ACFM0105.pdf (12.69Mb)

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ICES Advisory Committee on Fishery Management ICES CM 2005/ACFM:01

Report of the

Working Group on the Assessment of Northern Shelf Demersal Stocks (WGNSDS)

4 – 13 May 2004

ICES Headquarters, Copenhagen

This report is not to be quoted without prior consultation with the General 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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International Council for the Exploration of the Sea

Conseil International pour l’Exploration de la Mer

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CONTENTS

1 GENERAL ... 1

1.1 Participants... 1

1.2 Terms of reference ... 1

1.3 Working Documents Provided to WGNSDS 2004 ... 3

1.3.1 WD1: The Precision of Irish Market Sampling for Rockall Haddock ... 3

1.3.2 WD2: Revised Estimates of Annual Discards-at-Age for Haddock in ICES Division VIa ... 3

1.3.3 WD3: Revised Estimates of Annual Discards-at-Age for Haddock in ICES Division VIa: Resultant Effects on a TSA Assessment... 3

1.3.4 WD4: Bayesian Stock Assessment of Plaice in VIIa... 3

1.3.5 WD5: Testing TSA with Simulated Data ... 3

1.3.6 WD6: Rockall and the Haddock Fishery ... 4

1.3.7 WD7: Irish Sea Plaice Survey Tuning Indices... 4

1.3.8 WD8: Preliminary Assessment of the Rockall Haddock (Melanogrammus aeglefinus) Stock ... 4

1.3.9 WD9: Some Peculiarities of Distribution and Migration of Haddock (Melanogrammus aeglefinus) on the Rockall Bank... 4

1.3.10WD10: Results from the Rockall Haddock (Melanogrammus aeglefinus) Research and Fishery by Russia in 2003 ... 4

1.3.11WD11: Anglerfish in Norwegian Waters... 4

1.3.12WD12: Anglerfish (Lophius Spp.) in Nordic and European Waters: Status of Current Knowledge and Ongoing Research... 5

1.3.13WD13: Information on Irish Sea Cod from the UK Fisheries Science Partnership... 5

1.4 Report Structure and Implementation of the ICES Quality Control Handbook... 5

1.5 Recommendations... 6

1.6 References... 7

2 DATA AND METHODS... 8

2.1 Stocks and Assessments... 8

2.2 Sampling Data... 8

2.3 Data compilation and aggregation ... 8

2.4 Database Revisions ... 9

2.5 Misreported Landings ... 9

2.6 Sequential population analysis and recruit estimation ... 10

2.7 Short-term Predictions and Sensitivity Analyses ... 12

2.8 Reference Points ... 13

2.9 Quality control ... 14

2.10 Biological parameters of stocks ... 14

2.11 Total Allowable Catches... 14

2.12 Software ... 15

2.13 Research vessel surveys used by Working Group... 16

3 COD IN SUB-AREA VI ... 20

3.1 Cod in Division VIa ... 20

3.1.1 Stock definition and the fishery ... 20

3.1.1.1 ICES advice applicable to 2003 and 2004 ... 21

3.1.1.2 Management applicable to 2003 and 2004 ... 21

3.1.1.3 The fishery in 2003... 22

3.1.2 Commercial catch-effort series and research vessels surveys... 22

3.1.3 Age compositions and mean weights at age ... 23

3.1.4 Natural mortality and maturity at age ... 24

3.1.5 Historical stock analyses... 24

3.1.5.1 Data screening ... 24

3.1.5.2 Exploratory catch-at-age analyses ... 24

3.1.5.3 Final assessments... 27

3.1.6 Estimating recruiting year-class abundance... 28

3.1.7 Historical trends in biomass, fishing mortality and recruitment ... 28

3.1.8 Short-term catch projections... 28

3.1.9 Medium-term stock projections ... 30

3.1.10Yield and biomass per recruit ... 30

3.1.11Biological reference points ... 30

3.1.12Quality of the assessment ... 30

3.1.13Management considerations ... 31

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3.1.14Scottish west coast groundfish survey ... 35

4 HADDOCK IN SUB-AREA VI... 141

4.1 Haddock in Division VIa ... 141

4.1.1 The fishery... 141

4.1.1.2 Management applicable in 2003 and 2004 ... 142

4.1.1.3 The fishery in 2003... 142

4.1.2 Commercial catch-effort data and research vessel surveys... 143

4.1.4 Natural mortality, maturity and stock weights at age ... 144

4.1.5 Catch at age analysis... 144

4.1.5.2 Final runs 145 4.1.6 Estimating recruiting year class abundance ... 147

4.1.7 Long-term trends in biomass, fishing mortality and recruitment... 147

4.1.8 Short-term catch predictions... 147

4.1.9 Medium-term projections ... 149

4.1.11Reference points ... 149

4.1.12Quality of assessment ... 149

4.2 Haddock in Division VIb ... 246

4.2.1.1 ICES advice applicable to 2003 and 2004. ... 247

4.2.1.3 The fishery in 2003... 248

4.2.2 Commercial catch-effort and research vessel surveys ... 249

4.2.5 Catch at age analysis... 251

4.2.6 Reference Points ... 252

4.2.7 Management Considerations... 252

5 WHITING IN SUB-AREA VI ... 266

5.1 Whiting in Division VIa... 266

5.1.1 Stock definition and the fishery ... 266

5.1.1.3 The fishery in 2003... 267

5.1.2 Commercial catch-effort and research vessel surveys ... 267

5.1.3 Age composition and mean weights at age... 268

5.1.5 Catch-at-age analysis ... 268

5.1.5.2 Catch-at-age analyses ... 269

5.1.5.3 Final catch-at-age analyses ... 269

5.1.6 Estimating recruiting year-class abundance... 270

5.1.9 Medium-term predictions ... 272

5.2 Whiting in Division VIb ... 273

5.2.1 Catch trends ... 273

5.2.2 Age5 ... 297

6 ANGLERFISH ON THE NORTHERN SHELF... 357

6.1 Anglerfish in Sub-Area VI... 358

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6.1.1.3 The fishery in 2003... 359

6.1.3 Length and age compositions and mean weights at age ... 360

6.1.4 Natural mortality and maturity ... 360

6.2 Anglerfish in the North Sea ... 360

6.2.1.3 The fishery in 2003... 360

6.2.3 Length and age compositions and mean weights at age ... 361

6.2.4 Natural mortality and maturity ... 361

6.3 Anglerfish in Division IIIa... 361

6.4 Anglerfish on the Northern Shelf (combined IIIa, IV and VI)... 362

6.4.2 Catch-at-length analysis... 362

6.4.2.1 Exploratory analysis ... 362

6.4.3 Reference points ... 363

6.4.4 Assessment considerations ... 363

6.4.4.1 Data ... 363

6.4.4.2 Biological information... 364

6.4.4.3 Stock Structure ... 364

6.4.5 Management considerations ... 364

7 MEGRIM IN SUB-AREA VI ... 382

7.1 Megrim in Division VIa... 382

7.1.1 The stock structure and fishery... 382

7.1.2 ICES advice applicable to 2003 and 2004 ... 383

7.1.3 Management applicable in 2003 and 2004 ... 383

7.1.4 The fishery in 2003... 384

7.1.5 Commercial catch-effort data and research vessels survey... 384

7.1.6 Catch age compositions and mean weights at age ... 385

7.1.7 Natural mortality, maturity and stock weight at age... 385

7.1.8 Catch-at-age analysis ... 386

7.1.8.2 Conclusions regarding a final assessment... 388

7.1.9 Yield and Biomass per recruit ... 388

7.2 Megrim in Division VIb... 389

7.2.1.1 The fishery in 2003... 389

7.2.1.2 Management applicable to 2003 and 2004 ... 389

7.2.2 Commercial catch-effort data and research vessels survey... 390

7.2.3 Catch age compositions and mean weights at age ... 390

8 COD IN DIVISION VIIA ... 409

8.1 The Fishery ... 409

8.1.1 ICES advice applicable to 2003 and 2004 ... 409

8.1.3 The fishery in 2003... 411

8.2 Commercial catch-effort data and research vessel surveys... 412

8.3 Age composition and mean weights-at-age ... 412

8.4 Natural mortality and maturity at age ... 414

8.5 Catch-at-age analyses... 414

8.5.1 Data screening and exploratory runs... 414

8.5.1.1 Commercial catch data ... 414

8.5.1.2 Survey data ... 414

8.5.1.3 Exploratory assessment runs... 415

8.5.1.4 Final assessment run ... 419

8.5.1.5 Comparison with last years assessment ... 420

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8.5.2 Estimating recruiting year class abundance ... 420

8.5.5 Medium-term predictions ... 422

8.5.6 Yield and biomass per recruit ... 423

8.5.7 Reference points ... 423

8.5.8 Quality of the assessment ... 423

9 HADDOCK IN DIVISION VIIA... 490

9.1 The fishery ... 490

9.1.1 ICES advice applicable in 2003 and 2004 ... 491

9.1.3 The fishery in 2003... 492

9.2 Commercial catch-effort and research vessel surveys ... 492

9.3 Catch age composition and mean weights at age in the catch... 492

9.4 Natural mortality, maturity and stock weights at age... 493

9.5 Catch-at-age analysis ... 495

9.5.1 Data screening ... 495

9.5.2 Final assessment ... 496

9.5.3 Comparison with 2003 WG assessment ... 496

9.6 Estimating recruiting year class abundance ... 496

9.7 Long term trends of biomass, recruitment and fishing mortality ... 497

9.8 Short-term catch predictions ... 497

9.8.1 Survey based forecasts... 497

9.8.2 Extension of 2003 WG forecast... 498

9.8.3 TSA based forecast ... 499

9.9 Medium term predictions ... 499

9.10 Yield and biomass per recruit ... 500

9.11 Reference points... 500

9.12 Quality of the assessment... 500

10 WHITING IN DIVISION VIIA ... 540

10.1 The Fishery ... 540

10.1.1ICES advice applicable to 2003 and 2004 ... 540

10.1.2Management applicable in 2003 and 2004 ... 541

10.1.3The Fishery in 2003 ... 541

10.2.1Commercial catch and effort data ... 542

10.2.2Research vessel surveys... 542

10.3 Catch age compositions and mean weights at age ... 543

10.4 Natural mortality, maturity and stock weight at age ... 544

10.5.1Data Screening... 544

10.5.2Final Assessmen run ... 544

10.7 Long-term trends in biomass, fishing mortality and recruitment ... 544

10.9 Medium Term Projections ... 545

10.10 Yield and Biomass per Recruit ... 545

10.12 Quality of the Assessment... 545

10.13 Management considerations... 545

11 PLAICE IN SUB-DIVISION VII ... 577

11.1.3The fishery in 2003... 578

11.2 Commercial catch-effort data and research vessel surveys... 578

11.3 Age compositions and mean weights at age... 578

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11.5.1Data screening ... 580

11.5.2Final XSA run... 581

11.5.3Comparison with last year's assessment ... 582

11.6 Estimating recruiting year-class abundance... 582

11.7 Long-term trends in biomass, fishing mortality and recruitment ... 582

11.9 Medium-term projections... 584

11.10 Yield and Biomass Per Recruit ... 584

11.11 Reference points... 584

11.11.1 Quality of the assessment ... 584

12 SOLE IN SUB-AREA VII – SOLE IN DIVISION VIIA ... 634

12.1.3The fishery in 2003... 635

12.3 Age compositions and mean weights at age... 635

12.4 Natural mortality, maturity ... 636

12.5.1Data screening ... 636

12.5.2Exploratory catch-at-age analyses ... 636

12.5.3Final catch-at-age analysis... 637

12.7 Comparison between 2003WG and 2004WG... 639

12.8 Long-term trends... 639

12.10 Medium-term predictions... 640

12.11 Yield and biomass per recruit ... 640

12.13 Quality of assessment ... 641

12.14 Management considerations... 641

13 MIXED FISHERIES INTERACTIONS ... 679

14 EVALUATION OF MANAGEMENT MEASURES... 680

14.1 Medium term forecasts for cod using the CS5 software ... 680

15 ANGLERFISH STOCK STRUCTURE... 707

15.1 Anglerfish in Nordic waters... 707

15.1.1Norwegian waters north of N 62° (Division IIa NEZ)... 707

15.1.2Faeroese waters (Division Vb)... 707

15.1.3Icelandic waters (Division Va) ... 708

15.2 Spawning areas and drift of eggs and larvae... 708

15.3 Recent tagging programmes... 708

15.4 Genetic studies... 708

15.5 Discussion... 708

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1 GENERAL

1.1 Participants

Mike Armstrong United Kingdom (England and Wales)

Doug Beare United Kingdom (Scotland)

Otte Bjelland Norway

Helen Dobby United Kingdom (Scotland)

Richard Hillary United Kingdom (England and Wales)

Colm Lordan Ireland

Colin Millar United Kingdom (Scotland)

Sara-Jane Moore Ireland

Coby Needle United Kingdom (Scotland)

Rick Officer (Chair) Ireland

Pieter-Jan Schön United Kingdom (Northern Ireland)

Robert Scott United Kingdom (England and Wales)

Willy Vanhee Belgium

Vladimir Vinnichenko Russian Federation

1.2 Terms of reference

The Working Group on the Assessment of Northern Shelf Demersal Stocks [WGNSDS] (Chair: R. Officer, Ireland) will meet at ICES Headquarters from 4–13 May 2004 to:

a) assess the status of and provide catch options for 2005 for the stocks of cod, haddock, whiting, anglerfish, and megrim in Subarea VI, and for cod, haddock, whiting, plaice, and sole in Division VIIa;

b) assess the status of anglerfish stocks in Subarea IV and Divisions IIIa and VIa and provide catch options for each management area;

c) review information on the stock structure of anglerfish in Divisions IIa, IIIa, Va, Vb, VIa and in Subarea IV and define appropriate stock areas for fish stock assessment usage;

d) consider and implement the proposed methodology for projection of yield by fisheries made by the Study Group on the Development of Fishery-based Forecasts based on the data compiled through this Study Group. The Group should present a limited set of fisheries-based catch options;

e) provide specific information on possible deficiencies in the 2004 assessments including, at least, any major inadequacies in the data on catches, effort or discards; any major inadequacies in research vessel surveys data, and any major difficulties in model formulation, including inadequacies in available software. The consequences of these deficiencies for the assessment of the status of the stocks and for the projection should be clarified;

f) comment on this meeting’s assessments compared to the last assessment of the same stock, for stocks for which a full or update assessment is presented;

g) document fully the methods to be applied in subsequent update assessments and list factors that would warrant reconsideration of doing an update, and consider doing a benchmark ahead of schedule; for stocks for which benchmark assessments are done;

h) evaluate the effects of the existing recovery plans for cod in Division VIa and Irish Sea Cod.

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Terms of Reference a, b, e and f are dealt with in Sections 3 – 12 giving the results of the assessments of individual stocks. The issue mixed fisheries interactions (ToR d) is dealt with in Section 13. Term of Reference h (evaluation of existing recovery plans for VIa & VIIa Cod) is dealt with in Sections 3 and 8, respectively. Medium term recovery plan evaluations are presented in Section 14. Term of Reference c (Anglerfish Stock Structure) is dealt with in Section 15.

The Working Group’s approach to implementing Term of Reference g is described in Section 1.4.

VIb

VIa

VIIc VIIb VIIa

VIIk VIIj

VIIg

VIIf Vb2

Vb1 XII

18° 16° 14° 12° 10° 8° 6° 4° 2°

Longitude west

50°

51°

52°

53°

54°

55°

56°

57°

58°

59°

60°

61°

62°

Latitude no rth

Rising Ground 1 Butt of Lewis

South Minch

Clyde Outer

Hebrides

Malin Head Stanton Bank

Aranmore Island Rockall

Bank

Morecombe Bay

Donegal Bay

North Channel

The Smalls

St Georges Channel Republic

of Ireland Northern Ireland

Scotland

England

Wales

Figure 1.2.1. Map of the assessment areas showing the location of fishing grounds.

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1.3 Working Documents Provided to WGNSDS 2004

1.3.1 WD1: The Precision of Irish Market Sampling for Rockall Haddock Authors: R. Officer and S.-J. Moore

Summary: Preparation for the 2004 WGNSDS has involved a re-calculation of the time series of Rockall haddock assessment input data. An examination of the mean length-at-age of Rockall haddock sampled by Ireland, Scotland and Russia shows marked differences between countries. These differences were particularly apparent between the Scottish market sampling data and data from other sources. The precision of market sampling undertaken by Ireland was evaluated in order to determine whether the differences in mean length-at-age might be due to variability in the Irish sampling. Quarterly Irish market sampling data from 1995-2003 were analysed to obtain the precision of estimated catch numbers-at-age, catch-weight-at-age and mean length-at age.

1.3.2 WD2: Revised Estimates of Annual Discards-at-Age for Haddock in ICES Division VIa Authors: R. Fryer and C. Millar

Summary: Previous studies have found that collapsing the sampling stratification provides more robust estimators of total species discards because the chance of getting an unrealistically high stratum ratio is reduced. These studies of simulated data showed that the collapsed ratio estimator had negligible bias, was more precise and also provided a means of estimating total discards in un-sampled strata. The Working Document presents provisional revised annual discards-at-age estimates for VIa haddock and compares these to the current estimates.

1.3.3 WD3: Revised Estimates of Annual Discards-at-Age for Haddock in ICES Division VIa: Resultant Effects on a TSA Assessment

Author: C. Millar

Introduction: This Working Document is intended as a supplement to Working Document 2. It presents the summary plots from two runs of TSA using the current and revised estimates of VIa haddock discards-at-age. The TSA was run as it has been utilised to assess VIa haddock for the last 3 years. Only the summary plots are presented in the Working Document. The intention is to show the likely effects of the revised discard estimates on the historical and current perceptions of the stock.

1.3.4 WD4: Bayesian Stock Assessment of Plaice in VIIa Authors: R.M. Hillary and G.P. Kirkwood

Introduction: This alternative assessment of VIIa plaice is linked to the DEFRA project MO423. VIIa plaice are being used as a test stock for alternative stock assessment methods. The present assessment uses the same data available to the working group, but uses Bayesian state-space methods for estimation. This document contains the outline of the population dynamics model employed; the formulation of the Bayesian problem, given the data; the specifics of the MCMC (Markov chain Monte Carlo) method used in the estimation procedure; and the results of the assessment, and any predictive statements, congruous with the present WG methods.

1.3.5 WD5: Testing TSA with Simulated Data Author: C. Needle

Abstract: The formulation of a fisheries data simulator is specified, and one realisation of this simulator is used to test the ability of three assessment methods (TSA, XSA, SURBA) to a) reflect reality and b) detect model misspecification.

A general guide to setting up a TSA assessment is also included. None of the methods used performs particularly well in either test: all produce similar estimates of abundance which are wrong for much of the time-series, and none give sufficient diagnostics for misspecification detection. Firm conclusions would be unwise following a single test of this kind, but it is suggested that agreement between different assessment models does not necessarily indicate correct

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abundance estimates. Future work should investigate whether it is misspecification, data noise or incorrect model assumptions which cause these problems.

1.3.6 WD6: Rockall and the Haddock Fishery

Author: A. W. Newton, K. J. Peach, K. A. Coull, M. Gault and C. L. Needle

Summary: This Working Document presents the information that exists in the Marine Laboratory, Aberdeen concerning the haddock fishery at Rockall. The history of Rockall, the nature of the Plateau itself and the historical fishery are reviewed. Most of the information presented is based on historical data from a variety of sources and research vessel surveys but extra funding in 2001 has allowed some new information to be acquired.

1.3.7 WD7: Irish Sea Plaice Survey Tuning Indices Author: R. Scott

Summary: The Working Document describes the VIIa plaice survey tuning indices that are available to the Working Group. There is no survey index that can be considered to be truly representative of the Irish Sea plaice stock as a whole since each series is derived from only a portion of the total assessment area. Tuning indices at age are compared by year and by year-class to evaluate trends throughout each time series and the ability to track year-classes.

1.3.8 WD8: Preliminary Assessment of the Rockall Haddock (Melanogrammus aeglefinus) Stock Author: V.N. Khlivnoy

Summary: In recent years WGNSDS has repeatedly stated that a major impediment to making a reliable Rockall haddock assessment is the absence of information on discards and the lack of data on length-age composition of catches. A method for estimating historic discarded catch-at-age is presented. The Working Document also presents the results of a Rockall haddock stock assessment which takes account of discards and new data on length-age composition of catches.

1.3.9 WD9: Some Peculiarities of Distribution and Migration of Haddock (Melanogrammus aeglefinus) on the Rockall Bank

Authors: V. I. Vinnichenko and E. V. Sentyabov

Summary: The results of fisheries investigations on the Rockall Bank detailing the distribution of haddock are described. Changes in both biology and distribution of haddock are discussed with reference to their importance to the planning of stock assessments and, in particular, trawl surveys. The Working Document suggests the main environmental factors influencing the haddock distribution pattern and proposes improvements to demersal trawl surveys on the Rockall Bank.

1.3.10 WD10: Results from the Rockall Haddock (Melanogrammus aeglefinus) Research and Fishery by Russia in 2003

Authors: V.I. Vinnichenko and V.N. Khlivnoy

Summary: The Working Document shows results of the haddock research and fisheries on the Rockall Bank conducted by Russia in 2003. The aim is to summarise the fisheries and biological data collected and to present materials prepared for the Rockall haddock stock assessment.

1.3.11 WD11: Anglerfish in Norwegian Waters Authors: O. Bjelland and K. H. Nedreaas

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Summary: The Working Document details several aspects of the fishery for anglerfish in Norwegian waters: the type of vessels participating in the fishery, their gear and area of operation. Yield- and Spawning stock-per-recruit analyses are presented for the anglerfish caught in the Norwegian gillnet fishery, and total Norwegian fishery (including gillnet, trawl, and Danish seine).

1.3.12 WD12: Anglerfish (Lophius Spp.) in Nordic and European Waters: Status of Current Knowledge and Ongoing Research

Authors: T. Thangstad, J. E. Dyb, E. Jónsson, C. Laurenson, L. H. Ofstad and S. A. Reeves

Summary: The Working Document is a report prepared as a pilot study for a proposed 3-year research project aiming to co-ordinate a synoptic collection and analysis of anglerfish data in Nordic waters. The report describes the status with regard to research and knowledge about anglerfish in Nordic waters, as well as in other European regions. Proposed research topics and recommendations for a revised project application are given.

1.3.13 WD13: Information on Irish Sea Cod from the UK Fisheries Science Partnership Authors: C. Bannister, J. Cotter, M. Armstrong, T. Boon, J. Keable and P. Witthames

Summary: The Working Document describes the results of a UK government/industry partnership project which has used chartered fishing vessels to carry out scientifically-monitored commercial fishing in a number of priority fishing areas nominated by industry. Data on the catch rate and size distribution of target and by-catch species caught during these trips is compared with data collected by market sampling and by research vessel surveys.

1.4 Report Structure and Implementation of the ICES Quality Control Handbook In accordance with the guidelines in the ICES Quality Control Handbook the WGNSDS has:

• Drafted Stock Annexes for the stocks it assesses,

• Reduced the content of the Working Group report stock sections such that they concentrate more on the current assessment,

• Focussed attention on stocks designated as requiring full annual assessments, and,

• Attempted to apply update assessments to other stocks.

The WGNSDS notes that, of the stocks assessed by the WG, the ‘observation list’¹ of stocks subject to a benchmark assessment every year only includes West of Scotland Cod and Irish Sea Cod. The WGNSDS considers only two stocks (Irish Sea plaice and sole) which are not on the observation list, and for which analytical assessments exist. Update assessments were therefore anticipated for VIIa plaice and sole at WGNSDS2004. The Working Group intended to assess these stocks using the Same Procedure As Last Year (SPALY) with some exceptions (eg. tuning data alterations after preliminary analysis).

All of the other stocks considered by the WGNSDS were treated as experimental assessments at WGNSDS2004. The justification of a more thorough analysis for each stock is as follows:

• III, IV & VI Anglerfish: The estimation of recruitment was questioned by ACFM last year and the WG was requested to consider using additional survey indices in the WGNSDS2004 assessment. The very low credibility of the assessment was also raised prior to the WG at Industry meetings in two countries. Vessel logbook information was subsequently provided for consideration at this year’s WG.

1 The ‘observation list’ published by ICES in 2002 was the most recent list available to WGNSDS2004.

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• VIa Whiting and VIa Haddock: These stocks are connected to the cod rebuilding plan by association in mixed fisheries. The questions raised by ACFM about the TSA assessments applied to VIa gadoid stocks required detailed analyses of the assessment models applied to these stocks.

• VI Megrim: There is no accepted assessment. ACFM urged the WG to experiment with alternative assessments of this stock.

• VIb Haddock: There is no accepted assessment. ACFM urged the WG to experiment with alternative assessments of this stock. Furthermore, the stock is subject to area closures designed to protect the stock and a potential rebuilding plan is under discussion. Survey-based analyses suggest that the stock is at a relatively low level, and indicate major changes in the stock status.

• VIIa Whiting: The stock is connected to the cod rebuilding plan by association in mixed fisheries. ACFM considered the WGNSDS2003 assessment to be of poor quality but pragmatically adopted the assessment to facilitate management. Indications are that the stock is at very low level and a recovery plan was recommended by ACFM.

Furthermore, the denial of access to samples in 2003, required increased scrutiny of the adequacy of input data at WGNSDS2004.

• VIIa Haddock: Through association in mixed fisheries the stock is connected to the cod rebuilding plan and the whiting recovery plan recommended by ACFM. ACFM considered the WGNSDS2003 assessment to be of poor quality but pragmatically adopted the assessment to facilitate management. Furthermore, the denial of access to samples in 2003, required increased scrutiny of the adequacy of input data at WGNSDS2004.

Term of Reference g asks the Working Group to document fully the methods to be applied in subsequent update assessments. This documentation is provided for stocks subject to SPALY update assessments in the relevant Stock Annexes. For benchmark and experimental assessments it is not possible to describe the procedure to the same extent.

Elements of such assessments that remain relevant from year to year have been included in the Stock Annex for each stock. Other information is given in the WG report.

Term of Reference g also asks the Working Group to list factors that would warrant reconsideration of doing an update, and consider doing a benchmark ahead of schedule. These considerations are presented in the “Quality of the Assessment” section for relevant stocks.

1.5 Recommendations

1. The major deficiency facing many assessments this year is the poor quality of the input data. This was caused mainly by sectors of the fishing industry in the UK (Northern Ireland) and Ireland denying access to samples. The WG recommends that efforts be made by Research Institutes and Industry organisations to improve cooperation.

2. The use of existing maturity ogives for the full historic series may not be appropriate, particularly in view of the large changes in stock size over time. The WG recommends that a comprehensive review of the biological parameters of the stocks is carried out, including analysis of recent survey data and an evaluation of the information (if available) on which historic estimates have been based.

3. The WG noted a general lack of uniformity in data management procedures between the participating nations. This was particularly apparent in the compilation of age based data sets where consistency had not been maintained in the age range of the data. In some cases where the plus group used in the assessment had been reduced, the data sets contained information only up to the revised plus group age and information regarding the older ages was no longer recorded. Considerable work would be required to re-calculate the data for older ages should the plus group be revised up again at any point in the future. Furthermore, the methods by which data had been compiled in earlier years, within individual institutes, was sometimes not apparent and the WG was unable to determine the quality and integrity of these data and, consequently, their applicability to various assessment methods. It is likely that this problem effects many Working Groups. It is recommended that ICES considers this issue and makes proposals for future protocols regarding consistency in data storage.

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1.6 References

Anon. (2001). The distribution and biology of anglerfish and megrim in waters to the west of Scotland. EC Study Contract 98/096 Final Report August 2001.

Borges, L., Zuur, A., Rogan, E. and Officer, R. (Under review). Optimum sampling levels in discard sampling programmes. Submitted to the Canadian Journal of Fisheries and Aquatic Sciences.

Cook, R. M., P. A. Kunzlik and R. Fryer. 1991. On the quality of North Sea cod stock forecasts. ICES Journal of Marine Science, 48, 1-13.

Darby, C. and Flatman, S. (1994) Lowestoft VPA Suite Version 3.1 User Guide. MAFF: Lowestoft.

Hislop, J. R. G., J. C. Holst and D. Skagen. 2000. Near surface captures of post-juvenile anglerfish in the Northeast Atlantic – an unsolved mystery. Journal of Fish Biology, 57, 1083-1087.

Hislop, J. R. G., A. Gallego, M. R. Heath, F. M. Kennedy, S. A. Reeves and P. J. Wright. 2001. A synthesis of the early life history of anglerfish, Lophius piscatorius (Linnaeus, 1756) in northern British waters. ICES Journal of Marine Science, 58, 70-86.

ICES 2004. Report of an Expert Group on Rockall Haddock Recovery Plans following a request for advice made on behalf of the European Community and the Russian Federation. ICES CM 2004/ACFM:33.

ICES 2003. Report of the Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak, 2002.

ICES CM 2003/ACFM:02

ICES 2001. ACFM report - ICES Coop. Res. Rep. 246 (2) - Report of the ICES Advisory Committee on Fishery Management, 2001.

ICES 2001. Report of the Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak, 2000.

ICES CM 2001/ACFM:07.

ICES 2003. Report of the Working Group on the Assessment of Northern Shelf Demersal Stocks, 2003. ICES CM 2003/ACFM:04.

Kunzlik, P. (2003). Calculation of potential reduction in fishing mortality of North Sea and west of Scotland cod, haddock and whiting due to decommissioning of UK vessels in 2002. Working Document to the EU-Norway Expert Group Meeting, 28 May – 7 June 2003.

Kunzlik, P. A, A. W. Newton and S. Jermyn. 1995. Exploitation of monks (Lophius spp.) and megrims (Lepidorhombus spp.) by Scottish fishermen in ICES Division VIa (West of Scotland). EU FAR contract MA-2-250.

Mensil, B (March 2003). CSA Catch Survey Analysis Assessment Program Documentation. IFREMER - Laboratoire MAERHA Nantes (France).

Needle, C. L. (2003). Survey-based assessments with SURBA. Working Document to the ICES Working Group on Methods of Fish Stock Assessment, Copenhagen, Jan – Feb 2003.

Needle, C. L. (2004). Absolute abundance estimates and other developments in SURBA. Working Paper to the ICES Working Group on Methods of Fish Stock Assessment, IPIMAR, Lisbon, February 2004.

Sullivan, P. J., H-L Lai and V. F. Gallucci. 1990. A catch-at-length analysis that incorporates a stochastic model of growth. Can. J. Fish. Aquat. Sci. 47, 184-198.

Vinther, M, Reeves, S and Patterson, K (2003) From single-species advice to mixed-species management: taking the next step. ICES CM 2003/V:01

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2 DATA AND METHODS

2.1 Stocks and Assessments

The stocks within the remit of this Working Group are tabulated in Table 2.1.1 along with the type of assessment carried out and an indication of whether this reflects a change to previous practices.

2.2 Sampling Data

Table 2.2.1 shows which countries provided assessment data to the Working Group for the year 2003, and the form of data provided. An increased amount of discard data was provided to the WGNSDS2004 for several stocks. The level of sampling in 2003 for core assessment data (length measurements and age-length keys for landed catches) is indicated in Table 2.2.2, where data were available for individual countries. The data can be compared with the total international landings as used in the Working Group assessments. Deficiencies in sampling (if any) are discussed in the relevant stock Section.

2.3 Data compilation and aggregation

Institutes submitting data to the WGNSDS2004 were asked to provide data in a format that may better support mixed- fisheries analyses and assessments. For stocks in Divisions VIa and VIIa (where mixed fisheries analyses may be possible) institutes were asked to submit their 2003 catch-at-age data by fleet/fishery and species rather than by stock (as has been done up until now). The fleet/fishery groupings to be used were those agreed by the SGDFF2004 for demersal fisheries in VIa and VIIa. Institutes sometimes did not have sufficient sampling to support dis-aggregation into fleet specific catch-at-age datasets. In such cases the data co-ordinators allocated the most appropriate alternative age compositions and weights-at-age to the unsampled catch. The suitability of, and sensitivity to these allocations was assessed.

The stocks assessed by this Working Group have previously been split into three groups for which different data compilation and aggregation procedures were used. These groups were the Area VI gadoids, the Irish Sea gadoids and the Irish Sea flatfish. For the other stocks assessed by this WG, assessments are generally at a more preliminary stage and data compilation had been on a more ad hoc basis.

At the 2002 Working Group time was wasted correcting mistakes made in data collation for some stocks. It was thought that such problems would be best avoided if the data compilation and the assessment were co-ordinated by staff from the same laboratory. At the 2003 and 2004 Working Groups data compilation and the assessment of most stocks was co-ordinated by staff from the same laboratory, if not by the same person.

Much data was prepared in advance of the meeting but very late revisions to the Irish discard data, and Belgian and Scottish landings data necessitated a revision of input data at the Working Group. Revisions to the data are discussed in the appropriate stock Sections.

III, IV & VI Anglerfish

Data are supplied to the stock co-ordinators electronically. Data handling and aggregation is handled by standard spreadsheets that incorporate SOP checks at each stage. The files retain the full seasonal and gear disaggregation of the supplied data. Length compositions for landings where no length data are supplied are estimated using user-specified fill-in rules. Assessment files are updated manually and data are stored in spreadsheets with one worksheet per year.

Area VI Gadoids

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disaggregation in which the data are supplied. At present the file handling and data aggregation are done by a series of BASIC programs. The programs do not perform any checks on the data. SOP-correction is optional, but is usually applied to ensure consistency given SOP discrepancies in some fleets in the early years of the data. Age compositions for landings where no age data are supplied, are normally estimated using the total age composition across all fleets for which age data are available. More appropriate age compositions and weights-at-age can be allocated to the unsampled catch but this process has to be done externally to the data aggregation program. The programs write a complete set of assessment data files so it is straightforward to update the assessment data each year.

Irish Sea Gadoids and Area VI Megrim

Data are supplied to the stock co-ordinators electronically. Data handling and aggregation is handled by standard spreadsheets which incorporate SOP checks at each stage. The files retain the full seasonal and gear disaggregation of the supplied data. Age compositions for landings where no age data are supplied are estimated using user-specified fill- in rules. Assessment data files are updated manually. Data are stored in spreadsheets, with one worksheet per year.

Irish Sea Flatfish

Data are supplied to co-ordinators electronically, and the data handling and aggregation is handled by a series of spreadsheet macros. Some SOP checking is included in these macros. Raw data are not routinely SOP corrected, although SOP corrections are applied to the combined and smoothed total international weights at age. The files retain the full seasonal and gear disaggregation of the supplied data. Age compositions for landings where no age data are supplied are estimated using user-specified fill-in rules. The data for one year are stored in an individual spreadsheet file, making it less straightforward to update data for all years. The process includes independent checking of the data by two people.

2.4 Database Revisions

The assessment data files are retained on the ICES network in the ASCII format used by the stand-alone assessment packages. All revisions to these files for individual stocks are discussed in the separate stock sections. Revisions to the data to incorporate estimates of misreported landings are described in the following Section.

WGNSDS2004 made revisions to the 2002 Scottish landings used by the Working Group. Landings for particular gears and statistical rectangles had not been included in the previous extraction when effort was not reported for the strata.

The revised landings amount to an increase of about 10% on the previous estimates.

2.5 Misreported Landings

ACFM has in the past expressed concern over the increasing amount of “unallocated” landings for stocks assessed by Working Groups, and asked for a more detailed explanation of the derivation of these figures. These unallocated landings represent adjustments to nominal landings figures to correct either for misreporting or for differences between official statistics and data obtained by national scientists. The general term misreporting is used throughout this report to include misreporting by area, misreporting of landings by species and under- or over-reporting of landings. The history of WG attempts to quantify misreporting is given in the 2000 WG report (ICES CM:2001/ACFM:01). A summary of current practices is given below.

Stocks in Sub-Area VI

Previous Working Groups have expressed a view that misreporting of area VI gadoids has not been significant in recent years because of low availability of fish relative to quotas. This year’s Working Group was not able to make an informed judgement on misreporting of area VI gadoids. Values for misreported landings of VIa haddock in 1992 – 1994, inferred from survey data, are given in ICES CM 1996/Assess:1 and ICES CM 1997/Assess:2 and are included in the assessment files. The period 1992-95 has previously been treated explicitly in the TSA model for VIa cod as having unknown landings, which are estimated by the model conditional on the sample-based estimates of age compositions in the international landings and an assumption of no misreporting in 1996. Annual updates of the TSA assessment have made progressive downward estimates of these landings to levels very close to the reported values. The reported landings for these years are now included in the TSA assessment.

Misreporting of VIa whiting has historically not been considered a problem and no estimates are currently used. For anglerfish and megrim in Division VIa the existence of a restrictive precautionary TAC in Division VIa but no catch

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restrictions in the adjacent areas of the North Sea up until 1998 is suspected to have led to extensive reporting of catches from VIa into IVa. Such an effect is apparent in the reported distribution of catches by one nation where catches of anglerfish and megrim reported from the statistical rectangles immediately east of the 4^oW boundary (the E6 squares) have accounted for a disproportionate part of the combined VIa/North Sea catches of these species. This proportion has reached up to 57% in the case of anglerfish and 75% in the case of megrim. As it is strongly suspected that the large majority of catches reported from the E6 squares are actually taken in Division VIa the landings totals used in the assessments of these stocks have been corrected for this effect. The correction has been applied by first estimating a value for the true catch in each E6 square and then allocating the remainder of the catch into VIa squares in proportion to the reported catches in those squares. The ‘true’ catches in the E6 squares are estimated by replacing the reported values by the mean of the catches in the adjacent squares to the east and west. This mean is calculated iteratively to account for increases in catches in the VIa squares resulting from reallocation from the E6 squares.

Stocks in Division VIIa

Misreporting of cod, haddock and whiting in the Irish Sea has occurred during the 1990s due to restrictive quotas. This has mainly taken the form of misreporting between VIIa and surrounding regions (mainly from the Celtic Sea into the Irish Sea), and misreporting of species compositions (both over- and under-reporting). Reported (official) landings data from one country taking a significant part of the international catch have in the past been adjusted at source for area- misreporting based on local knowledge of fleet activities. Species-misreporting by another important national fleet has been estimated using a sampling method based on observations made by scientists taking length measurements in the ports. The mean observed weights of the three gadoid species per landing were calculated by port and gear type in 2002, and raised to the total number of landings for each port and gear in which at least one of the three species was recorded.

An analogous procedure was used for estimating haddock landings in 1993–2001 and landings of cod and whiting in 1998-2001. For cod and whiting in 1991–1997, observed and reported landings were compared and the mean proportion reported was calculated for different gear types. The mean proportions reported were used to correct the total reported landings for each species. Further details are given in ICES CM 1999/ACFM:1. The sample-based estimates of landings at official fish markets exclude any “black” landings made at non-designated ports or times and correct only for misreporting of species compositions. Possible increases in black landings may have occurred in the more recent years when some TACs have been set to achieve substantial reductions in fishing mortality without effective mechanisms for controlling fishing effort to the necessary extent. This is of concern not only for the accuracy of the assessments, but also for the appropriateness of assessment methods such as XSA in which survey and commercial CPUE data are evaluated against population numbers reconstructed from commercial catch data (see also Casey, J: Working Document 5; 2002 meeting of WGNSSK ICES CM 2003/ACFM:02).

2.6 Sequential population analysis and recruit estimation

Where a full analytical assessment was possible, the WG implemented either Extended Survivor’s Analysis (XSA) with shrinkage and recruit calibration or time-series analysis (TSA) as the baseline method. This follows the practices adopted at the 1993–2003 Working Group meetings. In some cases both methods were applied to examine the effect of model choice on the assessment.

The full sequence of analysis for application of XSA to each stock is given below. A complete exploratory analysis to determine q-plateau and/or appropriate level of shrinkage is only carried out if the values used at previous Working Groups are no longer considered appropriate, or if new tuning series are included. The choice of catchability model for the younger age classes was reviewed for each stock following advice from ACFM that the youngest age class should not automatically be treated as recruits, particularly when the time series is short.

a) A separable VPA was carried out to screen the catch at age data in order to detect if large residuals or unusual patterns reveal anomalies in the data from year to year. The separable VPA was used to select the range of ages over which to run XSA, and to investigate the exploitation pattern.

b) Tuning fleets were scrutinised in detail to address the advice from the 2002 meeting of the Working Group on Methods of Fish Stock Assessment (WGMG) that “Working groups…should favour fewer data of good quality (as evaluated independently of the assessment model) instead of large quantities of data of unknown properties”;

and that “The definition of fleets for tuning purposes should be improved, and stricter criteria should be used to select the catch and effort data retained for each fleet”. Tuning fleets were evaluated independently of XSA or TSA as follows:

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• The WG first considered if the survey or commercial CPUE fleet was potentially capable of providing an unbiased series of population indices for a given range of fish age classes. This was evaluated based on the distribution of fishing or survey stations relative to the known distribution of the stock; the type of fishing gear; the timing of a survey; whether or not changes in survey design or fishing gear over time, or in efficiency of fishing fleets, have been examined and their effect quantified; quality of sampling for length or age; and, in the case of commercial fleets, the absence of discards in the CPUE data at any age, the accuracy of the catch and effort data, and the targeting practices of the vessels. Where such evaluations were carried out in previous WG meetings, they were generally not repeated this year and any fleets previously excluded were not re-introduced unless there was a significant change in the data.

• The internal consistency of the data for each fleet was evaluated by examining the coherence of year- class effects at each age. For surveys with multiple ages, the separable model SURBA (survey based assessment) developed at the FRS Marine Laboratory in Aberdeen was run to examine how well the data conform to a simple model of separable year and age effects on mortality.

• The similarity of trends in the indices at each age was examined to check for consistency between fleets.

c) The consistency between the tuning data and the commercial catch at age data was examined by inspecting catchability residuals from single-fleet Laurec-Shepherd runs, or in some cases weakly-shrunk XSA (usually S.E. = 2.5), without taper and using the constant-catchability model for all ages. Age- and year- effects in log- catchability residuals over the entire time-series of data were examined. Based on the independent examination of tuning fleets, and the single-fleet L-S or XSA runs, a choice was then made on which fleets and age classes to be included in the multi-fleet XSA tuning. The period over which to tune the XSA was decided in such a way as to maximise the precision and minimise the bias in estimates of catchability in the final year, for those age classes where catchability was assumed constant. For a number of years the Working Group has avoided progressive down-weighting of data from earlier years using a tricubic taper and has instead used a fixed tuning window of 10 years. As many of the assessments are becoming more heavily dependent on survey data for tuning, the Working Group decided to abandon the 10-year fixed window approach and to use all years with data based on consistent survey methods. A further argument for this revised approach was to reduce variability introduced by the sudden exclusion of a year with influential catchability residuals. A 20-year tricubic taper was applied where progressive down-weighting of early year’s data was considered advisable. Time-series estimates from SURBA and from the catch-at-age analysis of relative spawning stock biomass, catch, and mean fishing mortality were compared.

The working group was aware of a lack of consistency in the value of F shrinkage standard error chosen for

"weakly shrunk" single fleet XSAs. A range of values between 2.0 and 3.0 were used at this year's meeting for exploratory analyses. Whilst it is accepted that the value chosen is very often subjective, the working group did not feel that standardisation to a fixed value would be an appropriate measure. The weighting applied to the F shrinkage estimates is also determined by the strength of the signal in the tuning data. For example the use of an F shrinkage standard error of 2.0 coupled with a tuning fleet which gives consistent information about year-class strength might result in very little weight being applied to shrinkage estimates and a weakly shrunk assessment.

On the other hand, the use of the same level of F shrinkage with a tuning fleet that gives less consistent year- class signals would result in a greater weighting being given to the F shrinkage estimates and a strongly shrunk assessment. Clearly, the value of the F shrinkage standard error on its own cannot be used to denote an assessment as either weakly or strongly shrunk.

d) Once the tuning fleets and the age range for XSA had been chosen, ages for which recruit calibration (RCT3- type calibration) is appropriate were identified. These were typically the youngest ages tuned mainly by surveys and for which F-shrinkage gave unstable estimates of survivors. In these circumstances, the XSA fit for these age classes treated catchability as a power function of population size only if the relationship between Ln (adjusted survey indices) and Ln (XSA estimates) in singe-fleet runs was well defined, with an adequate number of observations.

The age above which catchability can be assumed fixed (the q-plateau) was as determined for each stock in previous Working Groups.

e) The XSA and TSA outputs were examined for retrospective patterns in estimates of fishing mortality, SSB and recruitment. The possible sources of such patterns were investigated. If such patterns could not be resolved, additional tuning runs were carried out to investigate if increased shrinkage could reduce the bias in estimates of terminal F. Appropriate levels of shrinkage were also considered in the light of recent trends in F or the presence of individual high values of F over the period to which shrinkage is applied.

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f) The detailed diagnostic output of the XSA was inspected. This helped to determine which age groups in the final year should be replaced for input to prediction. Unless there was a good reason for doing otherwise, the XSA estimates for recruiting age groups were used for the stock predictions. In some cases, these values were overwritten using the geometric mean level of recruitment. The long term geometric mean was chosen unless strong recent trends in the recruitment time series indicated that this was inappropriate. In some cases including Irish Sea cod and plaice, where there was evidence of recent depression of recruitment (for example due to a stock-recruit relationship), the geometric mean was computed over a shorter recent period. If tuned values were to be overwritten and additional recent survey data were available, the RCT3 programme was used to calibrate recruitment levels using its default options. As XSA cannot incorporate survey indices collected after the last year of the catch-at-age data, previous WG’s have treated some spring surveys as if they were carried out at the end of the preceding year. The age ranges were then shifted down by one year. A consequence of this is the loss of tuning data for the oldest true age in the survey, which can cause problems for stocks with no other tuning data for these ages. The 2002 meeting of the WG avoided this problem and retained the original age- and year- ranges. However, at the 2003 meeting the WG had been explicitly asked to use the most recent available data in the assessments. The WG therefore reverted to its previous practice of treating some spring surveys as if they were carried out at the end of the preceding year.

Minor exceptions to the implementation of the procedure outlined above are described in the Sections for each stock.

In view of ACFM concerns about the use of recruit calibration in XSA where the use of such a model may not be justified, all cases where this catchability model was used were reviewed closely by the Working Group using the criteria outlined in paragraph d) above. For consistency of notation in the individual stock sections, ages which have been treated as recruits in this manner, and thus where catchability has been treated as a power function of population size are referred to as using the power model, whereas ages where this option has not been used are referred to as ages using the mean-q model.

The 1999 WG noted an apparent problem in the convergence of XSA on data for VIIa plaice, and highlighted significant differences in catchability residuals between two runs with identical parameters, one of which had been allowed to iterate until “converged” (41 iterations), while the other had been stopped at 30 iterations. The XSA algorithm contains a feature in the fitting procedure which is intended to reduce the risk of finding a local minimum, and is invoked for the first of each set of ten iterations chosen after the default of 30 have been completed. Results from XSA convergence on 31, 41, 51 etc. iterations should be viewed with caution, as occasionally the feature can have the opposite effect. Carrying out more than 30 iterations is usually unlikely to be very fruitful.

Age-based analytical assessments were attempted on all stocks other than anglerfish and megrim. For anglerfish the time-series of age compositions were considered unreliable due to difficulties in ageing. At the 2000 meeting the WG commenced using a modified catch-at-size analysis (CASA) in which estimates of recruitment, selectivity parameters and overall fishing mortality were obtained by fitting predicted length compositions from a size-transition matrix model to the observed annual length distributions of anglerfish landings. The assessment is based on that described in Sullivan et al. (1990), and has been further developed at successive Working Groups.

ACFM in 2003 urged the Working Group to consider alternative assessment approaches for megrim. The survey data were examined as a simple time-series of survey biomass, separated into indices of smaller (pre-recruit) and larger fish.

A proxy of exploitation history was derived by dividing the yield by the survey biomass. The utility of a Collie- Sissenwine Analysis (CSA) was also examined.

2.7 Short-term Predictions and Sensitivity Analyses

Short-term predictions and yield-per-recruit analyses were made for each stock subject to a full analytical assessment.

These analyses were carried out using either the Marine Lab (Aberdeen) programmes (MLA), or the MFDP / MFYPR software (Multi-fleet Deterministic Projection / Multi-fleet Yield-Per-Recruit).

As in previous years, SSB calculations for all stocks are set at 1 January (proportion of F and M before spawning = 0).

Short-term predictions were made after deciding on the most appropriate value for recruitment in both the recent period and over the prediction period. Tuned estimates of recruiting year classes, if considered unreliable, were overwritten by a geometric mean value. In some cases, including where 2004 survey data were available, recruitment estimates from the RCT3 recruit calibration program were used. Where tuned values were overwritten for prediction purposes, they

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The WG estimates of landings for most stocks can differ substantially from the TAC due to partial uptake of national quotas, misreporting or discarding. Unless there was strong evidence that the catch in the interim year of the short-term forecast would be constrained by the TAC or other measures, the WG continued its normal practice of assuming status quo F in the interim year. In other cases, the value chosen as status quo F for each stock was considered in the light of recent variations or trends in the estimates of F, as recommended by ACFM. The estimate of status quo F used by default in short-term predictions was the unscaled mean F at age for the last three years. This procedure stems from the consideration that while the point estimate of terminal F represents the best available estimate of F2003, it does not necessarily follow that it will also be appropriate as an estimate of F in 2004 and subsequent years. In the absence of any recent trends in F, an unscaled mean is considered a more appropriate estimate of status quo F than a scaled value.

The mean F vector was scaled to the mean F in the terminal year if there was clear evidence of a recent trend in F that is considered likely to continue or halt rather than increase again in the short term. A special case is a trend caused by retrospective bias. In this case, the true level of fishing mortality in the current year is essentially unknown, although it may still be possible to forecast the approximate status quo catch. To do this, the correlation between numbers and fishing mortality calculated from a given catch in the last year of the assessment must be retained otherwise the landings forecast may be seriously biased. In this case, a mean F over several years would be inappropriate. However, all forecasts based on assessments with strong retrospective bias must remain suspect.

Over-optimistic forecasts have been noted in some stocks assessed by ICES in which trends in weight-at-age are apparent and future weights are specified as an arithmetic mean of historic values. For most stocks assessed by the present Working Group, trends in weights at age were examined. For some stocks, the mean weights in the last year were used in forecasts if a recent trend was evident. Previous assessments of the VIIa haddock stock have taken year- class effects on growth into account when calculating stock weights for forecasts.

A detailed short-term prediction was made for each stock using the status quo F option. The contribution of recent year classes to future SSB and yields was tabulated, and the contribution of different sources of uncertainty to the variance of predicted SSB and yield was estimated where possible by means of sensitivity analysis. The sensitivity analysis programme WGFRAN4 gives estimates of the proportion of the total variance of predicted SSB and catch contributed by different inputs. The description of the abbreviated variable names on the Figures and Tables which show the results of sensitivity analyses for each stock is as follows (a is the age at recruitment, numerals indicate years):

Variable: Description:

Na Population number at age a in 2004 WSa Stock weights at age a in prediction

WHa Catch weights (landings) at age a in prediction WDa Catch weights (discards) at age a in prediction Ma Natural mortality at age a

MTa Proportion mature at age a

SHa Selectivity (human consumption fleets) at age a SDa Selectivity (discards) at age a

sIa Selectivity (bycatch) at age a

K04 Year effect on natural mortality in prediction in 2004

HF04 Year effect on (landings and discards) fishing mortality in 2004 R05 Recruitment in 2005

2.8 Reference Points

The terms of reference of the 1999 WG meeting requested the Group to review progress in determining reference points. This follows-on from the work done by the Group at its 1998 meeting, where following careful review, candidate reference points were proposed for each stock. These reference points were then considered, and in some cases revised, by ACFM, who have used them to frame management advice for the stocks for 1999 and subsequent years. WGNSDS2004 was asked to comment on the PA reference points proposed by the Study Group on Precautionary Reference Points for Advice on Fishery Management (SGPRP2004). The procedures used by the Working Group to select candidate reference points made use of the full time series of spawning stock, recruitment and fishing mortality estimates available for each stock.

The annual assessments made by the Working Group involve only the addition of a single year of data, and as a result would not normally result in a large change in the perception of the stock’s dynamics. Furthermore, it is desirable that once reference points are defined for a stock, they should remain stable, and should not be redefined without a firm

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basis for doing so. While it may be desirable to review reference points if e.g., there is a major change in assessment data or methodology, or a substantial change in the perception of the relationship between spawning stock and recruitment, these cases represent the exception rather than the rule. Hence, a review of precautionary reference points was not undertaken at WGNSDS2004.

2.9 Quality control

The terms of reference for the WG include identifying major deficiencies in assessments. The problems associated with individual assessments are discussed in the ‘quality of assessment’ sections within each individual stock section. In many cases, the problems are associated with data quality: e.g. due to misreporting; discard estimates of low precision;

survey data with catchability problems, etc. For some stocks such as Irish Sea haddock and plaice, and Rockall haddock, there are clear deficiencies in the data due to the absence of time series of discard estimates particularly for young fish for which survey indices are available. For anglerfish, and to a lesser extent megrim, there are major deficiencies in the understanding of the basic biology of the species that impede the development of appropriate stock assessments. In Rockall haddock and megrim there are major components of the catch for which there is no length or age sampling or a discontinuous time series of such data.

A major problem that affects many of the assessments is retrospective bias and how it should be dealt with in catch at age analysis and short-term predictions. Where retrospective bias was evident, the Working Group considered possible causes with reference to both catch and survey data. However, no attempt was made to correct any forecasts for bias. In general arbitrary shrinking of the assessment was avoided, as this generally does not deal with the underlying problem, but merely hides it. However, there are some stocks for which there are few age-classes in the stock and a short time series. In such cases XSA requires relatively high levels of F shrinkage.

2.10 Biological parameters of stocks

Previous ACFM reviewers have commented on the different methods used by the WG to estimate stock weights, and have been particularly concerned at using catch weights as the proxy for stock weights. The declining abundance and age composition in heavily exploited gadoids means that weights at age may be poorly estimated for the older ages where few fish may be represented in the age length keys for the catches. This adds un-necessarily to the uncertainties in mean weight at age in the forecast, both for catch and stock. In cases where catch (or even worse, landings weights) for partially recruited ages are used as stock weights, the biomass will be over-estimated for these ages. This can lead to incorrect total biomass estimates in the VPA output. There is a pressing need for this (and presumably other WGs) to develop a consistent methodology for (a) dealing with the variability introduced by small numbers of fish at the older ages in ALKs and (b) to develop robust and consistent methods for estimating stock weights that are not influenced unduly by sampling error and that track real changes in growth of different year classes.

The interaction between maturity ogives and stock weights influences the estimation of reference points for spawning stock biomass. The maturity ogives for some of the stocks assessed by the WG have remained unchanged for many years and may no longer be appropriate. The ogives for Irish Sea cod, plaice and sole were revised following sampling carried out as part of an EU contract to estimate SSB using the annual egg production method. However, the use of these ogives for the full historic series may not be appropriate, particularly in view of the large changes in stock size over time. The WG recommends that a comprehensive review of the biological parameters of the stocks should be carried out, including analysis of recent survey data and an evaluation of the information (if available) on which historic estimates have been based.

2.11 Total Allowable Catches

The Total Allowable Catches (TAC) by species and management area for stocks assessed by the Working Group were as follows: