ACOM1009.pdf (22.48Mb)

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ICES

ADVISORY

C

OMMITTEE

ICES CM 2009/ACOM:10

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

Skagerrak - Combined Spring and Autumn (WGNSSK)

6 - 12 May 2009

ICES Headquarters, Copenhagen

By Correspondence – September 2009

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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. 2009. Report of the Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak - Combined Spring and Autumn (WGNSSK), 6 - 12 May 2009, ICES Headquarters, Copenhagen.. 1028 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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Conten ts

0 Executive Summary ... 1

1 General ... 2

1.1 Terms of reference ... 2

1.2 InterCatch... 3

2 Overview ... 4

3 Nephrops (Norway lobster) in Division IIIa and Subarea IV ... 5

3.1 General comments relating to all Nephrops stocks... 5

3.1.1 Introduction... 5

3.1.2 WKNEPH... 5

3.2 Nephrops in Division IIIa ... 6

3.2.1 General ... 7

3.2.2 The Skagerrak (FU3) ...10

3.2.3 The Kattegat (FU4) ...11

3.2.4 Combined assessment (FU 3 & 4) ...12

3.3 Nephrops IN Subarea IV ... 12

3.3.1 Botney Gut/Silver Pit (FU 5) ...13

Landings. ...13

Discards. ...13

Length compositions ...13

Natural mortality, maturity at age and other biological parameters ...14

Commercial catch-effort data and research vessel surveys ...14

3.3.2 Farn Deeps (FU6)...15

3.3.3 Fladen Ground (FU7) ...19

3.3.4 Firth of Forth (FU 8)...25

3.3.5 Moray Firth (FU 9) ...30

3.3.6 Noup (FU 10)...35

3.3.7 Norwegian Deep (FU 32)...36

3.3.8 Off Horn Reef ...39

3.3.9 Other Rectangles in Subarea IV ...40

4 Sandeel in IV (WGNSSK Sep. 2009) ...133

4.1 General... 133

4.1.1 Ecosystem aspects ...133

4.1.2 Fisheries...133

4.1.3 ICES Advice...133

4.1.4 Management...134

4.2 Data available ... 134

4.2.1 Catch ...134

4.2.2 Age compositions...135

4.2.3 Weight at age...135

4.2.4 Maturity and natural mortality ...135

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4.2.5 Catch, effort and research vessel data ...136

4.3 Data analyses ... 137

4.3.1 Exploratory catch-at-age-based analyses ...137

4.3.2 Final assessment ...138

4.4 Historic Stock Trends... 139

4.5 Recruitment estimates ... 139

4.6 Short-term forecasts ... 139

4.6.1 Harvest control rule ...142

4.6.2 Stochastic short-term forecast...143

4.7 Medium-term forecasts ... 143

4.8 Biological reference points... 143

4.9 Quality of the assessment ... 143

4.10 Status of the Stock... 143

4.11 Management Considerations ... 143

5 Norway Pout in ICES Subarea IV and Division IIIa ...195

5.1 General... 195

5.1.3 ICES advice...197

5.1.4 Management up to 2009...198

5.2.1 Landings...198

5.2.2 Age compositions in Landings...198

5.2.5 Catch, Effort and Research Vessel Data ...201

5.3 Catch at Age Data Analyses ... 202

5.3.1 Review of last year’s assessment ...202

5.3.2 Final Assessment ...203

5.3.3 Comparison with 2008 assessment...203

5.4 Historical stock trends ... 203

5.5 Recruitment Estimates... 203

5.6 Short-term prognoses ... 204

5.7 Medium-term projections ... 205

5.10 Status of the stock ... 205

5.11 Management considerations ... 206

5.11.1Long term management strategies ...207

5.12 Other issues... 207

6 Plaice in Division VIId ...237

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6.1 General... 237

6.2.1 Catch ...239

6.3.1 Reviews of last years assessment...240

6.3.3 Exploratory survey-based analyses...242

6.3.4 Conclusions drawn from exploratory analyses ...242

6.10 Status of the stock ... 246

6.11 Management considerations ... 246

7 Plaice in IIIa ...297

7.1 Ecosystem aspects ... 297

7.2.1 Catch ...299

7.3.1 Reviews of last year’s assessment...301

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7.7 Medium-term forecasts - none... 302

7.12 References ... 303

8 Plaice in Subarea IV ...330

8.1 General... 330

No new information on ecosystem aspects was presented at the working group in 2009. All available information on ecosystem aspects can be found in the Stock Annex. ...330

8.2.1 Catch ...332

9 Sole in Subarea VIId ...406

9.1 General... 406

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9.2.1 Catch ...408

9.3.2 Exploratory catch at age analysis...410

9.3.4 Conclusion drawn from exploratory analyses...411

9.4 Historical Stock Trends ... 412

9.6 Short term forecasts... 413

9.7 Medium-term forecasts and Yield per recruit analyses ... 414

10 Sole in Subarea IV...459

10.1 General... 459

10.1.1Ecosystem aspects ...459

10.1.2Fisheries...459

10.1.3ICES Advice...460

10.1.4Management...461

10.2.1Catch ...465

10.2.2Age compositions...465

10.2.3Weight at age...465

10.2.4Maturity and natural mortality ...466

10.2.5Catch, effort and research vessel data ...466

10.3.1Exploratory catch-at-age-based analysis...467

10.3.2Exploratory survey-based analyses...467

10.3.3Conclusions drawn from exploratory analyses ...467

Final assessment ...468

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10.10Status of the Stock... 471

10.11Management Considerations ... 471

11 Saithe in Subareas IV, VI and Division IIIa ...508

11.1 Ecosystem aspects ... 508

11.1.3Evaluation of the Management plan...510

11.2.1Catch ...510

11.3.1Reviews of last year’s assessment...512

11.3.3Exploratory catch-at-age-based analyses ...513

11.3.5Final assessment ...514

12 Whiting in Subarea IV and Divisions VIId and IIIa ...545

12.1 General... 545

12.1.1Stock Definition ...545

12.1.2Ecosystem aspect ...545

12.1.4ICES Advice...547

12.2.1Catch data issues for 2008 ...548

12.2.2Catch ...549

12.3.1Summary of 2009 benchmark workshop ...552

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12.3.2Reviews of last year’s assessment – what were the

comments? ...552

12.12Whiting in Division IIIa ... 558

13 Haddock in Subarea IV and Division IIIa (N) ...610

13.1 General... 610

13.1.3ICES advice...611

13.2.1Catch ...614

13.4 Historical Stock Trends ... 618

13.7 Medium-term forecasts and yield-per-recruit analyses ... 621

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14 Cod ...684

14.1 General... 684

14.1.1Stock definition ...684

14.2.1Catch ...692

15 Cod management plan evaluations ...764

15.1 Background... 764

15.2 Review of the North Sea Cod Management Evaluation ... 768

15.3 Conclusions from WGNSSK 2009 for ACOM Advice ... 770

15.4 References: ... 770

16 Mixed fisheries ...773

Annex 1 – List of Participants ...775

Annex 2 – Update forecasts and assessments ...778

Annex 3 – Stock Annexes ...813

Stock Annex- Cod in Subarea IV, Division VIId and Division IIIa West (Skagerrak) ...813

Stock annex: Haddock in Subarea IV and Division IIIa(N) ...836

Stock Annex: WGNSSK – Norway pout ...845

Stock Annex: Plaice IIIA ...889

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Stock Annex: Plaice in area IV...895

Stock Annex: Plaice in Division VIId ...906

Stock Annex: Sole in Division VIId ...917

Stock Annex: Whiting in Subarea IV and Division VIId ...926

Stock Annex: FU6, Farn Deeps ...945

Stock Annex: FU7, Fladen Ground ...953

Stock Annex: FU8, Firth of Forth ...961

Stock Annex: FU9, Moray Firth ...969

Stock Annexes – Sandeel in IV...979

Annex 4: Assessment Methods and Software ...994

Annex 5 -Technical Minutes of the North Sea ecosystem Review Group ... 1005

Cod in Subarea IV (North Sea), Division VIId (Eastern Channel), and IIIa West (Skagerrak) cod_347d... 1006

Haddock in Subarea IV (North Sea) and Division IIIa (Skagerrak – Kattegat) had-34 ... 1007

Nephrops in Division IVb (Farn Deeps, FU6) nep-6... 1008

Nephrops in Division IVa (Fladen Ground, FU7) nep-7 ... 1010

Nephrops in Division IVb (Firth of Forth, FU8) nep-8... 1011

Nephrops in Division IVa (Moray Firth, FU9) nep-9 ... 1012

Norway Pout in ICES sub area IV and division IIIa nop-34... 1013

Plaice in Division VIId (Eastern Channel) ple-eche ... 1014

Plaice in Division IIIa (Skagerrak – Kattegat) ple-kask ... 1015

Plaice Sub-area IV (North Sea) ple-nsea ... 1016

Saithe in Sub-areas IV (North Sea), VI West of Scotland), and Division IIIa (Skagerrak) sai – 3a46... 1017

Sandeel in Subarea IV (North Sea excluding Shetland) san-nsea... 1018

Sole in Division VIId (Eastern Channel) sol- eche ... 1019

Sole in Sub-area IV (North Sea) sol-nsea ... 1020

Whiting Sub-area IV (North Sea) & Division VIId (Eastern Channel) whg-47d ... 1021

Whiting in Division IIIa (Skagerrak - Kattegat) whg-kask... 1022

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Nephrops in Division IVa (Noup, (FU 10) nep-10 ... 1023

Nephrops in Division IVa (Norwegian Deeps, (FU 32) nep-32 ... 1024

Nephrops in Division IVb (Off Horn Reef, FU 33) nep-33 ... 1025

Nephrops in Division IVbc (Botney Gut – Silver Pit, (FU 5) nep-5... 1026

Nephrops in Division IIIa (Skagerak Kattegat, (FU 3,4) nep-iiia... 1027

Sandeel in Subarea IVa (Shetland area ) san-shet ... 1028

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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 6-12 May 2009. The Working Group made stock assessments for demersal and industrial stocks in the North Sea, Skagerrak and Eastern Channel. These assessments included estimates of misreporting and discard and provided catch forecasts. Fur- thermore, stock recovery and management plans were evaluated and the Group commented on the outcome of existing management measures. Descriptions of fisheries were updated the report includes information on national sampling levels and data availability. The group also met by correspondence in September of 2009 to carry out assessments of the sandeel in the North Sea and the second of the biennial assessments of the Norway pout; and by correspondence in October of 2008 to provide update forecasts for stocks with survey information collected after the May meeting.

No update of the executive summary was provided for 2009. For information, please contact the Chair of the meeting.

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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] (Chair: Chris Darby, UK) will meet at ICES HQ, 6–12 May 2009 to:

a ) address generic ToRs for Fish Stock Assessment Working Groups (see table below). The Sandeel and Norway pout assessment 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.

WGNSSK will report by 18 May and 18 September 2009 (Sandeel/Norway pout) for the attention of ACOM.

Fish

Stock Stock Name Stock Coor-

dinator

Assessment Coord. 1

Assessment

Coord. 2 Advice cod-347d Cod in Subarea IV, Divison VIId &

Division IIIa (Skagerrak) UK(England) UK(England) Denmark Advice had-34 Haddock in Subarea IV (North Sea) and Division IIIa UK(Scotland) UK(Scotland) UK(England) Advice

nep-5 Nephrops in Division IVbc (Botney Gut

- Silver Pit, FU 5) UK(Scotland) UK(Scotland) Denmark No advice nep-6 Nephrops in Division IVb (Farn Deeps,

FU 6) UK(Scotland) UK(Scotland) Denmark Advice

nep-7 Nephrops in Division IVa (Fladen

Ground, FU 7) UK(Scotland) UK(Scotland) Denmark Advice

nep-8 Nephrops in Division IVb (Firth of

Forth, FU8) UK(Scotland) UK(Scotland) Denmark Advice

nep-9 Nephrops in Division IVa (Moray Firth,

FU9) UK(Scotland) UK(Scotland) Denmark Advice

nep-10 Nephrops in Division IVa (Noup, FU

10) UK(Scotland) UK(Scotland) Denmark No ad-

vice nep-32 Nephrops in Division IVa (Norwegian

Deeps, FU 32) UK(Scotland) UK(Scotland) Denmark No ad-

vice nep-33 Nephrops in Division IVb (Off Horn

Reef, FU 33) UK(Scotland) UK(Scotland) Denmark No ad-

vice nep-iiia Nephrops in Division IIIa (Skagerak

Kattegat, FU 3,4) Denmark Sweden UK(Scotland) No ad-vice nop-34 Norway Pout in Subarea IV and Divi-sion IIIa Denmark Denmark Norway Advice ple-eche Plaice in Division VIId (Eastern Chan-

nel) France France Belgium Advice

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ple-kask Plaice in Division IIIa (Skagerrak -

Kattegat) Denmark Denmark Sweden Advice

ple-nsea Plaice Subarea IV (North Sea) Netherlands Netherlands Belgium Advice

sai-3a46

Saithe in Subarea IV (North Sea) Divi- sion IIIa West (Skagerrak) and Subarea

VI (West of Scotland and Rockall) Norway Norway Germany Advice san-nsea Sandeel in Subarea IV excluding the

Shetland area Denmark Denmark Norway Advice

san-shet Sandeel in Division IVa North of 59° N

and West of 0 ° E – (Shetland area) UK/ Den-

mark No ad-

vice sol-eche Sole in Division VIId (Eastern Chan-

nel) Belgium Belgium France Advice

sol-nsea Sole in Subarea IV (North Sea) Netherlands Netherlands Belgium Advice

whg-47d Whiting Subarea IV (North Sea) &

Division VIId (Eastern Channel) UK(Scotland) UK(Scotland) UK(England) Advice whg-

kask Whiting in Division IIIa (Skagerrak -

Kattegat) Sweden Sweden Denmark Same

advice as last year

1.2 InterCatch

InterCatch is not used for the data collation of cod, haddock and whiting. The reason is that InterCatch cannot currently be used to generate discard estimates for countries for which no discard sampling data are available. This is a necessary part of the collation process for the three stocks mentioned above. As an interim measure, collation has been carried out for the last three years using a spreadsheet-based approach.

No further update of the 2008 general introduction was provided. For information, please contact the Chair of the meeting.

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2 Overview

No update of the 2008 overview was provided, for the most recent overview see Re- port of the Working Group on the Assessment of Demersal Stocks in the North Sea and Skagerrak (WGNSSK) 2008 CM 2008\ACOM:09, section 2.

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3 Nephrops (Norway lobster) in Division IIIa and Subarea IV 3.1 General comments relating to all Nephrops stocks

3.1.1 Introduction

Nephrops stocks have previously been identified by WGNEPH on the basis of popula- tion distribution and characteristics, and established as separate Functional Units.

The Functional Units (FU) are defined by the groupings of ICES statistical rectangles given in Table 3.1.1 and illustrated in Figure 3.1.1. The statistical rectangles making up each FU encompass the distribution of mud sediment on which Nephrops live.

There are two FUs in Division IIIa and eight FUs in Subarea IV. It is important to note that additional catches of Nephrops are also taken from smaller, isolated pockets of mud distributed throughout the ICES divisions. In recent years some of these areas have contributed significant landings despite their small size (eg Devils Hole). Man- agement of Nephrops currently operates at the ICES Subarea/Division level.

Functional Units were previously aggregated by WGNEPH into a series of nominal Management Areas (MA) intended to provide a pragmatic solution for more localised management. The Working Group agreed that this process had served no useful pur- pose and should be discontinued.

At the WG this year, advice was requested for FUs for which UWTV surveys are available (Farn Deeps, Fladen, Firth of Forth and Moray Firth) whilst the other FUs were given ‘No advice’ status. For those FUs requiring no advice, the report therefore consists solely of an update to available data and text describing the fishery, and no assessment or data analyses have been carried out.

The presentation of data and text relating to the Division IIIa FUs can be found as follows: Skagerrak (FU3) in Section 3.2.2; Kattegat (FU4) in Section 3.2.3; Divison IIIa overall in Section 3.2.3. The presentation of data and assessments for the Division IV FUs can be found as follows: Botney Gut – Silver Pit (FU 5) in Section 3.3.1; Farn Deeps (FU 6) in Section 3.3.2; Fladen (FU 7) in Section 3.3.3; Firth of Forth (FU 8) in Section 3.3.4; Moray Firth (FU 9) in Section 3.3.5; Noup (FU 10) in Section 3.3.6; Nor- wegian Deeps (FU 32) in Section 3.3.7; Off Horn Reef (FU 33) in Section 3.3.8; Other areas of Subarea IV in Section 3.3.9.

Overall landings for Divisions IIIa and IV reported to the WG are summarised by Functional Unit in Table 3.1.2 and Figure 3.1.2.

3.1.2 WKNEPH

General comments relating to Nephrops stocks with TV surveys, developments in assessments and the approaches employed are set out in the following section

A benchmark meeting for Nephrops stocks with TV surveys was held in March 2009.

This meeting was called in order to harmonise the approach taken between the different assessment working groups (WGCSE, WGNSSK). The principle task of the benchmark group was to decide how the TV indices should be used (i.e. as relative trends in abundance or absolute abundance estimates). There are several issues regarding the use of the indices as absolute abundance indices including edge effects, detection rate, confusion with other species and burrow occupancy. In the face of these potential biases it is perhaps desirable to use the indices simply as relative trends in abundance however this approach encounters even more serious problems

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in providing catch advice. It was not clear how proportional changes in a relative abundance index should be translated into proportional changes in TAC advice (is 1:1 correspondence suitable). Given the short time series of landings data which are considered reliable (2006 onwards) it was also not possible to determine if the relative harvest rates observed in this time period were likely to be sustainable or not. It was therefore decided that the TV indices should be used as absolute measures of abundance and that the potential biases should be estimated.

For each functional unit, bias estimates for edge effects, detection rate and confusion with other species were quantified using a combination of modelling, re-analyses and expert judgement resulting in a bias correction factor to be applied to the raw TV abundance indices. The area which resulted in the largest change to procedure was a re-evaluation of the size selectivity of the TV surveys. Previously there had been an implicit assumption that the TV surveys had the same size selectivity as the fishery, however after extensive debate the Group considered that the TV surveys were de- tecting burrows of individuals considerably smaller than the fishery can take. The proportion of the total abundance index which is available to the fishery is now considered to be a fraction of what it was and hence the harvest ratios equivalent to fishing at F0.1 (or any other proxy for sustainability) also require downward revision.

Failure to do this will result in fishing at a rate greater than the target.

New proxies for sustainable fishing (F^0.1 and F^max) were calculated for each FU and are given in the stock annexes. Two modelling approaches were used to derive harvest ratios equating to the candidate reference points under the new assumption of survey selectivity. Both approaches used the same growth, maturity and fishery selectivity data and were cross checked for consistency in the determination of the candidate reference points. The different assumptions in the models governing the length distributions at the time of the survey resulted in different harvest ratios for the given values of F0.1 and Fmax. Both modelling approaches appear to be reasonable simplifi- cations of a complex system and as such there is no a priori reason to believe that either model is more correct than the other. The benchmark group therefore decided that, for each candidate F-reference point, the mean harvest ratio between the two approaches should be taken for the point estimate for that FU. There is therefore no direct relationship between the harvest ratio presented and a particular F value (and additionally the fishing mortalities of male and female Nephrops are different). Refer- ence fishing mortality values are therefore not given in this report.

3.2 Nephrops in Division IIIa

Official landings supplied to ICES for Division IIIa are shown in Table 3.2.1.1.

Division IIIa includes FU 3 and 4, which are assessed together. This year’s assessment is an update of last year’s indicator assessment. Total Nephrops landings by FU and country are shown in Table 3.2.1.2 and Table 3.2.1.3.

FU 3 and FU 4 have for many years, mainly on basis on historical differences in the local fisheries, been maintained as separate stock units. The minor differences observed between the two areas in for instance size distributions may well have been due to area based differences in selectivity of fishing gear. However, for many years the trends both in fisheries data (LPUE) and size data have been very similar and do not indicate any significant differences between the two areas. Consequently, in the assessments and advice the two FUs have always been merged.

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Therefore, the WG suggests and recommends that both assessment data and assessments for these two FUs formally are merged into a single FU, comprising both Ska- gerrak and Kattegat (ICES Division. IIIa).

3.2.1 General

3.2.1.1 Ecosystem aspects

Nephrops lives in burrows in suitable muddy sediments and is characterised by being omnivorous and emerge out of the burrows to feed. It can, however, also sustain it- self as a suspension feeder (in the burrows) (Loo et al., 1993). This ability may contri- bute to maintaining a high production of this species in IIIa, due to increased organic production.

Severe depletion in oxygen content in the water can force the animals out of their burrows, thus temporarily increasing the trawl catchability of this species during such environmental changes (Bagge et al. 1979). A specially severe case was observed in the end of the 1980s in the southern part of IIIa in late summer, where initially un- usually high catch rates of Nephrops were observed. Eventually the increasing amount of dead specimens in the catches lead to the conclusion of severe oxygen deficiency in especially the southern part of IIIa (Kattegat) in late 1988 (Bagge et al., 1990).

No information is available on the extent to which larval mixing occurs between Nephrops stocks, but the similarity in stock indicator trends between FU 3 and 4 for both Denmark and Sweden indicates that recruitment has been similar in both areas.

These observations suggest they may be related to environmental influences.

3.2.1.2 Functional units a nd their fisheries.

Denmark

The restrictions in the fisheries for especially cod seem to have resulted in some significant changes in the Danish fisheries for Nephrops. Traditionally, Nephrops have mainly been caught in trawls using 70-89 mm mesh sizes. In the last five years an increasing proportion of total landings of Nephrops have been caught by vessels using gears with mesh sizes >89mm (which previously have been used in the fishery for cod, plaice and other demersal fish species). In Skagerrak and Kattegat it is since 2005 not allowed to use mesh sizes between 70-89 mm unless the codend and the extension piece is constructed of square meshed netting with a sorting grid (Council Regu- lation 27/2005). According to Council Regulation 51/2006 there is unlimited days when using this species selective trawl.

Those changes in fishing patterns may be seen in the light of the declines in most important demersal fish stocks in the North Sea, Skagerrak and Kattegat. Economically, Nephrops is one of the most important human consumption species in the Danish fi- shery in IIIa.

A new national management system was introduced in Denmark from the 1st of Jan- uary 2007. In this new, rather complex, fishing rights system (FKA, ‘vessel quota share’) each fisher is allocated an annual share of the national quota, which he can dispose of in a much more flexible way than previously. He may now trade his share, exchange it or pool it with other fishers share within the frames of the other regulations, e.g. total effort (fishing days) and national quotas and/or closed seasons.

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The sharp increase in LPUE observed for the Danish vessels both in IIIa and in the North Sea, mainly in the Norwegian Deep, may to some extent be explained as a consequence of the regulation system.

• One would expect that the shares targeting Nephrops gradually will be con- centrated among the more skilled Nephrops fishers.

• The fishers targeting Nephrops will optimise (minimise) their use of effort in catching their share of the FKA.

One consequence of this system is a more efficient use of the effort by the skilled fishers, which again renders the use of logbook recorded effort data even more proble- matic when using the data for tuning assessment models or for instance LPUEs directly as indicators of stock fluctuations for Nephrops stocks,

Sweden

The specialised Swedish Nephrops trawler (catching >3t/yr) shows a decrease in number during 2000 to 2004 (123 to 83 trawler) and an increase the last four years. The increase is mainly due to an increase of trawlers catching >10 t/yr (from 18 in 2004 to 45 in 2008) (see Figure 3.2.1.1). In 2008, mean length was 15m (ranging from 8 to 34m) and GRT of 46 (3 to 263).

Since 2004, new technical regulations were introduced for Swedish national waters in both FU 3 and FU 4. As Sweden has bilateral agreements with Denmark and Norway to fish inside the 12 NM limit, the regulations cover only waters exclusively fished by Swedish vessels (inside 3 NM in Kattegat and 4 NM in Skagerrak). The new regulations imply that it is mandatory to use a 35 mm species selective grid and 8 meter of 70 mm full square mesh codend and extension piece when trawling for Nephrops on Swedish national waters. The Swedish Nephrops landings from MA IIIa by gear during 1989 to 2008 are shown in Figure 3.2.1.2. Twin trawls were introduced in 1990 and the grid and square mesh trawls were legislated in Sweden during 2004 and show an increasing use since then. 65% of the Nephrops trawlers operating in IIIa used the grid at some time of the year during 2008.

A new coding of fishing gears in the Swedish log books has taken place since 2007 where the twin trawl code is phased out and the number of trawls of the new trawl codes should be registered. This mean that twin trawls in 2007 likely is included in other trawl categories that earlier was considered as single trawls. Since 2007, it is possible to distinguish between single and twin trawls in the new category with grid and square mesh targeting Nephrops. In recent three years, around 40% of the Neph- rops trawl landings in IIIa was caught with this new trawl. In the first quarter of 2008 a new effort regulation was introduced in the Kattegat, meaning that a “day at sea”

without the grid equipped trawl was counted as 2.5 This has further increased the incentives to use the sorting grid to the point were 80 % of all Kattegat Nephrops landings in the first quarter of 2008 were caught with sorting grids (compared to around 20% previous years).

The landings from the Swedish creel fishery show an increasing trend in recent years and comprise 26-29% of the Swedish Skagerrak landings in recent four years. The trends in effort and LPUE (g/creel) are shown in Figure 3.2.1.3 and show an increasing trend in effort during the last ten years while LPUE fluctuate without trend.

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Norway

In Skagerrak Nephrops is fished all year round. The largest part of the catches is taken with trawl (Nephrops and shrimp trawls (as bycatch)). In 2001 a creel fishery started developing with landings constituting about 12% of total annual landings.

Nephrops recordings in Norwegian log books from Skagerrak are incomplete. In 2004- 2006 logbook recorded catches constituted only 1% of the landings, but increased to 28% in 2008. Furthermore, records on the use of Nephrops trawl are lacking in the logbooks for 2006-2008. Norwegian trawlers fish in the whole Skagerrak. Catches from along the Norwegian coast are landed in Norway. Some catches are also landed in Sweden.

The following regulations apply: Fishing with mesh sizes down to 70 mm is legal, but requires square meshes in the cod end, and that the bycatch of other species should not exceed 70% of the total weight. The minimum legal size is 40 mm CL, but landings can none the less contain up to 10% animals (in weight) below the legal size. In Skagerrak in 2000-2005, 97% of Nephrops landings were taken by small-meshed trawls (<90 mm).

ICES Advice

In 2008 ICES concluded that:

‘Due to uncertainty in the available data ICES is not able to reliably forecast catch.

There are no signs of decline in the stocks and therefore current levels of exploitation and effort appear to be sustainable.’

No specific catch levels were recommended, but ICES gave the following comments:

The fishing effort on Nephrops has decreased since 2002 and is currently at a low level.

In recent years, lpue has shown an increasing trend but this is not necessarily an indi- cation of increase in stock abundance. There are no signs of overexploitation in Divi- sion IIIa.

ICES currently advises no catches for cod in Division IIIa, which is a significant bycatch species in the Nephrops fisheries. The current effort regulation (limiting days at sea for gears not using selective sorting grids) may increase the incentives to use sorting grids. This may reduce bycatch of cod.’

Management for FU 3 and FU 4

The 2008 and 2009 TAC for Nephrops in ICES area IIIa was set to 5170 tonnes, i.e. un- changed from 2006 and 2007. The minimum landings size for Nephrops in area IIIa is still 40mm carapace length. This high MLS for IIIa is maintained following advice from the industry. However, this leads to a high discard rate and at present 71% of the catch (N) in IIIa consists of undersized individuals (Figure 3.2.1.4). It is expected that ongoing experimental work on improved selectivity of the gear eventually will reduce the amounts of discards.

Days at sea limits restrict Nephrops trawlers to 19 days per month when using 90mm mesh with no square mesh panel, and 22 days with a square mesh panel. New gear regulations imply that it is mandatory to use a 35 mm species selective grid and 8 m of 70 mm full square mesh codend and extension piece when trawling for Nephrops in Swedish national waters. As Sweden has bilateral agreements with Denmark and Norway to fish inside the 12 nm limit, the regulations cover only waters exclusively fished by Swedish vessels (inside 3 nm in Kattegat and 4 nm in Skagerrak). Since

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2006, days at sea is unlimited for this species selective trawl (Council Regulation 51/2006). The changes in the national Danish regulation system from 2007 are described earlier in this section.

3.2.2 The Skagerrak (FU3) 3.2.2.1 Data availa ble

Landings

Denmark, Sweden and Norway exploit this FU. Denmark and Sweden dominate this fishery, with 61 % and 33 % by weight of the landings in 2008. Landings by the Swe- dish creel fishery represent 13-18 % of the total Swedish Nephrops landings from the Skagerrak in the period 1991 to 2002 and has increased to 29% in 2008 (Table 3.2.2.1) In the early 1980s, total Nephrops landings from the Skagerrak increased from around 1000 t to just over 2670 t. Since then they have been fluctuating around a mean of 2500 t (Figure 3.2.2.1)).

Length compositions

For the Skagerrak, size distributions of both the landings and discards are available from both Denmark and Sweden for 1991-2008. Of these, the Swedish data series can be considered as being the most complete, since sampling took place regularly throughout the time period and usually covered the whole year. In earlier years the Swedish discard samples were obtained by agreement with selected fishermen, and this might tempt fishermen to bias the samples. However, the reliability of the catch samplings is cross-checked by special discard sampling projects in both the Skagerrak and the Kattegat. In recent years the Swedish Nephrops sampling is carried out by on- board observers in both Skagerrak and Kattegat. Geographically, the samples from the Swedish fishery mainly cover the north-eastern part of the Skagerrak.

In 1991, a biological sampling programme of the Danish Nephrops fishery was started on board the fishing vessels, in order to also cover the discards in this fishery. Due to its high cost and the lack of manpower, Danish sampling intensity in the early years was in general not satisfactory, and seasonal variations were not often adequately covered. Due to increasing lack of resources the Danish at sea sampling in Skagerrak was at unsatisfactory low level in 2007 and 2008, and for these years the length composition data for Skagerrak are based on Swedish samples only. The Norwegian Nephrops fishery is small and has not been sampled. Trends in mean size in catch and landings are shown in Figure 3.2.2.1 and Table 3.2.2.2. Mean sizes in landings, in both sexes are fluctuating without trend while there is a slightly decreasing trend for discards.

Maturity and natural mortality

Data on size at maturity for males and females were presented at the ICES Workshop on Nephrops Stocks in January 2006 (ICES WKNEPH, 2006) but since no estimates of SSB has been made, these data were not used in this year’s analysis of these stocks.

Catch, effort and research vessel data

Effort data for the Swedish fleet are available from logbooks for 1978-2008 (Figure 3.2.2.1 and Table 3.2.2.3). In recent years the twin trawlers have shifted to target both fish and Nephrops, and this shift has resulted in a decreasing trend in LPUE from 1998 to 2005 for this gear (Table 3.2.2.3). In the most recent years LPUEs have increased for both gear types. The long term trend in LPUEs (an increase from 1992 to 1998, a de-

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crease from 1999 to 2001 and a subsequent increase in the last 6 years) is similar in the Swedish and Danish fisheries. Total Swedish trawl effort shows a decreasing trend since 1992. From 2004 onwards total Swedish trawl effort has been estimated from LPUEs from the grid single trawl (targeting only Nephrops) and total trawl landings.

Danish effort figures for the Skagerrak (Table 3.2.2.4 and Figure 3.2.2.1) were estimated from logbook data. For the whole period, it is assumed that effort is exerted mainly by vessels using twin trawls. The overall trend in effort for the Danish fleet is similar to that in the Swedish fishery. After having been at a relatively low level in 1994-97, effort did increase again in the next five years followed by a decrease in recent six years. Also the trend in LPUE is similar to that in the Swedish single trawl fishery, however with a much more marked increase in the Danish LPUE for 2007 and 2008. This high LPUE level may be partly a consequence of the new national (Danish) management system introduced in 2007 (see Sect. Fisheries) (Figure 3.2.2.2).

It has not been possible to incorporate ‘technological creeping’ in a further evaluation of the Danish effort data. However, use of twin trawls has been widespread for many years. In 2008 the Danish logbook data was analysed in various ways to elucidate the effect of some factors likely to influence the effort/LPUE (Figure 3.2.2.3):

• Incorporation of kW (HP) in the effort measure

• Vessel size (GLM to standardise LPUE regarding vessel size)

• Degree of targeting Nephrops (measured as value of Nephrops in landing).

Note, that the trends in the resulting LPUE (relative indices) are very similar. How- ever, this may merely reflect that vessels catching Nephrops in this area are very similar with respect to e.g. size and HP.

Norwegian logbook records of Nephrops trawl are lacking for the last five years. Addi- tionally, LPUE data for all trawl gears have covered 9% on the average of the Norwe- gian landings in the last 8 years. Norwegian data are therefore not included in the analysis.

3.2.3 The Kattegat (FU4) 3.2.3.1 Data availa ble

Catch

Both Denmark and Sweden have Nephrops directed fisheries in the Kattegat. In 2008, Denmark accounted for about 71 % of total landings, while Sweden took remaining 28 % (Table 3.2.3.1). Minor landings are taken by Germany.

After the low that was observed in 1994, total Nephrops landings from the Kattegat increased again until 1998. Since then, they have fluctuated around 1500 t. However, landings increased markedly in 2008 to more than 2000 t, the highest observed landings since 1984 (Figure 3.2.3.1).

For the Kattegat, size distributions of both the landings and discards are available from Sweden for 1990-1992 and 2004-2008, and from Denmark for 1992-2008. The at- sea-sampling intensity has generally increased since 1999, but the Danish sampling decreased in 2007 and 2008. Information on mean size is given in Table 3.2.3.2. Trends in mean size are shown in Figure 3.2.3.1 and after some years of small mean sizes 1993 to 1996 all categories are fluctuating without trend the last 12 years.

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Maturity and natural mortality

Data on size at maturity for males and females were presented at the ICES Workshop on Nephrops Stocks in January 2006 (ICES WKNEPH, 2006) but since no estimates of SSB has been made, these data were not used in this year’s analysis of this stock.

Swedish total effort, converted to single trawl effort, has been relatively stable over the period 1978-90. An increase is noted in 1993 and 1994, followed by a decrease to 1996, and a stabilisation at intermediate levels in recent years (Figure 3.2.3.1 and Ta- ble 3.2.3.3)). Figures for total Danish effort are based on logbook records since 1987.

Danish effort increased during 1995 to 2001, but since then it has been showing a decreasing trend until 2007. In 2008 the effort increased slightly (Figure 3.2.3.1 and Ta- ble 3.2.3.4).

It has not been possible to incorporate ‘technological creeping’ in a further evaluation of the Danish effort data. However, use of twin trawls has been widespread for many years. In 2008 the Danish logbook data were analysed in various ways to elucidate the effect of some factors likely to influence the effort/LPUE (Figure 3.2.3.2):

• Incorporation of kW (HP) in the effort measure

• Vessel size (GLM to standardise LPUE regarding vessel size)

• Degree of targeting Nephrops (measured as value of Nephrops in landing).

Note, that the trends in the resulting LPUE (relative indices) are very similar. How- ever, this may merely reflect that vessels catching Nephrops in this area are very simi- lar with respect to e.g. size and HP.

The Swedish single trawl LPUE and Danish annual LPUEs have shown similar trends since 1990. Both series show a marked increase in the last 2 years (Tables 3.2.3.3 &

3.2.3.4; Figure 3.2.3.3).

3.2.4 Com bined assessm ent (FU 3 & 4)

No advice was requested in 2009, so no assessment was carried out this year.

3.2.4.1 Status of the Stock

The 2008 assessment lead to the conclusion for the two FUs in Division IIIa that, given the apparent stability of the stocks, the current levels of exploitation appeared to be sustainable. The most recent assessment data compiled in 2009 do not indicate any changes in the state of the stock

3.3 Nephrops IN Subarea IV

Division IV contains eight FUs 5, 6, 7, 8, 9, 10, 32, and 33. Management is applied at the scale of ICES Division through the use of a TAC and an effort regime.

Management at ICES Subarea Level

The 2008 EC TAC for Nephrops in ICES Subarea IIa and IV was 26144 tonnes in EC waters (plus 1300 tonnes in Norwegian waters). For 2009, this has been reduced to 24837 tonnes in EC waters and 1210 tonnes in Norwegian waters.

The minimum landings size (MLS) for Nephrops in Subarea IV (EC) is 25mm carapace length. Denmark, Sweden and Norway apply a national MLS of 40mm.

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Days at sea limits apply to Nephrops trawlers when using mesh sizes 70-99 mm and in 2009, under the Scottish Conservation Credits Scheme (CCS), the number of days available to Scottish vessels is the same as 2008 and 2007. EU catch composition regulations apply to Nephrops trawlers.

UK legislation (SI 2001/649, SSI 2000/227) requires at least a 90 mm square mesh panel in trawls from 80 to 119mm, where the rear of the panel should be not more than 15 m from the cod-line. The length of the panel must be 3 m if the engine power of the vessel exceeds 112 kW, otherwise a 2 m panel may be used. Under UK legislation, when fishing for Nephrops, the cod-end, extension and any square mesh panel must be constructed of single twine, of a thickness not exceeding 4 mm for mesh sizes 70- 99 mm, while EU legislation restricts twine thickness to a maximum of 8 mm single or 6 mm double.

Under EU legislation, a maximum of 120 meshes round the cod-end circumference is permissible for all mesh sizes less than 90 mm. For this mesh size range, an additional panel must also be inserted at the rear of the headline of the trawl. UK legislation also prohibits twin or multiple rig trawling with a diamond cod end mesh smaller that 100 mm in the North Sea south of 57^o30’N.

Official catch statistics for Subarea IV are presented in Table 3.3.1. The preliminary officially reported landings in 2008 are just over 22,000 tonnes which is around 2,500 tonnes lower than in 2007. Minor updates have been made to landings in previous years.

Table 3.1.2 shows landings by FU as reported to the WG. It also shows that a small but significant proportion of the landings from Subarea IV come from outside the defined Nephrops FUs.

The trends observed in the 2008 North Sea Commission Fisheries Partnership (NSCFP) stock survey for Nephrops are discussed in the Quality of Assessment sec- tions.

3.3.1 Botney Gut/Silver Pit (FU 5) 3.3.1.1 Data Available

Landings.

Table 3.3.1.1 shows the landings from this FU. For many years total landings have been at a level of 1000 t. Up to 1995, the Belgian fleet took more than 75% of the international Nephrops landings from this FU/stock, but since then, the Belgian landings have declined drastically, and since 2006 there has been no directed Belgian Nephrops fishery. Danish landings have been at low levels in recent years. In the most recent years UK, Netherlands and Germany have accounted for most of the landings from this FU.

Discards.

Discard data were available for the Belgian Nephrops fleet for the period 2002 - 2005.

Since 2006, because of no directed fisheries, there has been no data collection from the Belgian Nephrops landings. No discard data are available from the other fisheries.

Danish sampling of landed Nephrops has taken place 2005-2007, mainly as a compen- sation for inadequate at-sea-sampling.

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Data on mean sizes of male and female Nephrops in the Belgian landings (1991- 2005) are shown in Table 3.3.1.2 and Figure 3.3.1.1. The mean sizes of males show evidence of an overall downward trend, while mean sizes of females seem to be stable. There is little evidence in these of a notable change in sizes and the maximum sizes have remained quite constant during this period.

Natural mortality, maturity at age and other biological parameters

In previous analytical assessments (see e.g. WGNEPH, 2003), natural mortality was assumed to be 0.3 for males of all ages and in all years. Natural mortality was assumed to be 0.3 for immature females, and 0.2 for mature females. Discard survival was assumed to be 0.25 for both males and females (after Gueguen & Charuau, 1975, and Redant & Polet, 1994).

• Growth parameters are as follows:

• Males: L∞ = 62mm CL, k = 0.165.

• Immature females: L∞ = 62mm CL, k = 0.165.

• Mature females: L∞ = 60mm CL, k = 0.080, Size at 50% maturity = 27mm CL.

• Growth parameters have been assumed to be similar to those of Scottish Nephrops stocks with similar overall size distributions of the landings (see e.g. WGNEPH, 2003). Female size at 50% maturity was taken from Redant (1994).

Commercial catch-effort data and research vessel surveys

Effort and LPUE figures are available for Belgian Nephrops specialist trawlers (1985- 2005), the Dutch fleet (all vessels catching Nephrops for the period 2000-2005) and the Danish bottom trawlers with mesh size > 70 mm (1996-2008), Table 3.3.1.3 and Figure 3.3.1.1.

The effort of the Belgian Nephrops fleet has shown an almost continuous decrease since the all times high in the early 1990s. In 2005, effort was at the lowest level in the time series No data are available for the 2006-2007

The effort of the Dutch (Nephrops) fleet was relatively stable, between 7900 and 9800 days at sea annually. Danish Nephrops effort in the Botney Gut was always low but has decreased drastically in recent years. The very high LPUE in 2008 may reflect both technological creep and increasing efficiency due to the FKA agreement for fishing industry described in Section 3.2.1.2.

There are no fishery-independent survey data for FU 5.

3.3.1.2 Status of stock

The shortage of information on this stock in the recent 2 years makes an evaluation of stock condition difficult. The high value of the Danish LPUE in 2008 may reflect technological creep, and since the Danish fishery is very small, the LPUE should be viewed with caution. There is no other evidence of significant downward movements trends in LPUE or in mean size, but the lack of more substantial data for the 3 recent years gives rise for concern about the status of this and the stock.

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3.3.2 Farn Deeps (FU6) 3.3.2.1 Fishery in 2007 & 2008

Since the beginning of the time-series, the UK fleet has accounted for virtually all landings from the Farn Deeps (Table 3.3.2.1). In 2008 total landings were 1,213 tonnes, significantly down from both 2,966 t in 2007 and the historical maximum observed in 2006 of 4,858t (Figure 3.3.2.1). The introduction of the buyers and sellers legislation in 2006 precludes direct comparison with previous years because the resulting improvement in reporting levels has created a discontinuity in the data. Effort also decreased sharply in 2008 and has been generally declining since the early 1990s although again the change in legislation in 2006 complicates the interpretation of any trends. Effort trends in terms of KW hours are further complicated by moves towards multi-rig fishing gears which generally have a higher fishing power. The proportion of landings by multi-rig gears (mainly twin riggers) has risen steadily through time and reached just under 40% in 2008 (Figure 3.3.2.2). Historically the fishery is prosecuted by a combination of local English boats (smaller vessels under- taking day-trips) and larger vessels from Scotland with occasional influxes of effort by Northern Irish vessels. The number of vessels in the fishery from Scotland and Northern Ireland decreased in 2008.

The Farn Deeps fishery is essentially a winter fishery commencing in September and running through to March, hence the 2008 fishery comprises the end of the 2007-2008 fishery and the start of the 2008-2009 fishery. Effort in the first and fourth quarters of 2008 was considerably lower than previous years whilst effort in the second and third quarters remained relatively stable (Figure 3.3.2.3).

3.3.2.2 ICES ADVICE in 2006

The last assessment of Nephrops in FU6 was in 2008.

State of the stock. (from ACOM advice sheet)

“The TV survey and lpue data indicate a decline in abundance from the highest estimate in the time-series in 2006 to levels comparable to 1997 and 2002. Mean length in the catches has increased which could indicate that recruitment in 2007 is low, or it could indicate a reduc- tion in fishing mortality. However, there is no apparent trend over the available time-series of relative abundance and mean length and the stock appears to be stable .”

It would appear that there was an error in the final composition of this advice because this is contradictory to both the WGNSSK report for 2008 and the “management considerations” section states of the ACOM advice sheet which states:

“All available indices point to the stock in 2007 having been reduced to a low level following the high abundances in 2005–2006. Latest recruitment signals are low. This is consistent with the industry’s perception of the stock.”

”ICES recommends that the Nephrops fisheries should not be allowed to increase relative to 2007. This corresponds to landings of no more than 3000 t for the Farn Deeps stock. “

3.3.2.3 Mana gement

Management is at the ICES Subarea level as described at the beginning of Section 3.3.

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3.3.2.4 Assessment

Review of the 2008 assessment.

May 2008:

“FU6: the RG agrees with views of EG which are well explained as in terms of assessment as advice. Recent UWTV indices are suggesting a decline in abundance.

For this FU and with the same concerns mentioned before about the general procedure fol- lowed for these stocks, the EG are conscious on delicate of situation and the proposal achieved seems to be more prudent (2,800 t in 2009). It is not recommended to use the average of recent landings, since 2006 landings dropped from 4,858 to 2,966 in 2007. The RG agrees with EG that this stock is in a transition phase and it should be re-assessed in 2009..”

Data available

Three types of sampling occur on this stock, landings sampling, catch sampling and discard sampling providing information on size distribution and sex ratio. The sampling intensity is considered to be generally good (see section 2.????).

Two different procedures have been used to estimate discards with a change in method in 2002. These are described in detail in the Stock Annex.

LPUE had remained relatively stable between 1993-2000, at a relatively high level around 26 kg.hour^-1 (Table 3.3.2.2 & Figure 3.3.2.1). Since 2000 annual LPUE has sharply increased to its highest value in the series in 2006 (38 kg.hour^-1). Since 2006, effort has decreased by 53%, landings by 75% and LPUE by 49%.. The introduction of the buyers and sellers legislation in 2006 precludes comparison with previous years.

Males generally predominate in the landings, averaging about 70% (range 64%-79%) by biomass in the period 1992-2005. The fishery in 2008 continued this trend and there was no repeat of the anomaly in sex ratio (high proportion of females) observed during the 06/07 winter fishery (Figure 3.3.2.3).

Effort is generally highest in the 1^st and 4^th quarter of the year in this fishery (Figure 3.3.2.3) with landings correspondingly highest in these quarters. In 2008 effort was down on recent levels with the exception of quarter 2. The reduced number of larger vessels in 2008 may have a disproportional negative impact on CPUE measures in that the larger vessels are likely to have a higher efficiency. Quarterly LPUE values were more variable than the annual trends, but overall the same pattern is apparent.

LPUEs of males are typically highest in the 1st and 4th quarters. LPUE for males was slightly reduced from 2007 levels in all quarters. The seasonal pattern of LPUE for females is much more variable ranging from very strong seasonality (1998) to almost none (2002). The extremely high LPUE for Females in quarter 4 in 2006 appears to be genuine and not an artefact of sampling. LPUE on Females was considerably reduced on 2007 levels for the first three quarters but increased for the 4^th quarter.

Trends in the mean lengths for the <35mm categories (Figure 3.3.2.1) are used to infer possible changes to recruitment. Changes to the raising procedure in 2000 and 2002 confound comparison with years prior to 2002, but clear upward trends can be seen for both sexes between 2002 and 2007 implying a trend towards lower recruitments.

There was a reduction in mean length in 2005 which corresponded with the high abundance index in 2006. The mean length for the <35mm categories in 2008 are the

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same or lower than 2007 implying improved recruitment for 2008. Length distributions of landed and estimated discarded portions of the catch are shown in Figure 3.3.2.4. Catches of smaller size males are very similar to 2007, however there were considerably more small females.

Analysis of individual vessel records indicates an increase in directed Nephrops fishing since around 2000. Restrictions on both quota and effort for directed finfish fishing over the last seven years will have restricted the more casual effort on Nephrops.

Further research is needed to better define directed fishing effort and thereby im- prove on this series.

Underwater TV surveys of the Farn Deeps grounds have been conducted at least once in each year from 1996 onwards. The most consistent series, and the one used in the assessment is the autumn survey which coincides with the start of the winter fishery.

A time series of indices is given in figure 3.3.2.5 and table 3.3.2.4. Figure 3.3.2.6 shows the distribution of stations and relative density in the most recent 8 TV surveys.

Discard survival is set to zero for this FU in contrast to the 25% used in many other FUs. This is due to the practice of catch sorting and tailing whilst steaming back to port when the vessel passes over ground not suitable for Nephrops habitation.

Natural mortality, maturity at age and other biological parameters Biological parameter values are included in the Stock Annex.

Exploratory analyses of RV data

A comprehensive review of the use of underwater TV surveys for Nephrops stock assessment was undertaken by WKNeph (ICES 2009). This covered the range of potential biases resulting from factors including edge effects, species mis-identification, burrow occupancy. Cumulative bias factors were estimated for each FU and for FU6 the bias correction factor is 1.2 meaning that the TV estimate is likely to overestimate absolute abundance of Nephrops by 20%. Estimates of mean burrow density and the resulting bias-corrected abundance estimates (with confidence estimates) are given in table 3.3.2.4. The confidence estimates presented are a product of the within-strata variance which only partially takes into account the spatial structure of the data.

Analyses which take spatial structuring of the counts into account (such as geo- statistical methods) have been carried out for other FUs and indicate that uncertainty in the estimates of abundance from these underwater TV surveys is considerably overestimated.

Final Assessment.

Mean size of Nephrops <35mm carapace length (CL) in the catch has been generally increasing for both sexes since 2002, peaking in 2007 with similar or lower values in 2008. Mean size above 35mm has been comparatively static from 2002-2007. The im- plication of the increase in mean size for the smaller size classes is that there has been either a significant improvement in survivorship of the older classes or a progressive reduction in recruitment. Given the reduced TV abundances and poor fishing in both 2007 and 2008, a reduction in recruitment would seem the more likely scenario. The TV index for 2008 is at a similar low level to that of the absolute minimum observed in 2007 reflecting the low level of the fishery in these years.

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3.3.2.5 Historic stock tr ends.

The time series of TV surveys is short compared to the IBTS (8 consecutive years) but estimates that the stock has fluctuated between 900 and 1700 million individuals with the most recent two estimates being at the bottom of this range.

Estimates of historic harvest ratio (the proportion of the stock which is removed) range from 7.4% to 24.6% (Table 3.3.2.5). The harvest ratio jumped from around 12%

in 2004-2005 to 24.6% in 2006 when the new reporting legislation came in.

3.3.2.6 Short ter m for eca sts.

Catch and landing predictions for 2010 are given in the text table below. This as- sumes that the bias corrected survey index made in October 2008 is relevant to the stock status for 2010. The harvest ratio estimated to be equivalent to fishing at F0.1 was calculated by WKNeph (2009) to be 8.2%. This is significantly lower than the value used in previous advice due to a revision of the assumptions regarding the sizes of Nephrops observed by the TV survey.

Discard rate = 29.5%, mean weight in retained portion (2006-2008)=23.4g Harvest ratio Bias co rrected

survey index Retained

number Landings

0% 965 0 0

2% 14 318

4% 27 637

6% 41 955

F2008 7.6% 52 1210

8% 54 1274

F0.1 8.2% 56 1305

10% 68 1592

12% 82 1910

13% 90 2117

14% 95 2229

16% 109 2547

Fmax 18% 122 2866

20% 136 3184

3.3.2.7 BRPs

No biological reference points have been determined for Nephrops in FU6.

3.3.2.8 Quality of assessment

Changes to the legislation regarding the reporting of catches in 2006 means that the levels of reported landings from this point forward are considered to better reflect the true landings and hence effort input into this fishery. This does mean that comparison of LPUE with previous years is inadvisable and the independence of the final assessment from these data is likely to continue for some time.

The length and sex compositions arising from the land-based catch sampling programme are considered to be representative of the fishery. Estimates of discarded and retained length frequencies arising from the discard sampling programme are also considered robust since 2002.

The TV survey in this area has a high density of survey stations compared to other TV surveys and the abundance estimates are considered robust.