Fishery Resources Monitoring System

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Sole - North Sea, 2008
Marine Resource  Fact Sheet
ICES Advice 2009
Sole - North Sea, 2008
Fact Sheet Citation  
Sole in Sub-area IV (North Sea)
Owned byInternational Council for the Exploration of the Sea (ICES) – More
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Species:
FAO Names: en - Common sole, fr - Sole commune, es - Lenguado común, zh - 欧洲鳎
Geographic extent of Sole - North Sea
Main Descriptors
Considered a single stock: Yes        Spatial Scale: Sub-Regional
Management unit: Yes
 
 
Habitat and Biology
Depth zone: Shelf (50 m - 200 m).   Vertical distribution: Demersal/Benthic.  

Geographical Distribution
Jurisdictional distribution: National

Water Area Overview
Spatial Scale: Sub-Regional

Geo References
Resource Structure
Considered a single stock: Yes
Exploitation
 

Factors affecting the fisheries and the stock

The effects of regulations

Due to a range of factors such as TAC constraints on plaice, effort limitations, and increases in fuel prices, the fishing effort of the major fleets targeting sole has decreased since the mid-1990s and concentrated in the southern part of the North Sea. This is the area where a large part of the juvenile plaice in the North Sea is found. The combination of a change in fishing pattern and the spatial distribution of juvenile plaice has led to an apparent increase in discarding of plaice.

The plaice box was established in 1989, and the area has been closed in all quarters since 1995. The closed area applies to vessels using towed gears, but vessels smaller than 300 HP are exempted from the regulation. The effectiveness of the plaice box has been evaluated by a STECF expert group which concluded that the proportion of undersized sole inside the plaice box did not change after the closure and remained stable at 60–70% (Grift et al., 2004).

Changes in fishing technology and fishing patterns

Due to a range of factors such as effort limitations, increases in fuel prices, and disproportionate changes in the TACs for the two main target species sole and plaice, the fishing effort of the major fleets has concentrated in the southern part of the North Sea. This is the area where many juvenile plaice are found. In addition, juvenile plaice has shown a more offshore distribution in recent years. The combination of a change in fishing pattern and the spatial distribution of juvenile plaice has lead to an increase in discarding of plaice.

The technical efficiency has increased by about 2.8% per year in the sole fishery since 1990, which could have counteracted part of the decrease in effort (Rijnsdorp et al., 2006).

Impacts of the environment on the fish stocks

There has been an overall increase in growth of North Sea sole, followed by a decline correlated with the temporal patterns in eutrophication, in particular the discharge of dissolved phosphates by the Rhine. The spatial distribution of juvenile and adult sole remains constant within the Plaice Box (Grift et al., 2004), following the removal of a large amount of effort. The proportion of undersized sole (<24 cm) did not change after closure and remained stable at a level of 60–70% (Grift et al., 2004). Different length groups showed different patterns in abundance. Sole of around 5 cm showed a decrease in abundance from 2000 onwards, while the groups of 10 and 15 cm seemed rather stable. The largest groups showed a declining trend in abundance, which had already set in years before the closure.
Assessment
 
Assessment Model
Methodology

Scientific basis

Data and methods

The stock assessment is based on an XSA assessment, calibrated with two survey indices and one commercial lpue index.

Discards are not included in the assessment. Routine discard sampling since 2003 under the EU Data Collection Regulations indicates overall discarding of sole in the order of 10% in weight and discards are considered to be a minor bias to the assessment results.

Uncertainties in assessment and forecast

There are indications of recurring underestimation of fishing mortality and overestimation of abundance. A discrepancy between the trends in surveys and the commercial cpue is apparent in the results of comparisons between survey and commercial time-series of information. The survey information indicates higher mortality rates and lower stock abundance, consistent with the more recent values to which the assessment estimates are revised each year.

There are indications that discarding may have an impact on the assessment results

Information from the fishing industry

The North Sea Fisher’s survey took place in 2008. A total of 195 responses were collected for sole. As in the 2007 survey, the beam trawl and gill net fishing groups indicated that sole were “more” abundant and the fishers felt that discarding had reduced slightly with respect to 2007. Of those that expressed an opinion on recruitment the majority considered it to be “moderate” or “high”. These observations are consistent with the assessment results.

Comparison with previous assessment and advice

The estimate of F for 2007 from the 2008 assessment was revised down by 5% in the 2009 assessment. The SSB estimate for 2007 increased 3% in the 2009 assessment. The revisions are different from retrospective bias in the assessment results previously.

Fishing mortality in the intermediate year used in the 2009 forecast was the average F (2006-2008) scaled to F (2008). The basis for this forecast is different from the previous forecasts, where the average F over the final 3 years was used to counteract the retrospective change from year to year.

The advice this year is based on the EU management plan, similar to last year.
Overall Assessment Results


Table 6.4.10.1 Sole in Subarea IV (North Sea). Single stock exploitation boundaries (advice), management and landings
Year ICES Advice Single-stock exploitation boundaries Predicted catch corresponding to advice Predicted catch corresponding to single-stock Agreed TAC Official landings ICES landings
1987 Rebuild SSB to 40 000 t; TAC   11.0   14.0 13.8 17.4
1988 Increase SSB towards 50 000 t; TAC   11.0   14.0 13.4 21.6
1989 Increase SSB towards 50 000 t; TAC   14.0   14.0 14.5 21.8
1990 80% of F(88); TAC   25.0   25.0 26.5 35.1
1991 SSB>50 000 t ; TAC   27.0   27.0 27.6 33.5
1992 TAC   21.0   25.0 26 29.3
1993 no long-term gains in increased F   29.01   32.0 29.8 31.5
1994 no long-term gains in increased F   31.01   32.0 31.3 33
1995 no long-term gains in increased F   28.01   28.0 28.8 30.5
1996 Mixed fishery, link plaice advice   23.01   23.0 20.4 22.7
1997 <80% of F(95)   14.6   18.0 13.7 15
1998 75% of F(96)   18.1   19.1 19.7 20.9
1999 F<Fpa (80% of F(97))   20.3   22.0 22 23.5
2000 F< Fpa   <19.8   22.0 20.7 22.5
2001 F< Fpa   <17.7   19.0 16.4 19.9
2002 F<0.37   <14.3   16.0 16 16.9
2003 F< Fpa   <14.6   15.85 17.1 17.9
2004 2 F< Fpa 2 <17.9 17.0 17.8 17.1
2005   F< Fpa   <17.3 18.6 15.6 16.4
2006   Keep SSB above Bpa   <11.9 17.67 11,9 12.6
2007   SSB above Bpa   <10.8 15.0 13.8 14.6
2008   SSB above Bpa   <9.8 12.8 13.4 14.1
2009   Apply management plan   <14.0 14.0    
2010   Apply management plan   <14.1      

Weights in ‘000 t.

1 Catch status quo F.

2 Single-stock boundary and the exploitation of this stock should be conducted in the context of mixed fisheries protecting stocks outside safe biological limits.



Figure 6.4.10.1 Sole in Subarea IV (North Sea). Landings, fishing mortality, recruitment, and biomass (SSB). Predicted values are shaded.


Figure 6.4.10.2 Sole in Subarea IV (North Sea). Stock recruitment, yield, and precautionary approach.


Figure 6.4.10.3 Sole in Subarea IV (North Sea). Results of the North Sea Commission fishers’ survey 2008.


Figure 6.4.10.4 Sole in Subarea IV (North Sea). Historical performance of the assessments.


Table 6.4.10.2 Sole in Subarea IV (North Sea). Nominal landings and landings as estimated by the Working Group (tonnes).
Year Belgium Denmark France Germany Netherlands

UK

(E/W/NI)

Other

countries

Total

reported

Unallocated

landings

WG

Total

TAC
1982 1900 524 686 266 17686 403 2 21467 112 21579 21000
1983 1740 730 332 619 16101 435   19957 4970 24927 20000
1984 1771 818 400 1034 14330 586 1 18940 7899 26839 20000
1985 2390 692 875 303 14897 774 3 19934 4314 24248 22000
1986 1833 443 296 155 9558 647 2 12934 5266 18200 20000
1987 1644 342 318 210 10635 676 4 13829 3539 17368 14000
1988 1199 616 487 452 9841 740 28 13363 8227 21590 14000
1989 1596 1020 312 864 9620 1033 50 14495 7311 21806 14000
1990 2389 1427 352 2296 18202 1614 263 26543 8577 35120 25000
1991 2977 1307 465 2107 18758 1723 271 27608 5905 33513 27000
1992 2058 1359 548 1880 18601 1281 277 26004 3337 29341 25000
1993 2783 1661 490 1379 22015 1149 298 29775 1716 31491 32000
1994 2935 1804 499 1744 22874 1137 298 31291 1711 33002 32000
1995 2624 1673 640 1564 20927 1040 312 28780 1687 30467 28000
1996 2555 1018 535 670 15344 848 229 21199 1452 22651 23000
1997 1519 689 99 510 10241 479 204 13741 1160 14901 18000
1998 1844 520 510 782 15198 549 339 19742 1126 20868 19100
1999 1919 828   1458 16283 645 501 21634 1841 23475 22000
2000 1806 1069 362 1280 15273 600 539 20929 1603 22532 22000
2001 1874 772 411 958 13345 597 394 18351 1593 19944 19000
2002 1437 644 266 759 12120 451 292 15969 976 16945 16000
2003 1605 703 728 749 12469 521 363 17138 782 17920 15850
2004 1477 808 655 949 12860 535 544 17828 -681 17147 17000
2005 1374 831 676 756 10917 667 357 15579 776 16355 18600
2006 980 585 648 475 8299 910   11933 667 12600 17670
2007 955 413 401 458 10365 1203 5 13800 835 14635 15000
2008 1379 507 714 513 9456 851 15 13435 710 14145 12800


Table 6.4.10.3 Sole in Subarea IV (North Sea). Summary of stock assessment.
Year

Recruitment

Age 1

thousands

SSB

tonnes

Landings

tonnes

Mean F Ages 2-6

1957 128909 55107 12067 0.18
1958 128643 60919 14287 0.21
1959 488757 65580 13832 0.17
1960 61714 73398 18620 0.20
1961 99488 117099 23566 0.19
1962 22895 116830 26877 0.21
1963 20420 113628 26164 0.31
1964 539075 37127 11342 0.29
1965 121959 30029 17043 0.32
1966 39901 84243 33340 0.33
1967 75135 82958 33439 0.41
1968 99262 72306 33179 0.49
1969 50787 55267 27559 0.55
1970 137795 50680 19685 0.40
1971 42148 43742 23652 0.51
1972 76525 47437 21086 0.46
1973 104859 36775 19309 0.50
1974 109939 36110 17989 0.49
1975 40816 38365 20773 0.50
1976 113311 38944 17326 0.42
1977 140375 34623 18003 0.46
1978 47256 36195 20280 0.48
1979 11723 44954 22598 0.49
1980 151694 33584 15807 0.45
1981 149346 22921 15403 0.50
1982 152751 32855 21579 0.54
1983 142179 39956 24927 0.49
1984 70791 43464 26839 0.61
1985 80833 41082 24248 0.60
1986 159654 34554 18201 0.57
1987 72553 29658 17368 0.49
1988 454627 38765 21590 0.57
1989 108296 34075 21805 0.45
1990 177757 89643 35120 0.45
1991 70476 77479 33513 0.45
1992 354171 76772 29341 0.43
1993 69289 54752 31491 0.51
1994 57057 74337 33002 0.56
1995 96104 58934 30467 0.53
1996 49508 38310 22651 0.70
1997 271749 28071 14901 0.60
1998 114161 20882 20868 0.64
1999 82581 41918 23475 0.57
2000 123824 39217 22641 0.60
2001 63480 30762 19944 0.57
2002 187821 31412 16945 0.56
2003 85663 25758 17920 0.57
2004 46679 38402 18757 0.50
2005 49955 33520 16355 0.55
2006 221770 25778 12594 0.41
2007 60383 19585 14635 0.41
2008 90949 40676 14144 0.34
2009 93786* 37670    
Average 124747 49191 21703 0.46
* GM (1957–2006)

Reference Point
 


  Type Value Technical basis
Precautionary approach Blim 25,000 t Bloss
  Bpa 35,000 t Bpa1.4 *Blim
  Flim Not defined  
  Fpa 0.40 Fpa = 0.4 impliesBeq > Bpa and P(SSBMT<Bpa) < 10%.
Targets Fmgt 0.2 EU management plan
(unchanged since 1998, target added in 2008)


Yield and spawning biomass per Recruit F-reference points (2009):
  Fish Mort Ages 2–6 Yield/R SSB/R
Average last 3 years 0.39 0.17 0.36
Fmax * 0.59 0.17 0.24
F0.1 0.11 0.14 1.02
Fmed 0.32 0.17 0.43
*Poorly defined



Candidates for reference points consistent with high long-term yields and a low risk of depleting the productive potential of the stock are in the range of F0.1–Fpa.
Projection


Outlook for 2010 Basis: F(2009) = Fsq = mean F(2006–2008) scaled to 2008 = 0.34 ; R(2009) = GM (1957–2006) =93.800 mln; SSB(2010) = 37.660 ; landings (2009) = 15.140
Rationale Landings (2010) Basis F (2010)

SSB

(2011)

%SSB

change1)

%TAC change2)
Zero catch 0 F=0 0 53 +41% -100%
High long-term yield 5.6 F0.1 0.11 48 +27% -60%
Management plan 14.1 Fsq*0.9 0.3 40 +5% +1%
  4.4 Fsq*0.25 0.08 49 +30% -69%
Status quo 8.4 Fsq*0.5 0.17 45 +20% -40%
  9.8 Fsq*0.59 0.2 44 +16% -30%
  11.9 Fsq*0.74 0.25 42 +11% -15%
  15.5 Fsq*1 0.34 38 +2% +10%
  16.1 Fsq*1.05 0.36 38 0 +15%
  17.8 Fsq*1.18 = Fpa 0.4 36 -4% +27%
  18.6 Fsq*1.25 0.42 35 -6% +33%
  21.4 Fsq*1.5 0.51 33 -13% +53%
  24 Fsq*1.75 0.59 30 -20% +72%
  26.5 Fsq*2 0.68 28 -26% +89%

Weights in ‘000 t.

Scenarios in italics are not considered consistent with the precautionary approach.

1) SSB(2011) relative to SSB(2010).

2) Calculated landings (2010) relative to TAC 2009 (14 000 t).



Scientific Advice

Single-stock exploitation boundaries

Considering the options below, ICES advises on the basis of exploitation boundaries in relation to the agreed management plan that landings should be less that 14 100 t in 2010.

Exploitation boundaries in relation to the agreed management plan

According to the management plan adopted by the EC in 2007, fishing mortality in 2010 should be reduced by 10% compared to the fishing mortality estimated for the preceding year (F2008=F2009=0.34) with the constraints that the TAC should not be changed by more than 15%. A 10% reduction in fishing mortality corresponds to an F of 0.304 and landings of 14 100t in 2010 which is within the 15% change (TAC 2009=14 000t).

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

The current fishing mortality is within the range that is expected to lead to high long-term yields and low risk to stock depletion.

Exploitation boundaries in relation to precautionary limits

The fishing mortality in 2010 should be no more than Fpa, corresponding to landings of less than 17 800 t.
Management
Management unit: Yes

Management Objectives

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 Advice

Management 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., 2000).

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., 2007)
Biological State and Trend
Exploitation rate: Harvested sustainability
Abundance level: Full reproductive capacity


Spawning biomass in relation to precautionary limits Fishing mortality in relation to precautionary limits Fishing mortality in relation to high long-term yield Fishing mortality in relation to agreed target

Comment

Full reproductive capacity Harvested sustainably Appropriate Above target  



Based on the most recent estimate of SSB (in 2009) and fishing mortality (in 2008), ICES classifies the stock as having full reproductive capacity and is being harvested sustainably. SSB has fluctuated around the precautionary reference points for the last decade, but has increased since 2008 owing to a large incoming 2005 year class and reduced fishing mortality. Fishing mortality has shown a declining trend since 1995 and is currently estimated to be below Fpa. The assessment suggests that the 2006 year class was below average, and 2007 average.
Source of information
 
ICES.2009.Report of the ICES Advisory Committee, 2009. ICES Advice, 2009.
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