Combined Ex post and ex ante evaluation of the long term management plan for sole and plaice in the North Sea.pdf (1.34 MB)
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Combined Ex post and ex ante evaluation of the long term management plan for sole and plaice in the North Sea, including responses to ICES review

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posted on 2023-03-17, 11:36 authored by David C. M. Miller, J.J. (Jan Jaap) Poos

This report describes the combined ex post and ex ante evaluation of the multiannual plan for sole and plaice in the North Sea as laid out in Council Regulation (EC) No 676/2007. It extends previous reports by IMARES (C114/09, C104/10). This plan has been in place since 2007. The plan aims to ensure, in its first stage, that stocks of plaice and sole in the North Sea are brought within safe biological limits, i.e. above Bpa and below Fpa. Following this, and after due consideration by the Council on the implementation methods for doing so, the plan will ensure that the stocks are exploited on the basis of maximum sustainable yield and under sustainable economic, environmental and social conditions. This report extends work previously carried out by Wageningen IMARES evaluating the multiannual plan for plaice and sole in the North Sea as described in Council Regulation EC 676/2007 (Machiels et al. 2008). That evaluation has been redesigned to take into account comments and criticisms highlighted by two ICES reviewers.

The ex ante evaluation of the plan includes an examination of its implementation, and the stock and fleet dynamics in relation to the measures and objectives of the plan. The stock dynamics were evaluated using two different stock assessment models: an XSA model and a statistical catch at age (SCA) model. Changes in fleet dynamics were evaluated in terms of overall fishing effort and number of vessels in the Dutch beam trawl and otter trawl fleets. 

The regulations in the Council Regulation have been used as the basis for establishing TACs for North Sea plaice and sole for the last two years. However, the actual implementation of these regulations has required a degree of interpretation by those implementing them. The regulations by themselves lack transparency, without proper specification of how they are to be implemented (e.g. how Fsq is to be calculated) and how the achievement of objectives is to be assessed (neither in terms of the model used to specify this, nor in the level of uncertainty that is acceptable).  

The multiannual plan, without further specifications than the regulations alone, can appear to lack direction. A simple 10% reduction from an unclearly defined Fsq does not necessarily steer the stock towards the objectives. This is especially true given retrospective problems in the North Sea plaice and sole stock assessment. A stepwise decrease in the ‘distance’ between current F and the target F, while potentially invoking the 15% TAC change more often, may be more successful in assuring progress towards the objectives and prevent issues such as the projected increase in sole F from 2008 to 2009.

The rationale for the target fishing mortality reference points in the long term management plan is not given in the Council Regulation. The objective F for the North Sea plaice stock is similar to, though slightly high than, the current proxy of FMSY for this stock as used by ICES. In the case of North Sea sole, the management objective F lies within the broad range of potential proxies for FMSY for the stock. North Sea plaice F is currently below the target F level while, a significant decrease in the F of North Sea sole is still required, especially considering that the current management F for the stock is likely to represent an increase in F from 2008 to 2009. In this context, the long term management plan will steer towards a further decrease in fishing effort by the main fleets targeting sole and plaice.

Under the multiannual plan, the North Sea plaice TAC has been increasing. This increasing trend is likely to persist as long as the stock continues to recover because fishing in the near future should fluctuate around what is considered to be the optimum F for long term sustainable yields. Discarding levels remain high, but currently they are near the lowest level of the past ten years and show a downward trend. North Sea sole TACs have stabilised to a degree under the multiannual plan in the most recent years, but these are likely to continue to fluctuate depending on the strength of incoming year classes. 

The data available to analyse the stock dynamics under the multiannual plan allow the following conclusions to been drawn with respect to the chosen reference points and the safe biological limits of the stock: 

  • Spawning Stock Biomass (SSB) of both species have increased since the implementation of the plan. The XSA assessment for plaice (giving only point estimates), indicates that SSB has been larger than Bpa for two consecutive years. The SCA assessment (following Aarts and Poos 2009) indicates that plaice indeed has a larger than 95% probability of having reached a stage where the SSB is above Bpa for two consecutive years. For sole, the XSA stock assessment indicates that SSB has been above Bpa for two consecutive years. An alternative assessment including uncertainty estimates indicate that the probability of being above Bpa in not yet larger than 95%.  
  • The annual fishing mortality rates (F) of the two stocks have been declining in recent years. The North Sea plaice stock is now fished at a level below the management regulation target for this stock (<0.3 per year). The annual rate of F for the North Sea sole stock remains above the management regulation target for this stock (>0.2 per year). 
  • According to the latest assessment results, both stocks appear to be within the precautionary zone with regards to SSB and F in 2008.  

Hence, despite a lack of clarity in the implementation of the management regulations, the multiannual plan, given the current perception of the stock, appears to be resulting in stock trajectories and fishing levels moving towards the desired objectives and is, therefore, in line with the principles of the precautionary approach.  

In the first year of its implementation (2008), it appears that the effort regulations defined for the multiannual plan have been having the desired effect. Overall fishing effort has declined, along with a decrease in the size of the main fishing fleets utilising these resources. These reductions in effort seem to be compensating for issues relating to the practical application of management regulations on the setting of TACs.  

In the ex‐ante evaluation, three different approaches were used to test the effects of the management plan. The first was a yield curve analysis for the two stocks under different stock and recruit relationship assumptions to assess the equilibrium fishing mortality targets in the plan. The second approach is a projection of the two stocks under the rules of the plan, varying only future recruitment under different assumptions. Finally, the third approach is a full feedback MSE approach where in addition to the biology the fisheries system is also modelled. In order to show that the management plan is precautionary for the two species under consideration according to ICES, we use the Criteria agreed during WKOMSE to be applied in the evaluation of Harvest Control Rules/Management Plans in relation to precautionary reference points.

The results presented here suggest that the multiannual plan can be considered to be precautionary for both of the managed stocks according to the criteria described by WKOMSE for the evaluation of multiannual plans. The plan allows for increases in yield to 2015 and in the long term while reducing the current levels of F. There is a very high likelihood of stock growth in terms of SSB for both stocks. Both the simple stock projections and full feedback MSE analysis showed that F is likely to remain at low levels allowing for increases in stock biomass. Caution needs to be taken in the interpretation of the MSE, and stock projection results because future projections take the stock to outside the range of historic observations. But by examining the performance of the plan at the lower ends of the simulation ranges and considering ‘worst case’ recruitment scenarios the likely risk of a management failure can be considered to be adequately estimated.  

The results presented show that the plan is very likely to be precautionary but it is more difficult to assess whether it achieves the goals of long term yields and sustained healthy populations. This is essentially a question over whether the F targets specified for the two stocks are reasonable and whether in practice they can be achieved simultaneously. The former relies of the definition of MSY for these two stocks and the corresponding stock sizes that can deliver these yields while the latter depends on how fisheries behaviour and gear selection changes into the future. Given the uncertainty associated in the estimation of Fmsy reference points and that expert opinion has been incorporated into the determination of target F points, the targets as they stand seem plausible. Regardless, it is clear that both stock growth and long term increases in current yield levels are likely should the multiannual plan be implemented. 


Published under the auspices of the following ICES Steering Group or Committee

  • ACOM

Published under the auspices of the following ICES Expert Group or Strategic Initiative



ICES Expert Group Reports

Recommended citation

Miller, D.C.M. and Poos, J.J. 2010. Combined Ex post and ex ante evaluation of the long term management plan for sole and plaice in the North Sea, including responses to ICES review (ADHOC Report). ICES CM 2010/ACOM:62. 87 pp.