The EC has adopted a management plan for flatfish in the North Sea in June 2007 (Council Regulation (EC) No. 676/2007, see Annex 6.4.10 in the source of information). This plan has two stages. The first stage aims at an annual 10% reduction of fishing mortality in relation to the fishing mortality estimated for the preceding year until an F of 0.2 is reached, with a maximum change in TAC of 15% until the precautionary reference points are reached for both sole and plaice for two successive years. ICES interprets the F for the preceding year as the estimate of F for the year in which the assessment is carried out. The basis for this F estimate in the preceding year will be a constant application of the procedure used by ICES in 2007. In the second stage, the management plan aims for exploitation at F = 0.2.
ICES has evaluated the long-term management plan and concluded that it leads on average to a low risk of B < Blim
within the next 10 years. ICES concludes that for sole the management plan can be provisionally accepted as precautionary. According to the evaluation the agreed management plan can be provisionally accepted as precautionary for sole and could be used as a basis for the management of the stock in the short term. Additional evaluations of the management plan are necessary to take into account retrospective bias of the assessment and the sporadic nature of recruitmentIn
Management AdviceManagement considerations
Sole are mainly caught in a mixed beam trawl fishery with plaice and other flatfish using 80 mm mesh in the southern North Sea. The minimum mesh size in the mixed beam trawl fishery in the southern North Sea means that large numbers of undersized plaice and cod are discarded. There are indications that in recent years sole discarding has taken place. Measures to reduce discarding in the mixed beam trawl fishery would greatly benefit these stocks. An increase in the minimum landing size of sole could provide an incentive to fish with larger mesh sizes and would therefore mean a reduction in the discarding of plaice. The minimum landing size of North Sea sole is 24 cm. An increased mesh size in the fishery would reduce the catch of undersized plaice and cod, but would also result in a short-term loss of marketable sole.
For two successive years, the stock has been classified within safe precautionary boundaries and thus fulfilled the 1st
phase of the management plan. The increase in SSB is mainly achieved by the strong 2005 year class. The main explanation for the reduction of fishing mortality seems to be a proper implementation of the current management plan complemented with a reduction of capacity in the beam trawl fleet and a limitation of fishing effort. Also high fuel prices have contributed to the decrease in fishing mortality.
The peaks in the historical time-series of SSB of North Sea sole correspond with the occasional occurrence of strong year classes. Due to a high fishing mortality the SSB has declined during the nineties. The fishery opportunities and SSB are now dependent on incoming year classes and can therefore fluctuate considerably between years. The SSB and landings in recent years have been dominated by the 2001 and 2005 year classes. The predicted SSB in 2011 is still largely dependent on the above-average recruitment of the 2005 year class.
ICES has developed a generic approach to evaluate whether new survey information that becomes available in September forms a basis to update the advice. If this is the case, ICES will publish new advice in November 2009. Management plan evaluation
According to evaluation of the agreed management plan it can be provisionally accepted as precautionary for sole. Estimations of sole stock status appear to have a retrospective under-estimation of fishing mortality and over-estimation of SSB, which have resulted in forecast bias. The probability of successfully attaining the objectives of the first stage of the management plan is dependent on the assumption of a stock–recruitment relationship, which is not well founded. The dynamics of the stock is driven by the sporadic strong year classes that lead to wide fluctuations in the SSB. Additional evaluations of the management plan are necessary to take into account retrospective bias of the assessment and the nature of recruitment to conclude on the precautionary nature of the plan in the long term.Impacts of fisheries on the ecosystem
Currently the mixed plaice and sole fishery is dominated by bottom trawls, with bycatch of both commercial and non-commercial species and with physical impact on the seabed. Bottom trawling reduces biomass, production, and species richness. For the North Sea, an ecosystem model showed that the bottom trawl fleet reduced benthic biomass and production by 56% and 21%, respectively, compared with an un-fished situation (Hiddink et al.
, 2006; Hinz et al.
, 2008). Chronic fishing has caused a shift from communities dominated by relatively sessile, emergent, high biomass species to communities dominated by infaunal, smaller-bodied fauna (Kaiser et al.
Within species, the size selectivity may lead to a shift in the age and size at maturation. For example, plaice and sole become mature at younger ages and at smaller sizes in recent years than in the past. There is a risk that this shift is a genetic fisheries-induced change (Grift et al.
, 2004; Mollet et al.