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ICES A

DVISORY

C

OMMITTEE

ICES CM 2010/ACOM:13

Report of the Working Group on the Assess- ment of Demersal Stocks in the North Sea and

Skagerrak (WGNSSK)

5 -11 May 2010

ICES Headquarters, Copenhagen

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International Council for the Exploration of the Sea Conseil International pour l’Exploration de la Mer

H. C. Andersens Boulevard 44–46 DK-1553 Copenhagen V

Denmark

Telephone (+45) 33 38 67 00 Telefax (+45) 33 93 42 15 www.ices.dk

[email protected]

Recommended format for purposes of citation:

ICES. 2010. Report of the Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak (WGNSSK), 5 -11 May 2010, ICES Headquarters, Co- penhagen. ICES CM 2010/ACOM:13. 1058 pp.

For permission to reproduce material from this publication, please apply to the Gen- eral Secretary.

The document is a report of an Expert Group under the auspices of the International Council for the Exploration of the Sea and does not necessarily represent the views of the Council.

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Contents

0 Executive summary ... 1

0.1 Working procedures ... 1

0.2 State of the stocks ... 2

0.3 Environmental and ecosystem considerations ... 4

0.4 Mixed-fisheries data collation and modelling ... 4

1 General ... 6

1.1 Terms of Reference ... 6

1.2 InterCatch ... 8

1.3 MSY reference points ... 9

1.3.1 Estimating Fmsy using AD model builder ... 9

1.3.2 Estimating Fmsy using FLR ... 10

1.3.3 Estimating Fmsy using a stand-alone R script ... 10

1.3.4 Estimating Fmsy for Nephrops ... 10

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4 Sandeel in IV (WGNSSK Sep. 2010) ... 172

4.1 General ... 172

4.1.1 Ecosystem aspects ... 172

4.1.2 Fisheries... 172

4.1.3 ICES Advice ... 173

4.1.4 Management ... 173

4.1.5 Catch ... 174

4.2 Sandeel in Area-1 ... 175

4.2.1 Catch data ... 175

4.2.2 Weight at age ... 175

4.2.3 Maturity ... 175

4.2.4 Natural mortality ... 175

4.2.5 Effort and research vessel data ... 175

4.2.6 Data analysis ... 176

4.2.7 Final assessment ... 177

4.2.8 Historic Stock Trends ... 177

4.2.9 Recruitment estimates ... 177

4.2.10Short-term forecasts ... 177

4.2.11Stochastic short-term forecast. ... 178

4.2.12Biological reference points ... 178

4.2.13Quality of the assessment ... 178

4.2.14Status of the Stock ... 178

4.2.15Management Considerations ... 178

4.3.1 Catch data ... 179

4.3.3 Maturity ... 179

4.4.1 Catch data ... 180

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4.4.3 Maturity ... 180

4.5.1 Catch data ... 180

4.6.1 Catch data ... 180

4.7.1 Catch data ... 180

4.8.1 Catch data ... 180

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Annex 05 Recommendations ... 1057

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0 Executive summary

The ICES Working Group for the Assessment of Demersal Stocks in the North Sea and Skagerrak (WGNSSK) met at ICES Headquarters in Copenhagen, Denmark, during 5-11 May 2010. There were 24 participants from 9 countries. The main terms of reference for the Working Group were: to Produce a first draft of the advice on the fish stocks and fisheries under considerations, to update, quality check and report relevant data for the working group, to produce an overview of the sampling activities on a national basis to update the description of major regulatory changes and comment on the potential effects of such changes, to update the assessment of the stocks and to set MSY reference points (F^MSY and MSY B^trigger) The group also met by correspondence in September 2010 to carry out assessments of the sandeel in the North Sea and the second of the biannual assessments of the North Sea Pout; and by correspondence in October of 2010 to provide update forecasts for stocks with survey information collected after the May meeting.

0.1 Working procedures

A number of issues were encountered by the WG to meet its objectives as a result of 1) The addition of new ToRs of primary importance, without any additional time

available to the WG for its May meeting. In particular, the process of providing new reference points for the MSY framework as well as the requested changes to the format of advice sheets were experienced to be highly time-consuming by the WG members.

2) Data quality issues arising from

a. the scheduling of the meeting in May imposing severe stress at some national laboratories as a result of the concentration of the majority of ICES assessment working groups into May

b. the timing of the meeting being close to the date at which survey information from the IBTS quarter 1 survey was first complete

c. a number of important sources of data, including both commercial tuning series and scientific surveys, having no estimates for 2009

d. sometimes severe inconsistencies in the stock trends coming out of the various sources of information.

3) The requirement for update advice in September after the autumn surveys The point raised in a) was the main issue encountered by the WG. The 7 days duration of the WG meeting (which had been scheduled in September 2009, prior to the addition of the ToR related to MSY) is considered appropriate by the WG to address the more routine ToRs dealing with stock assessment and draft advice. However, providing relevant MSY reference points building on a sound a thorough scientific analysis required considerable time during the WG meeting, in spite of significant amount of work performed by a number of WG members prior to the meeting. This is due to the high level of uncertainty and variability linked to the estimation and inter- pretation of long-term yield, which leads to necessary discussions about the number of arbitrary choices that must be made.

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While the WG generally supports the transition of ICES advice to the MSY framework, it is also strongly concerned by the very short time frame that has been allo- cated to its testing prior actual implementation in the advice. It is the opinion of the WG that a thorough and generic review of the outcomes of the various assessment EGs should take place in order to adjust for potential inconsistencies and gaps between stocks, before than this framework can serve as the basis for advice. The WG wishes to underline that the estimates provided this year during the May meeting are considered as preliminary estimates and are likely to be revised during future meet- ings.

With regards to the point raised in b), considerable effort was made by the WG this year to provide the data in due time ahead of the meeting, and improve the quantity and quality of data being included in the InterCatch database. Much effort was also done by the various labs to provide their IBTS estimates to ICES secretariat, timely for the WG. However, a number of data issues still remained; These are described in the relevant parts of the report.

As in previous years, the system of benchmark/update assessments could not be en- tirely followed by the WG. Various changes in data availability and/or consistency raised important issues for the assessment of a number of stocks, leading to some hindrance to produce an updated advice draft during the May meeting

As previously, stock annexes for where available were included in the main report within Appendix 3. The stock annexes will be updated each time that the stock is ana- lysed within a benchmark review.

0.2 State of the stocks

The yields for stocks of Nephrops are fairly stable from year to year. Reported landings for FU 3 (Skagerrak) and FU 4 (Kattegat) have averaged 2500t and 1500t respec- tively since 2000 with relatively little variation. There are no signs of overexploitation in IIIa and given the apparent stability of the stock, the current levels of exploitation appear to be sustainable.

FU 7 (Fladen, 13300 t), FU 8 (Firth of Forth, 2600 t) and landings from outside the FUs (2367t) were all at their highest recorded landings TV surveys for FUs 7, 8 and 9 all decreased in 2009 following several years of increases in observed abundance. The TV survey in FU6 also decreased but this stock is considered to have been in a de- pleted state for the last 3 years due to high levels of fishing effort. Extended effort was performed in order to provide a sound basis for Fmsy reference points, thus re- placing the previous F0.1 standard approach.

The Norway Pout fishery has fluctuated considerably in recent years with full or par- tial closures in 2005, 2006, and 2007 due to very low recruitments in 2003 & 2004. The mid-year update of the Norway Pout assessment shows the stock to be well above Btrigger at the start of 2010 and projected to remain above Btrigger at the start of 2011. The first indications of the 2010 year class (from the 2010 IBTS 3^rd quarter survey), is for a record low recruitment. For this reason, short term forecasts indicate that even in a total absence of fishing mortality in 2011 the stock will fall back below Btrigger by the start of 2012.

The sandeel assessment was benchmarked in September 2010. This resulted from a move in the assessment from a single region to 7 distinct regions, for which analytical assessments can be undertaken for 3 areas (covering the majority of the fishery).

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The sandeel fishery targets 1 and 2 year old fish and by October there are no data upon which to gauge the size of the incoming 0-group. The DTU-Aqua dredge survey undertaken in December will provide sufficient data to estimate the 0-group in areas SA1 (Dogger) and SA2 (SE North Sea). ICES will be in a position to give advice for these areas in late January. The data for SA3 (NE North Sea) is not yet robust enough to provide a reliable estimate and in-year monitoring will probably be required in this area for a few more years and therefore final advice for this stock will be available in April 2011.

Assessment of cod in Sub-area IV and Divisions IIIa and VIId has been particularly difficult for 2009. Estimates of abundance (and consequently mortality) from the two IBTS surveys have continued to diverge to the extent that they are not considered reliable enough to provide a precise assessment of the stock status although the main trends can be estimated with some degree of certainty. Estimated spawning-stock biomass reached a low in 2006 but has subsequently increased. Fishing mortality is now estimated to have declined since 2000 (~ 0.64 in 2007). Recruitment since 2000 has been well below average. The higher levels of discarding observed since 2007 is maintaining the fishery induced mortality at a high level.

The fishing mortality for the stock of Haddock in Subarea IV and Division IIIaN in 2009 is close to the historical low. The decline in abundance of the dominant 1999 year-class has been offset to a certain extent by an improved 2005 year class. How- ever, the reduction in mortality rate has not prevented a continued decline in SSB.

The 2005 year-class is estimated to be quite abundant (39 000 million) and the largest since the 1999 year-class.

The assessment of whiting in Sub-area IV and Division VIId remains problematic in that the historic estimates of biomass derived from surveys exhibit differing trends from those based on catch data. However the recent trends are consistent and the WG accepted that assessment based on data from 1990. There have been substantial revisions in estimates of recent recruitment and, in conjunction with low fishing mortality, the stock is considered to be increasing from its recent low level. Survey estimates of the youngest ages in the recent years appear to have either under- estimated the incoming recruitment or imply that mortality has effectively been zero (which is unlikely).

The assessment of saithe in Sub-areas IV and VI and Division IIIa was hampered by the loss of several tuning indices for the year 2009, hence it was not possible to run an update assessment. As the assessment results of saithe tend to be relatively stable between years catch option for saithe were generated using a 3 year forecast from the assessment results of ICES WGNSSK 2009. Landings of saithe in Sub-areas IV and VI and Division IIIa have been stable for several years at a level well-below the permitted TAC. Fishing mortality has now remained at or below 0.3 (Fmsy) for nine years while SSB has stabilised at around 260 kt. Recruitment is fluctuating about the mean level.

The reported landings for sole in Subarea IV in 2009 (13.9 kt) were almost the same as 2008 (14.0kt). SSB has fluctuated around a moderate-to-low level for several years and is currently around Bpa. Fishing mortality has been generally falling since the late 1990’s and is now below Bpa. However, the updated recruitment estimate based on the latest BTS data in September suggests a high 2009 year class abundance, and hence a potential for slight TAC increase compared to the June advice.

Landings of plaice in Subarea IV increased slightly in 2009 but are low compared to historical levels. SSB has increased dramatically over the last three years, well above

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Bpa and is currently close to the historical maximum. Fishing mortality has decreased to its lowest observed level. Recent year-class strength has been at the long-term mean.

Discrepancies between catch-at-age based analyses and survey-based analyses have still prevented the WG from providing a definitive assessment the state of plaice in Division VIId, in spite of significant improvements gained during the benchmark procedure in 2010. F has been stable for the last five years. The spawning stock biomass has followed a stepped decline in the last 10 years, following a peek generated by the strong 1996 year class. The current level of SSB is stable at a low level.

It has been postulated that a mismatch between the biological entity of the Plaice stock in Division IIIa and the defined management area might exist. Most catches are taken at the boundary with the North Sea where some mixing with North Sea plaice may occur, and this may undermine the quality of age-based information. Fur- thermore, the limited survey coverage of main fishing grounds has regularly prevented the presentation of a stock assessment. There is evidence for sustained biomass in the Kattegat and in Eastern Skagerrak, where the populations intermingle between both areas. But the status of the stock in the Southwestern Skagerrak, cannot be determined.

Landings for sole in Division VIId have fluctuated around a mean level for many years, and show no significant trends. Fishing mortality has been stable between 2000 and 2005 around Fpa. In the last 4 years fishing mortality has increased to values between Fpa (0.4) and Flim (0.57). The spawning stock biomass has been stable for most of the time series and SSB is presently well above Bpa. The strong 2004 and 2005 year class increased SSB to around record high level of the time series in 2008. The potentially very strong 2008 year class could even increase SSB in the future.

0.3 Environmental and ecosystem considerations

The WG was asked to summarise, when relevant, species interactions and ecosystem drivers, and ecosystem effects of fisheries. Potential updates of relevant information were done within each stock section, but no significant changes have been considered compared to the previous reports. The main adjustment so far has been the inclusion of natural mortality estimates accounting for multispecies interactions, and provided by ICEs WGSAM, in the single-stock assessment models.

Beside this, only few quantitative modifications have been made so far to assessments or forecasts to account for environmental information. As a general basis, the lack of firm understanding on causative mechanisms linking fish stocks and the envi- ronment, the poor predictability of ecosystems and the difficult coupling between environmental models and assessment models are the main reasons advocated to explain this,. The exceptions were those stocks for which recent recruitment is clearly different (in some way) to historical recruitment, in which case the recent recruitment estimates only were used to generate recruitment forecasts. Apart from this, the report is limited to comments on potentially-important ecosystem impacts.

0.4 Mixed-fisheries data collation and modelling

Since 2006, most of the analyses of mixed-fisheries interactions were undertaken outside of the WGNSSK, both within the Study Group on Simple Mixed-Fisheries Man- agement models (ICES SGMIXMAN) which met between 2006 and 2008 (ICES, 2006- 2008), and within various research projects, but each time with the demersal fisheries of the North Sea as the primary case study. This resulted of the setup of a workshop

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(ICES WKMIXFISH 2009) and a adhoc group (ICES AGMIXNS 2009) which aimed at providing a draft mixed-fisheries advice for the North Sea, based on single-stock exploitation boundaries produced by WGNSSK in 2009. The results of this were presented to WGNSSK during its May meeting this year. This workshop reconvened in August 2010 as Working Group (WGMIXFISH), and the information collected by this group in terms of trends in catches and fishing effort have been summarised in the overview section. Mixed fisheries issues are also raised in management considerations for the individual stocks where appropriate.

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1 General

1.1 Terms of Reference

The Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak [WGNSSK] (Chaired by: Clara Ulrich*, Denmark and Ewen Bell*, UK) met at ICES HQ, 5–11 May 2010 to:

a ) address generic ToRs for Fish Stock Assessment Working Groups (see table below). The Sandeel and Norway pout assessments shall be developed by correspondence;

The assessments will be carried out on the basis of the stock annex in National Labo- ratories, prior to the meeting. This will be coordinated as indicated in the table below.

Material and data relevant for the meeting must be available to the group no later than 14 days prior to the starting date.

WGNSSK will report by 18 May and 17 September 2010 (Sandeel/Norway pout) for the attention of ACOM. The group will report on the AGCREMP 2008 procedure on reopening of the advice before 8 October and will report on reopened advice before 29 October.

Fish

Stock Stock Name Stock Coor-

dinator Assessment

Coord. 1 Assessment Coord. 2

Perform assess-

ment

Advice

cod-

347d Cod in Subarea IV, Divison

VIId & Division IIIa (Skagerrak) UK(Scotland) UK(England) Denmark Y Update had-

34 Haddock in Subarea IV (North

Sea) and Division IIIa UK(Scotland) UK(Scotland) UK(Englan

d) Y Update

nep-5 Nephrops in Division IVbc (Bot-

ney Gut - Silver Pit, FU 5) Denmark Denmark Denmark Y Update nep-6 Nephrops in Division IVb (Farn

Deeps, FU 6) UK(England) UK(England) Denmark Y Update

nep-7 Nephrops in Division IVa

(Fladen Ground, FU 7) UK(Scotland) UK(Scotland) Denmark Y Update nep-8 Nephrops in Division IVb (Firth

of Forth, FU8) UK(Scotland) UK(Scotland) Denmark Y Update nep-9 Nephrops in Division IVa (Mo-

ray Firth, FU9) UK(Scotland) UK(Scotland) Denmark Y Update nep-

10

Nephrops in Division IVa (Noup,

FU 10) UK(Scotland) UK(Scotland) Denmark Y Update

nep-

32 Nephrops in Division IVa (Nor-

wegian Deeps, FU 32) Norway Norway Denmark Y Update

nep-

33 Nephrops in Division IVb (Off

Horn Reef, FU 33) Denmark Denmark Norway Y Update

nep-

iiia Nephrops in Division IIIa

(Skagerak Kattegat, FU 3,4) Denmark Denmark Sweden Y Update nop-

34 Norway Pout in Subarea IV and

Division IIIa Denmark Denmark Norway Y Update

ple-

eche Plaice in Division VIId (Eastern

Channel) France France Belgium Y Update

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ple-

kask Plaice in Division IIIa

(Skagerrak - Kattegat) Denmark Denmark Sweden Y Same ad-

vice as last year ple-

nsea Plaice Subarea IV (North Sea) Netherlands Netherlands Belgium Y Update sai-

3a46

Saithe in Subarea IV (North Sea) Division IIIa West (Skagerrak) and Subarea VI (West of Scot- land and Rockall)

Norway Norway Germany Y

Update

san-

nsea Sandeel in Subarea IV exclud-

ing the Shetland area Denmark Denmark Norway Y Update

san- shet

Sandeel in Division IVa North of 59° N and West of 0 ° E – (Shetland area)

UK/ Den-

mark N Catch sta-

tistics only san-

kask Sandeel in Division IIIa (Skager-

rak – Kattegatt DK N Catch statis-

tics only san-

scow Sandeel in Division VIa DK N Catch sta-

tistics only sol-

eche

Sole in Division VIId (Eastern

Channel) Belgium Belgium France Y Update

sol-

nsea Sole in Subarea IV (North Sea) Netherlands Netherlands Belgium Y Update whg-

47d

Whiting Subarea IV (North Sea)

& Division VIId (Eastern Chan-

nel) UK(Scotland) UK(Scotland) UK(Englan

d) Y Update

whg-

kask Whiting in Division IIIa

(Skagerrak - Kattegat) Sweden Sweden Denmark N Catch sta- tistics only

The generic ToRs applying to assessment Expert Groups were the following : The working group should focus on:

ToRs a) to h) for stocks that will have advice ToRs b) to f) and h) for stocks with

,

same advice as last year ToRs b) to c) and f) for stocks with

. no advice

a) Produce a first draft of the advice on the fish stocks and fisheries under considerations and the regional overview according to ACOM guidelines.

.

b) Update, quality check and report relevant data for the working group:

b ) Load fisheries data on effort and catches (landings, discards, bycatch, including estimates of misreporting when appropriate) in the IN- TERCATCH database by fisheries/fleets. Data should be provided to the data coordinators at deadlines specified in the ToRs of the individual groups. Data submitted after the deadlines can be incorporated in the assessments at the discretion of the Expert Group chair;

c ) Abundance survey results;

d ) Environmental drivers.

e ) Propose specific actions to be taken to improve the quality of the data (including improvements in data collection).

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c) Produce an overview of the sampling activities on a national basis based on the INTERCATCH database);

d) In cooperation with the Secretariat, update the description of major regulatory changes (technical measures, TACs, effort control and management plans) and comment on the potential effects of such changes including the effects of newly agreed management and recovery plans.

e) For each stock update the assessment by applying the agreed assessment method (analytical, forecast or trends indicators) as described in the stock annex. If no stock annex is available this should be prepared prior to the meeting.

f) Produce a brief report of the work carried out by the Working Group. This report should summarise for the stocks and fisheries where the item is relevant:

Input data (including information from the fishing industry and NGO that is pertinent to the assessments and projections);

f ) Where misreporting of catches is significant, provide qualitative and where possible quantitative information and describe the methods used to obtain the information;

g ) Stock status and 2011 catch options;

h ) Historical performance of the assessment and brief description of quality issues with the assessment;

i ) Mixed fisheries overview and considerations;

j ) Species interaction effects and ecosystem drivers;

k ) Ecosystem effects of fisheries;

l ) Effects of regulatory changes on the assessment or projections;

g) Where appropriate, check for the need to reopen the advice in autumn based on the new survey information and the guidelines in AGCREFA

h) Set MSY reference points (FMSY and MSY B^trigger) according to the ICES MSY framework and following the guidelines developed by WKFRAME.

1.2 InterCatch

The InterCatch database has historically not been widely used by the WGNSSK. In particular for the stocks including discards estimates, a repeated concern has been the incapacity of InterCatch to raise discards data to sampling strata with missing data (e.g. countries not providing discards estimates, where estimates must then be ap- proximated externally based on landings figures). In 2009, only one stock was using InterCatch up to the final level, and some data sets were also uploaded into the database for some other stocks, but not used for generating assessment data.

During the 2010 meeting, a specific effort was made to try improving the coverage of the data uploaded in InterCatch, through short workshops dedicated to particular stocks in order to identify the potential issues in the use of InterCatch. It has though not been possible to spend much time actually uploading new data during the meeting itself because of time pressure, but it is expected that further follow-up will take place intersessionally and improvements will be achieved by 2011. The actual level of InterCatch use by stock is described within each stock section.

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1.3 MSY reference points

The WGNSSK spent a considerable share of its May meeting addressing this ToR, which was added after decision on the 7-days duration of the meeting. Important work had been prepared by some WG members in advance of the meeting, in order to provide generic exploratory and estimation tools to the group (see below). Many preliminary analyses could be performed using these tools on a variety of stocks, and improving the scripts available alongside.

The WG considered there to be fundamental differences between the PA framework, which worked out limits reference points largely based on observed historical data, and the MSY framework, which builds on fuzzier potential future targets. There is thus more inherent uncertainty in Fmsy than in Fpa. However, the limited duration of the WG meeting did not allow sufficient analyses of the preliminary results obtained. As underlined during ICES WKFRAME, and as experienced by the WG members, the MSY reference points estimates are highly dependent of the underlying hypotheses. In a single-stock context, Fmsy estimates are mostly sensitive to:

• The form of the Stock-Recruitment relationships chosen, and the choice of the fitting algorithm and software,

• The number of years used for averaging the weight-at-age and selectivity-at- age values in a deterministic approach, or for estimating the variability around these in a stochastic context.

Because of this inherent uncertainty, the WG does not consider that the results included in the stock sections are definitive. In particular, the priority has been towards the estimation of Fmsy, and little time has been left to the estimation of MSY Btrigger.

Therefore, it is inevitable that revisions will occur before the next WG meeting if further work takes place intersessionally and that some changes may be substantial. It is the WG opinion that 1-2 additional meeting days would have allowed more appropriate analyses and adequate discussion of the results obtained, and would have lead to more robust estimates in the first place.

Four different approaches were developed by WG members, largely developed around ICES WKFRAME and further used and developed during the WGNSSK meeting. The first three deal with stocks for which age-based information exist, and present many similarities in their standard combinations of YPR, SRR and SPR relationships. The fourth one is an approach specifically developed for Nephrops stocks ahead of the WG meeting.

It is to be noted that the approaches 1, 2 and 4 were later used by the WGCSE, which met after WGNSSK. The four approaches are briefly summarised below :

1.3.1 Estimating Fmsy using AD model builder

(Further information available from Jose DeOliveira, Timothy Earl, Chris Darby) AD Model Builder (admb-project.org) is a highly efficient, freely available software for implementing non-linear statistical models. One of the principal advantages of this software is the ability to carry out automatic differentiation which speeds up the convergence of any model fit and calculates the derivatives as accurately as if the analytical derivatives were implemented. It also produces several different estimates of the uncertainties of model parameters and selected derived quantities.

During ICES WKFRAME, a suite of programmes in AD model builder was developed, in order to estimate Fmsy and some components of its uncertainty from the

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outputs of a standard ICES stock assessment, and in particular the ICES *.sum and

*.sen files. The suite is described in details in ICES WKFRAME (2010), Case Study 2.

This suite of programs was successfully tested and used for a number of stocks during WGNSSK meeting, and served as the primary tool for providing final Fmsy estimates.

1.3.2 Estimating Fmsy using FLR

(Further information available from Clara Ulrich, John Simmonds, Jan-Jaap Poos) A number of R scripts using the FLR framework (www.flr-project.org) were developed ahead and during ICES WKFRAME 2010 (Case Studies 3 and 6). These scripts were later merged into a single generic R-FLR program (Finding Fmsy with FLR_v4.r), in order to explore and compare various methods for estimating Fmsy using a single FLStock object as input. As no adequate documentation is to be found in ICES WKFRAME (2010), this script is briefly summarised here. The script investi- gates the following steps:

• Fitting and comparing various Stock Recruitment Relationships with FLSR,

• Estimating usual deterministic biological Reference Points (Fmax, F0.1, Fspr30%, Fmsy) using these SRR and the standard equilibrium equations from the FLBRP package,

• Exploring the variability in time of these reference points, using the default 3-years average for weight-at-age and selectivity-at-age or using a longer time span for averaging

• Fitting stochastic SRR, either through bootstrapping of the variance- covariance matrix of the parameters of the SRR (if positive), or alterna- tively using boostrapping og jaknifing of the observations

• Estimating stochastic BRP using equilibrium equations as above (Analyti- cal estimation of Fmsy)

• Estimating maximum yield out of long-term projections of the stock under various levels of fishing mortality (Empirical estimation of Fmsy)

This flexible script was tested and further developed on a number of stocks during WGNSSK, and served mostly as a valuable exploratory tool for understanding the importance of the choices around input parameters (cf figure 1.3.1).

1.3.3 Estimating Fmsy using a stand-alone R script (Further information available from Coby Needle)

An alternative R script was also developed around ICES WKFRAME 2010 (Case Study 5), using a analytical combination of fitted stock-recruit, yield-per-recruit and SSB-per-recruit curves. This script was used during WGNSSK for estimating Fmsy for the haddock stock, and is thus described in section 13.7

1.3.4 Estimating Fmsy for Nephrops

(Further information available from Ewen Bell and Helen Dobby)

The different Nephrops stocks (Functional Units, FUs) for which ICES delivers advice cover a wide range of fisheries including single, twin, triple and even quadruple trawls, creeling (potting), with activity covering inshore and offshore grounds. The timing of these fisheries varies, which due to the different emergence patterns of the

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different sexes due to moulting and egg-brooding, leads to very different relative exploitation rates (between the sexes) in different FUs. Local ecosystem type is also highly variable with a range of Nephrops densities, different composition and density of organisms competing for space as well as different assemblages of predators.

Ground types also cover a wide range including large contiguous sediment beds, fragmented patches of suitable sediment in rocky areas, shallow sea-lochs and patches of mud on relatively deep shelf-edges. Given these differences in fishery and ecology it is inevitable that estimates of the exploitation rate leading to long term MSY will vary between the FUs, the difficulty for scientists is how to estimate these rates given the inherent difficulty in assessing crustacean stocks, for which no practi- cal method routine of age determination is available. Some assessments take the observed length frequency data and slice it into age-classes according to the Von- Bertalanffy growth parameters. These numbers at age are then taken forward into standard stock-assessment packages. This practice was ceased in 2005 within this Group due to concerns over both the reliability of reported landings in some FUs (particularly the UK fisheries) and the use of the ‘pseudo’ age-structured data in an age-based assessment was deemed untenable. As a result of this, no dynamic population model is fitted to the data and consequently there are no estimates of spawning stock and recruitment which are fundamental to the determination of F^msy and proxies for Fmsy must therefore be sought. ICES WKFRAME (ICES 2010) made several recommendations for defining F^msy proxies where no direct estimation of F^msy was possible (i.e. for stocks for which there is no analytic assessment, but length- or age- structured catch data are available). The suggested approach focussed on per-recruit analysis with the following guidelines:

• Use input parameters which reflects the current situation (selection and discard ogive, maturity and weight at age/length)

• If there is clear peak at low F in the YPR analysis and no evidence of recruitment dependence on biomass, then F^max may be an appropriate proxy.

• Where Fmax, is undefined then F0.1 might be considered as a ‘lower bound’

to the range of F suitable for F^msy, as it is assumed to be low risk.

• Spawning biomass per recruit analysis should be routinely evaluated in addition to YPR. There is not a single level of % SPR that is optimal for all stocks and the proposal for F^msy should include some consideration of life history. Studies by Clark (1991, 1993) concluded that F35% and higher were robust proxies for Fmsy, considering uncertainty in stock-recruitment functions and or recruitment variability.

• Conduct a sensitivity analysis to the input parameters and consider the variability of estimates over time.

WKFRAME also emphasized that given the substantial amount of data exploration and sensitivity analysis that would be required in defining appropriate F^msy proxies, the process was likely to be iterative and that ICES and its clients should be willing to work with recursively updated targets.

Within the North Sea, Skagerrak and Kattegat areas, assessment of Nephrops stocks falls into three categories, those with TV surveys, those monitored by LPUE / mean size and those with only landing information. Only for those stocks with TV surveys is the catch advice determined by an exploitation rate, advice for the other stocks is based on changes to landings. For those stocks with a TV survey, the Harvest Rates (removals divided by abundance as estimated by the TV survey) associated with fishing at F^0.1 and F^max were estimated at the 2009 benchmark meeting WKNEPH (ICES

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2009). In response to the recommendations of WKFRAME, estimates of F^35%SpR and the corresponding Harvest Rate have also been determined and these estimates typically lie between the estimates of F^0.1 and F^max. Suggestions for a TV-abundance based proxy for B^trigger have been made on the basis of the lowest observed TV-abundance (median survey value) unless the stock has shown signs of stress at a higher TV- abundance in which case this value becomes B^trigger.

The remaining challenge is determining which F^msy proxy is appropriate for which stock and this becomes an exercise in expert judgement based upon knowledge of the fishery and the ecosystem. The implications for exploitation rate can vary considerably depending upon which proxy is chosen (F^0.1, F^35%SpR or F^max). Given that there is often a distinct difference in the exploitation rate between the two sexes (males>females) it is usually impossible to simultaneously achieve the target fishing mortality on both sexes (i.e. the stock cannot be fished such that both the male and female YPRs are maximised simultaneously). Different F^msyproxies are therefore obtained by conducting male, female or a combined sex per-recruit analysis. The following text-table shows the F-multipliers required to achieve various Fmsy proxies for the sexes of a typical Nephrops stock (FU 8 in this example), the Harvest Rates which correspond to those F multipliers and the resulting level of spawner-per-recruit expressed as a percentage of the virgin level.

Fbar(20-40 mm)

HR (%) SPR (%)

Fmult Male Female Male Female Combined

F0.1

Male 0.2 0.13 0.06 7.47 42.33 64.50 51.72

Female 0.43 0.29 0.13 14.23 22.96 44.80 32.21

Combined 0.24 0.16 0.07 8.75 37.29 60.04 46.92

F^max

Male 0.36 0.24 0.11 12.31 26.94 49.50 36.49

Female 0.81 0.54 0.24 23.38 12.11 28.95 19.24

Combined 0.46 0.31 0.14 15.03 21.55 43.02 30.64

F^35%SpR

Male 0.27 0.18 0.08 9.67 34.13 57.04 43.83

Female 0.63 0.42 0.19 19.28 15.79 34.96 23.91

Combined 0.39 0.26 0.12 13.15 25.10 47.38 34.53

The yield per-recruit and spawner per-recruit plots for this stock are shown in figure 1.3.2, emphasising the disparity in f-multipliers required to achieve F^max. The general tradition in fisheries science is to concentrate on the mortality on females because in a freely distributing population, one male should be able to fertilise several females and therefore a higher exploitation rate on males should not affect spawning potential. Nephrops are slightly different in that the adults have a fairly limited range of movement (100’s of metres) and therefore very low densities of males could result in sperm limitation. Ensuring that the fishing mortality target on males is not exceeded will usually result in an under-utilisation of the females, but due to the faster growth rate of males the under-utilisation of total yield is not likely to be large. The alternative, of trying to achieve Fmsy on females, carries a potentially serious risk to the pro- duction of future recruits and may result in very high exploitation of males. The Working Group suggested that a combined sex F^msy proxy should be considered appropriate provided that the resulting percentage of virgin spawner per-recruit for males does not fall below 20%. In such a case the male Fmsy proxy should be chosen in preference to the combined proxy.

In cases where recruitment rates are typically low and/or highly variable then a more cautious F^msy proxy would be appropriate as the stock may have reduced resilience to

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periods of poor recruitment and in this case F^0.1 is recommended. Conversely where recruitment rates are considered to be regularly high and the stock appears to have supported a harvest rate at or above F^max, (or in the case of a short TV time series a particular landing level) without showing signs of recruitment overfishing, then F^max is recommended. In all other cases F35%SpR should deliver high long term yield with a low probability of recruitment overfishing and is recommended as the “default”

value.

In order to assist communication of the decision process the following bullet list is suggested as a standard checklist for describing the rationale behind the choice of a particular F^msy.

• Describe the absolute density. Is it high (i.e. >1 per m²), medium (i.e. 1.0 – 0.2 per m²) or low (i.e. <0.2 per m²)

• Variability in density. Is there large interannual variability, spatial com- plexity?

• Understanding of biological parameters. Is the growth rate particularly fast or slow, high or low estimates of natural mortality?

• Fishery timing & operation. Is there a strong seasonal pattern leading to different exploitation rates on the sexes, does this pattern vary much between years?

• Observed Harvest Rate or landings compared to stock status. Is the harvest rate consistently around or above F^max? Have landings been stable?

Have the indicators of stock status shown signs of difficulty?

Accompanying this text should be a table listing the F^msy proxies F^max, F^35%SpR and F^0.1 for males and females, the Harvest Rates they correspond to along with the implied

%spawner per recruit for males and females.

Following changes to UK legislation in 2006 the reliability of UK landings data is considered to have significantly improved (representing ~80% of the landings). Provided that this is both true and continues into the future, assessment scientists will eventu- ally have data which could be used to parameterise dynamic stock assessment models which in turn will enable estimation of F^msy directly rather than have to rely upon proxies thereof. Until this point the decision of which F^msy proxy is suitable for which FU will inherently be a subjective process but the process outlined above should provide sufficient justification to support the decision.

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Figure 1.3.1. Example of variability in time of standard Biological Reference Points using 3-years average for input parameters (Top left), 4-years average (Bottom left), 5-years average (Top right) and 10-years average (Bottom right) for saithe, using a Beverton-Holt Stock Recruitment relation- ships.

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Figure 1.3.2. Yield-per-recruit and spawning stock biomass-per recruit for males, females (dotted line) and combined (bold) with F^max and F^35%spr reference points.

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2 Overview

The overview section was not updated in the 2009 WGNSSK report.

Some parts of the overview for the 2008 WGNSSK report were updated in 2010, with additional insights from other EGs.

2.1 Stocks in the North Sea (Subarea IV) 2.1.1 Introduction

The demersal fisheries in the North Sea can be categorised as a) human consumption fisheries, and b) industrial fisheries which land the majority of their catch for reduction purposes. Demersal human consumption fisheries usually either target a mixture of roundfish species (cod, haddock, whiting), a mixture of flatfish species (plaice and sole) with a by-catch of roundfish, or Nephrops with a bycatch of roundfish and flatfish. A fishery directed at saithe exists along the shelf edge. Landings used by the WG for each North Sea stock are summarised in Table 2.1.1.

The industrial fisheries which used to dominate the North Sea catch in weight have become much less prominent. Human consumption landings have steadily declined over the last 30 years, with an intermediate high in the early 80’s. The landings of the industrial fisheries show the largest annual variations, resulting from variable recruitment and the short life span of the main target species. The total demersal landings from the North Sea reached over 2 million t in 1974, and have been around 1.5 million t in the 1990s.

For some stocks, the North Sea assessment area may also cover other regions adjacent to ICES Sub-area IV. Thus, combined assessments were made for cod including IIIaN (Skagerrak) and VIId, for haddock and Norway pout including IIIa, for whiting including VIId, and for saithe including IIIa and VI. Advice for the sandeel stocks at the Shetland Islands and in IIIa is provided separately by ICES, and there are no analytic assessments for them. The state of Nephrops stocks are evaluated on the basis of dis- crete Functional Units (FU) on which estimates of appropriate removals are founded.

Quota management for Nephrops is still carried out at the Sub-Area and Division level, however.

Biological interactions are not dynamically incorporated in the assessments or the forecasts for the North Sea stocks. However, average values of natural mortalities estimated by multispecies assessments for cod, haddock, whiting and sandeel are incorporated in the assessments of these species, and exploratory runs using updated natural mortality estimates are presented for some stocks.

Gear types vary between fisheries. Human consumption fisheries use otter trawls, pair trawls, Nephrops trawls, seines, gill nets, or beam trawls, while industrial fisheries use small meshed otter trawls. Trends in reported effort in the major fleets fishing in the North Sea are described annually by the STECF¹

1 Scientific, Technical and Economic Committee for Fisheries (STECF) Report of the SG-MOS-09-05 Working Group on Fishing Effort Regime Edited by Nick Bailey & Hans-Joachim Rätz 28 september – 2 october 2009, Ispra, Italy.

; Quantitative description of the main fleets and fisheries and their recent trends was also summarised in the ICES WG report on Mixed Fisheries Advice for the North Sea (ICES WGMIXFISH 2010),

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largely based on the data collected for STECF SGMOS 10-05 for the evaluation of effort management, with additional data provided for some countries. The main trends are summarised below:

Some discards data were available for some of the fleet segments. French data are missing for 2009 in these tables.

The data distinguish between two basic concepts, the Fleet (or fleet segment), and the Métier. Their definition has evolved with time, but the most recent official definitions are those from the CEC’s Data Collection Framework (DCF, Reg. (EC) No 949/2008), which we adopt here:

• A Fleet segment is a group of vessels with the same length class and pre- dominant fishing gear during the year. Vessels may have different fishing activities during the reference period, but might be classified in only one fleet segment.

• A Métier is a group of fishing operations targeting a similar (assemblage of) species, using similar gear, during the same period of the year and/or within the same area and which are characterized by a similar exploitation pattern.

Fleets and métiers were defined to match with the available economic data and the cod long term management plan. WGMIXFISH defined 27 national fleets from nine countries. These fleets engaged in one to five different métiers each, resulting in 73 combinations of country*fleet*métier catching cod, haddock, whiting, saithe, plaice, sole and Nephrops.

ICES WGMIXFISH produced a number of synthetic figures describing main trends, between 2003 and 2009, of effort by fleet in absolute levels (Figure 2.1.2.1) and relative trends (Figure 2.1.2.2), effort share by fleet (Figure 2.1.2.3) and landings by fleet and stock (Figure 2.1.2.4). Data are also summarized by main metier and stock in the table 2.1.2.4.

The total effort (expressed in KW*days at sea) for these 27 fleets decreased by 25%

between 2003 and 2009, with largest decreases between 2006 and 2008, but less that 2% decrease between 2008 and 2009.

2.1.2 Main management regulations

The near-collapse of the North Sea cod stock in the beginning of the 2000s led to the introduction of effort restrictions alongside TACs as a management measure within EU fisheries. There has also been an increasing use of single-species multi-annual management plans, partly in relation to cod recovery, but also more generally. These management frames can be summarised as such :

2.1.2.1 Effort limitations

For vessels registered in EU member states, effort restrictions in terms of days at sea were introduced in 2003 and subsequently revised annually (Table 2.1.2.1). Initially days at sea allowances were defined by calendar month. From 2006 the limit was defined on an annual basis. The maximum number of days a fishing vessel could be absent from port varied according to gear type, mesh size (where applicable) and region. A complex system of ‘special conditions’ (SPECONs) developed upon request from the Member States, whereby vessels could qualify for extra days at sea if special conditions (specified in the Annexes) were met. The evolution of the number of gear categories and special conditions used in these regulations are given in Table 2.1.2.2,

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illustrating the trend towards increasingly detailed micromanagement that has taken place until 2008. A detailed description of these categories as well as the corresponding days at sea can be found in STECF (2008).

In 2008 the system was radically redesigned. For 2009, a total effort limit (measured in kW days) was set and divided up between the various nation’s fleet effort categories. The baselines assigned in 2009 were based on track record per fleet effort cate- gory averaged over 2004-2006 or 2005-2007 depending on national preference. Table 2.1.2.3 lists the new fleet effort categories and shows how they map to the previous gear groups. The effort allocations available by nation and gear are given in Appen- dix 1A of Annex IIa of Council Regulation 43/2009. In relation to this, some member states have implemented real-time closure schemes. The closures apply to areas with high cod catch rates with the intention that closing these will lead to an overall reduction in the catchability of cod.

In addition to the restrictions on effort, a number of other measures have been introduced during 2009 to help ensure that the cod quota is not exceeded. For instance, if a nation’s uptake of its cod quota reaches 90% on or before 15 November 2009, this will trigger a requirement for that nation’s vessels to use highly selective gears (Regula- tion 43/2009, Annexe III, para. 5a). This is associated with a ban on high-grading (Regulation 43/2009, Annexe III, para. 5c).

2.1.2.2 Stock-based management plans

Cod, saithe, haddock, plaice and sole are now subject to multi-annual management plans (the latter two, being EU plans, not EU-Norway agreements) . These plans all consist of harvest rules to derive annual TACs depending on the state of the stock relative to biomass reference points and target fishing mortality. The harvest rules also impose constraints on the annual percentage change in TAC. These plans have been discussed, evaluated and adopted on a stock-by-stock basis, involving different timing, procedures, stakeholders and scientists involved, and have never been evaluated in an integrated mixed-fisheries approach (ICES WKMIXFISH 2009). The technical basis of the individual management plans is detailed in the relevant stock section.

2.1.3 Additional Technical measures

The national management measures with regard to the implementation of the available quota in the fisheries differ between species and countries. The industrial fisheries are subject to regulations for the by-catches of other species (e.g. herring, whiting, haddock, cod). Quotas for these fisheries have only recently been introduced. Tech- nical measures relevant to each stock are listed in each stock section – for conven- ience, the recent history of technical measures in the area as a whole is also summarised here.

Until 2001, the technical measures applicable to the North Sea demersal stocks in EU waters were laid down in the Council Regulation (EC) No 850/98. Additional technical measures have been established in 2001 by the Commission Regulation (EC) No 2056/2001, for the recovery of the stocks of cod in the North Sea and to the west of Scotland. In 2001, an emergency measure was enforced by the Commission to enhance cod spawning (Commission Regulation EC No 259/2001).

2.1.3.1 Minimum landing size

“Undersized marine organisms must not be retained on board or be transhipped, landed, transported, stored, sold, displayed or offered for sale, but must be discarded

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immediately to the sea” (EC 850/98). Minimum landing sizes in the North Sea are the same as in all European waters (except in Skagerrak and Kattegat, where minimum sizes are slightly smaller for fin fish and larger for Nephrops). The value for demersal stocks is shown below.

Species MLS

Cod 35 cm

Haddock 30 cm

Saithe 35 cm

Whiting 27 cm

Sole 24 cm

Plaice 27 cm

Nephrops 24mm ( carapace length)

2.1.3.2 Minimum mesh size

Regulations on mesh sizes are more complex than those on landing sizes, as they differ depending on gears used, target species and fishing areas. Many other accompanying measures are implemented simultaneously with mesh sizes. They include regulations on gear dimensions (e.g. number of meshes on the circumference), square-meshed panels, and netting material. The most relevant mesh size regulations of EC No 2056/2001 are presented below.

Towed nets excluding beam trawls

Since January 2002, the minimum mesh size for towed nets fishing for human consumption demersal species in the North Sea is 120 mm. There are however many derogations to this general rule, and the most important are given below:

• Nephrops fishing. It is possible to use a mesh size in range 70-99 mm, pro- vided catches retained on board consist of at least 30% of Nephrops. How- ever, the net needs to be equipped with a 80 mm square-meshed panel if a mesh size of 70-99 mm is to be used in the North Sea and if a mesh size of 70-89 mm is to be used in the Skagerrak and Kattegatt the codend has to be square meshed.

• Saithe fishing. It is possible to use a mesh size range of 110-119 mm, pro- vided catches consist of at least 70% of saithe and less than 3% of cod. This exception however does not apply to Norwegian waters, where the minimum mesh size for all human consumption fishing is 120 mm. Since Janu- ary 2002 Norwegian trawlers (human consumption) have had a minimum mesh size of 120 mm in EU-waters. However, since August 2004 they have been allowed to use down to 110 mm mesh size in EU-waters (but minimum mesh size is still 120 mm in Norwegian waters).

• Fishing for other stocks. It is possible to use a mesh size range of 100-119 mm, provided the net is equipped with a square-meshed panel of at least 90 mm mesh size and the catch composition retained on board consists of no more than 3 % of cod.

• 2002 exemption. In 2002 only, it was possible to use a mesh size range of 110-119 mm, provided catches retained on board consist of at least 50% of a mixture of haddock, whiting, plaice sole, lemon sole, skates and anglerfish, and no more than 25% of cod.

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Beam trawls

• Northern North Sea. It is prohibited to use any beam trawl of mesh size range 32 to 119 mm in that part of ICES Sub-area IV to the north of 56° 00' N. However, it is permitted to use any beam trawl of mesh size range 100 to 119 mm within the area enclosed by the east coast of the United King- dom between 55° 00' N and 56° 00' N and by straight lines sequentially joining the following geographical coordinates: a point on the east coast of the United Kingdom at 55° 00' N, 55° 00' N 05° 00' E, 56° 00' N 05° 00' E, a point on the east coast of the United Kingdom at 56° 00' N, provided that the catches taken within this area with such a fishing gear and retained on board consist of no more than 5 % of cod.

• Southern North Sea. It is possible to fish for sole south of 56° N with 80-99 mm meshes in the cod end, provided that at least 40 % of the catch is sole, and no more than 5 % of the catch is composed of cod, haddock and saithe.

Combined nets

It is prohibited to simultaneously carry on board beam trawls of more than two of the mesh size ranges 32 to 99 mm, 100 to 119 mm and equal to or greater than 120 mm.

Fixed gears

The minimum mesh size of fixed gears is of 140 mm when targeting cod, that is when the proportion of cod catches retained exceeds 30% of total catches.

2.1.3.3 Closed areas Twelve mile zone

Beam trawling is not allowed in a 12 nm wide zone along the British coast, except for vessel having an engine power not exceeding 221 kW and an overall length of 24 m maximum. In the 12 mile zone extending from the French coast at 51°N to Hirtshals in Denmark trawling is not allowed to vessels over 8m overall length. However, otter trawling is allowed to vessels of maximum 221 kW and 24 m overall length, provided that catches of plaice and sole do not exceed 5% of the total catch. Beam trawling is only allowed to vessels included in a list that has been drawn up for the purposes.

The number of vessels on this list is bound to a maximum, but the vessels on it may be replaced by other ones, provided that their engine power does not exceed 221 kW and their overall length is 24 m maximum. Vessels on the list are allowed to fish within the twelve miles zone with beam trawls having an aggregate width of 9 m maximum. To this rule there is a further derogation for vessels having shrimping as their main occupation. Such vessels may be included in annually revised second list and are allowed to use beam trawls exceeding 9 m total width.

Plaice box

To reduce the discarding of plaice in the nursery grounds along the continental coast of the North Sea, an area between 53°N and 57°N has been closed to fishing for trawlers with engine power of more than 221 kw (300 hp) in the second and third quarter since 1989, and for the whole year since 1995. Beare et al. (2010) conducted a thorough analysis of the potential effect of the plaice box on the stock of plaice, and concluded that no significant effect, neither positive nor negative, could be related to the implementation of the plaice box.

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Cod box

An emergency measure to enhance cod spawning in the North Sea was enforced in January 2001. The EU and Norway agreed on a temporary closure of the demersal fishery in the main spawning grounds from February 15 until 30 April 2001.

Sandeel box

In the light of studies linking low sandeel availability to poor breeding success of kittiwake, ICES advised in 2000 for a closure of the sandeel fisheries in the Firth of Forth area east of Scotland. All commercial fishing was excluded, except for a maximum of 10 boat days in each of May and June for stock monitoring purposes. The closure was initially designated to last for three years but has been repeatedly extended and remains in force. The level of effort of the monitoring fishery was increased in 2006.

Cod protection area in the North Sea

The cod protection area defined in Council Regulation (EC) No 2287/2003 Annex IV was intended to enhance the TAC uptake of haddock in the North Sea while prevent- ing cod by-catches. It regulated fishing of haddock of licensed vessels for a maximum of 3 months under the conditions that there was no fishing inside or transiting the cod protection area, that cod did not contribute more than 5 % to the total catch retained on board, that no transhipment of fish at sea occurred, that trawl gear of less than 100 mm mesh size was carried on board or deployed, and that a number of special landing regulations were complied with. It was discontinued at the end of 2004.

2.1.4 Environmental considerations

The WG considers that although it is clear that the North Sea ecosystem is undergo- ing change and this will affect fish stocks, the causal mechanisms linking the envi- ronment with fish stock dynamics are not yet clearly-enough understood for such information to be used as part of fisheries management advice.

2.1.5 Human consumption fisheries 2.1.5.1 Data

Estimates of discarding rates provided by a number of countries through observer sampling programme were used in the assessments of cod, haddock, whiting and some Nephrops FUs in the North Sea, to raise landings to catch. A combination of observed and reconstructed discard rates was used in the North Sea plaice assessment.

Other discard sampling programmes have been in place in recent years, but have not been used in the assessments yet because of short time-series or because of collation problems. In general, some discarding occurs in most human-consumption fisheries, particularly when strong year classes are approaching the minimum landing size.

For a number of years there have been indications that substantial under-reporting of roundfish and flatfish landings is likely to have occurred. Anecdotal evidence for this is particularly strong for cod during 2001–2003, when the agreed TAC implied a reduction in effort of more than 50% which the WG suggests probably did not occur. In the absence of information from the industry on the likely scale of this under- reporting, the WG has used a modified assessment method for North Sea cod (Section 14) which estimates unallocated removals on the basis of research-vessel survey data.

Such removals may be due to reporting problems, unrecorded discards, changes in