to remain reasonably low. This strategy is also sensible in relation to potential risks of regime shifts in the stock-recruitment-relationship.
Conclusions from management strategy evaluations
Not any particular of the management strategies presented above is recommended. All strategies that have a low risk of depleting the stock below Blim are considered to be in accordance with the precautionary approach. The choice between different strategies depends on the requirements that fisheries managers and stakeholders have regarding stability in catches or the overall level of the catches.
The evaluation shows that all three types of management strategies (escapement, fixed effort, fixed TAC) are capable of generating stock trends that stay away from Blim with a high probability.
The escapement strategy has a higher long-term yield (110 kt) compared to the fixed effort strategy (90 kt) and the fixed TAC strategy (50 kt) but at the cost of having closures in the fishery with a substantially higher probability. If the continuity of the fishery is an important property, then the fixed effort strategy performs better.
The simulations deal with observation error and implementation error of the management strategies but do not take into account process error in relation to natural mortality, maturity-at-age, or mean weight-at-age in the stock, which could have a significant impact.
The fixed effort strategy does not rely critically on the results of stock assessment models in any particular year. On the other hand, that strategy is very dependent on the possibility of actually implementing an effort scheme, including an account of the bycatch fisheries (e.g. for blue whiting) and ways to deal with effort creep.
The fixed effort strategy and the fixed TAC strategy are likely to imply a substantial loss in long-term yield compared to the escapement strategy if the risk of SSB falling below Blim is to remain reasonably low. These strategies are also sensible in relation to potential risks of regime shifts in the stock-recruitment-relationship.