ICES ADHOC REPORT 2015

Transcription

1 ICES ADHOC REPORT 2015 ICES ACOM COMMITTEE ICES CM 2015\ACOM:45 Report on the request to ICES on impacts of SELTRA mounted trawl on catch efficiency, assessment and catch forecast for sole in Division IIIa and Subdivisions By J. Boje, M. Vinther, and L. Krag DTU Aqua, Denmark

2 International Council for the Exploration of the Sea Conseil International pour l Exploration de la Mer H. C. Andersens Boulevard DK-1553 Copenhagen V Denmark Telephone (+45) Telefax (+45) info@ices.dk Recommended format for purposes of citation: ICES Report on the request to ICES on impacts of SELTRA mounted trawl on catch efficiency, assessment and catch forecast for sole in Division IIIa and Subdivisions 22-24, By J. Boje, M. Vinther, and L. Krag, DTU Aqua, Denmark. ICES CM 2015\ACOM: pp. For permission to reproduce material from this publication, please apply to 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 International Council for the Exploration of the Sea

3 ICES ADHOC REPORT 2015 i Contents 1 EC Request Summary Background and assumptions Input to assessment Assessment Short-term projection Quality considerations References Annex 1 Technical Minutes... 16

4 2 ICES ADHOC REPORT EC Request In its 2014 advice for sole in Division IIIa and Subdivisions 22-24, ICES pointed out that the assessment did not take into account the reduction in catch per unit of effort (cpue) that results from the use of the SELTRA trawl. ICES is requested to quantify this reduction in cpue if possible, or else to give is expert judgment on the likely reduction in cpue. It is also requested to re-assess the stock based on the assumed reduction in cpue, and provide the corresponding TAC for 2015 based on the ICES MSY approach. 2 Summary The reduction in cpue caused by a shift from 90 mm codend trawl to SELTRA trawls has not been directly measured for sole, but ICES assumes a 20% reduction as measured for plaice. The assessment and TAC advice for sole in IIIa is sensitive to changes in catchability of the trawl fishery, due to the use of SELTRA trawl. With an assumed 20% reduction in catchability for the trawl fishery, the estimated SSB in 2013 increases by 18% and fishing mortality decreases by 18%. Accordingly, the MSY advice for 2015 is changed from 211t (ICES, 2014a) to 249 t. 3 Background and assumptions Catch per unit of effort (cpue) from commercial trawlers (April-August) are currently used in the ICES assessment of the sole stock with an assumption of an unchanged catchability since Cease of the scientific sole survey in 2011 means that the assessment is highly dependent of the quality of the commercial cpue (ICES, 2014a). Several changes in trawl selectivity have been made since 2007 to protect cod in the Kattegat, gear changes which might also influence cpue of sole. The usage of an exit window with square meshes at a minimum of 120mm became mandatory in 2008 in the Danish fisheries. Further, new trawls with sorting windows with various designs and mesh sizes (named SELTRA) have been introduced since then (Madsen and Valentinsson, 2010; Madsen et al., 2010). In 2009, as part of the attempts to rebuild the cod stock in the Kattegat, Sweden and Denmark introduced protected areas on historically important spawning grounds. The protected zone consists of four different areas in which the fisheries are either not allowed or limited to certain selective gears (e.g. Danish SELTRA with 300mm mesh size in exit window (SELTRA300)) throughout part, or all of the year. Since 2011, the use of various types of SELTRA trawls has become mandatory in the Danish fisheries in the Kattegat. To conduct a profitable sole fishery in the fourth quarter it is allowed to use less selective trawls in that period, however that period of the year is not included in the cpue time-series. For this analysis it is assumed that it is the change to SELTRA trawls that mainly affect catchability of sole. SELTRA trawls were introduced and used in 2009, but have mainly been implemented since The change in catchability due to the shift to SELTRA trawls has not been quantified for sole. Experiments conducted on plaice and catch efficiency by SELTRA trawls estimated the loss to be 21% (Krag et al., 2015). Although plaice and sole may behave differently in the catching process in SELTRA gears, plaice is considered the most likely species to compare with sole (assuming common flatfish behaviour). Therefore the most likely assumption for sole catchability is considered around 20%.

5 ICES ADHOC REPORT In order to mimic a reduction in catch efficiency in SELTRA trawl, effort used for cpue calculation was reduced to reflect the change in gear selectivity. Assessment and catch forecast scenarios are here conducted with a 20% reduction in effort for the relevant trawl tuning fleet since Input to assessment In Table 1 is provided the revised input to this assessment, the trawler tuning series. Effort (1 st column) is reduced by 20% since 2010 when the SELTRA gear was mainly implemented in the fishery. The remaining input is unchanged from the 2014 assessment (ICES, 2014a).Table 1. Input data. Revised trawler tuning series assuming 20% less effort in (grey shaded). Official logbooks TR All combined

6 4 ICES ADHOC REPORT Assessment The present assessment is conducted according to stock annex for sole in Division IIIa and Subdivisions (ICES, 2014a). The State-space Assessment Model (SAM) is used and input data, diagnostics and results from this assessment is available at stockassessment ( Table 2 provides diagnostics (sd s) by fleet and age and Figure 1 compares model residuals from present assessment and the June 2014 assessment. No major changes in diagnostics from present assessment are evident. The retrospective behaviour of the revised assessment vs. the June 2014 assessment is shown in Figure 3. The revision of the effort for the trawl fleet does not seem to have improved the retro-behaviour significantly. Estimated fishing mortality and stock numbers is given in Table 3 and 4. The stock summary is provided in Table 5 and Figure 2 illustrates estimates of F and SSB compared to the June 2014 assessment. The revised assessment estimates higher SSB in recent years than the June 2014 assessment (1222 t vs t) and a lower F (0.34 vs. 0.41; Figure 2). The lower estimated Fs are evident for all ages (Table 3). Also estimates of recruitment are slightly higher in the revised assessment (Table 5).

7 ICES ADHOC REPORT Table 2. Sole IIIa. SAM diagnostics. Estimated s.d. of log observations. (Fleet1: Catch in numbers, fleet2: Survey, fleet3: trawlers, fleet4: private logbooks from gillnetters, fleet5: private logbooks from trawlers) FLEET AGE SD(LOGOBS) LOW HIGH

8 6 ICES ADHOC REPORT 2015 Table 3. Sole IIIa. Fishing mortality-at-age (age 6-9 assumed constant). YEAR\AGE

9 ICES ADHOC REPORT Table 4. Sole IIIa. Stock number-at-age from SAM. YEAR\AGE * *Estimated by simple forward projection of 2012 stock

10 8 ICES ADHOC REPORT 2015 Table 5. Estimated recruitment, total-stock biomass (TBS), spawning-stock biomass (SSB), and average fishing mortality for ages 4 to 8 (F48) from new assessment run with 20% decreased effort in YEAR RECRUITS LOW HIGH TSB LOW HIGH SSB LOW HIGH F48 LOW HIGH

11 ICES ADHOC REPORT Figure 1. Model residuals from SAM for the trawler tuning fleet (upper: ICES June 2014 assessment, lower: new assessment with 20% reduction in gear efficiency).

12 10 ICES ADHOC REPORT 2015 Figure 2. Stock spawning biomass (SSB) and fishing mortality (F) assuming a reduction of 20% in trawl catching efficiency since The bold black curve is assessment with new settings (i.e. the proxy for 20% reduction in gear efficiency), while grey curve is accepted assessment from June 2014 ICES advice. Associated uncertainty is indicated.

13 ICES ADHOC REPORT Figure 3. Retrospective plots of SSB and Fbar; upper row is June 2014 ICES assessment and lower row is with an assumption of a 20% reduction in catch efficiency in for the trawl tuning fleet.

14 12 ICES ADHOC REPORT Short-term projection The STF is conducted using the ICES standard assessment tools (MFDP1a). Medians from SAM are used as input. The input to the forecast is given in Table 6. Exploitation pattern was unscaled due to lack of trend. Applying Fsq (0.32) in 2014 will provide landings of 350 t (preliminary landing figures in 2014 are 282 t). The forecast predicts that a fishing mortality at Fsq (F4 8 = 0.32 equals FMSY) will lead to yields of 346 t in 2014 and 374 t in 2015 (Table 7). At this level of exploitation, spawningstock biomass is estimated at 1446t in 2015 and 1638 t in SSB in 2015 is still estimated below Btrigger, therefore using the ICES MSY approach implies fishing at 0.23 in 2015 (0.32*1446/2000). At this exploitation level landings are estimated to 249 t and SSB by the end of 2015 estimated to 1765 t. This advice corresponds to a 30% decrease in TAC from The present ICES June 2014 MSY approach advice is provided below together with the forecast from the sensitivity assessment. Outlook table of advice according the MSY approach Weights in tonnes. Rationale Catches (2015) a) Landings (2015) Basis F (2015) SSB (2016) %SSB change b) %TAC change c) MSY approach (ICES, June advice) MSY framework assuming 20% reduction in catch efficiency FMSY SSB2015/MSY Btrigger FMSY *SSB2015/MSY Btrigger % -42% % -30% a) Catches are calculated based on landings and 3% discard rate (in weight), the same as 2013 discard rate. b) SSB 2016 relative to SSB c) Landings 2015 relative to TAC Although the estimated SSB in 2015 is higher than provided in June 2014 ICES assessment (1446t vs. 1230t), it is still lower than MSY Btrigger (2000 t) and the MSY approach therefore still requires a reduction from FMSY.

15 ICES ADHOC REPORT Table 6. Sole IIIa. Input to short-term prediction. MFDP version 1a Run: run2_lowr Time and date: 11:46 04/04/2014 Fbar age range: Age N M Mat PF PM SWt Sel CWt Age N M Mat PF PM SWt Sel CWt Age N M Mat PF PM SWt Sel CWt Input units are thousands and kg - output in tonnes

16 14 ICES ADHOC REPORT 2015 Table 7. Sole IIIa. Management options table for short-term prediction. Basis: F(2014) = Fsq = mean F(11-13) = ; R94-13 = GM = 2.6 million; SSB(2014) = 1.23 kt; SSB(2015) = 1.45 kt; landings (2014) = 0.35 kt The maximum fishing mortality which would be in accordance with msy limits (Fmsy) is 0.32 The fishing mortality which is consistent with taking high long-term yield and achieving low risk of depleting the productive potential of the stock (F(long-term yield)) is 0.2 Rationale Catches (2015) 3) Landings (2015) Basis F (2015) SSB (2016) %SSB change 1) %TAC change 2) MSY framework FMSY *SSB2015/MSY Btrigger % -30% Precautionary approach TAC(Fpa) * % -96% Zero catch F= % -100% Status quo % Fsq (F2014*0.5) -15% TAC(F2014*0.76) F2014*0.99 (FMSY) % -44% % -15% % 5% No change TAC (F2014*1.1) % 0% Fsq (F2014) % 6% % TAC(F2014*1.08) % 15% Weights in 000 tonnes. 1) SSB 2014 relative to SSB ) Catches 2015 relative to TAC ) Catches are calculated based on landings and 3% discard rate (in weight), the same as 2013 discard rate.

17 ICES ADHOC REPORT Quality considerations In the present scenario is assumed that all trawling gear uses the SELTRA300 device and therefore that the reduced catching efficiency is applicable to all trawling in Kattegat and Skagerrak. This is probably only partly correct since SELTRA exists in more forms, and SELTRA300 which is only mandatory in the closed areas in Kattegat is probably associated with the most significant loss in catch efficiency. SELTRA devices implemented in the rest of Kattegat and in Skagerrak are likely associated with less escapement (i.e. smaller mesh sizes). Since a major part of the fishery for sole takes place outside the closed areas in Kattegat, the estimates of predicted catches in 2016 are considered slight overestimates given the assumed loss in gear efficiency. No direct observations or measurements have been made on the escapement of sole in the upper window of the SELTRA trawl. Among flatfish only plaice behaviour has been studied (Krag et al., 2015). Krag and his colleagues found that for plaice the catch efficiency was reduced by 21% in the SELTRA300. Present assessment assumes a similar behaviour for sole and the reduction in catching efficiency is therefore entirely based on this assumption. Diagnostics from the revised assessment (retrospective behaviour of F and SSB) do not show any significant reduction in the retrospective bias in the most recent years. The bias in the assessment therefore still persists; consequent underestimation of SSB and similar overestimation of F. 8 References ICES. 2014a. Sole in Division IIIa and Subdivisions (Skagerrak, Kattegat, and the Belts). Report of the ICES Advisory Committee, ICES Advice Section ICES. 2014b. Report of the Baltic Fisheries Assessment Working Group. ICES 2014 CM/ACOM:10. Krag, LA, Herrmann, B., Feekings, J., Karlsen, JD., Escape panels in trawls a consistent management tool? Paper under submission to ICES JMS. Madsen, N., Frandsen, R., Holst, R., and Krag, L. A Development of new concepts for escape windows to minimize cod catches in Norway lobster fisheries. Fisheries Research, 103: Madsen, N., and Valentinsson, D Use of selective devices in trawls to support recovery of the Kattegat cod stock: a review of experiments and experience. ICES Journal of Marine Science, 67:

18 16 ICES ADHOC REPORT 2015 Annex 1 Technical Minutes Review of Report on the request to ICES on impacts of SELTRA mounted trawl on catch efficiency, assessment and catch forecast for sole in Division IIIa and Subdivisions by J. Boje, M. Vinther, and L. Krag ICES CM 2015\ACOM:45 Reviewer: Dankert Skagen, Norway The review considers documentation provided to address the following special request from European Commission, DG MARE, Unit E2: In its 2014 advice for sole in Division IIIa and Subdivisions 22-24, ICES pointed out that the assessment did not take into account the reduction in catch per unit effort (CPUE) that results from the use of the SELTRA trawl. ICES is requested to quantify this reduction in CPUE if possible, or else to give is expert judgement on the likely reduction in CPUE. It is also requested to re-assess the stock based on the assumed reduction in CPUE, and provide the corresponding TAC for 2015 based on the ICES MSY approach. The background for the request is that technical measures (SELTRA trawl) have been introduced in the trawl fisheries in Division IIIa, partly in combination with protected areas, primarily to protect cod. These measures may have led to a reduction in catchability for sole that has so far not been included in the calculation of effort in this fishery. Since CPUE is a major source of information in the assessment of this stock, this should have an impact on the assessed stock abundance, and further on the advised TAC. The document describes a new assessment, where the CPUE for the years at all ages has been increased by 20%. The main results are properly documented. The report states that the full output is available at At the time of writing, this was not the case, so the comments to the assessment are based on the material in the report. The change in CPUE leads to a higher stock abundance estimate in the most recent period, and a higher TAC advice. With this assessment, a new prediction has been made according to the ICES MSY approach that leads to a higher TAC than previously advised. The procedure in the new assessment and prediction is, as far as I can see, an exact repetition of the WG assessment (ICES, 2014b), except that the effort values for the 'Official logbooks TR All combined' tuning series were decreased by 20% for the years , corresponding to a 20% increase in CPUE for all ages. Two questions stand out as important: 1. How much has the introduction of the technical measures (SELTRA-trawl) reduced the effective effort? 2. How has the perception of the stock abundance changed because of this.

19 ICES ADHOC REPORT The answer to the first question is not conclusive. The report states: The change in catchability due to the shift to SELTRA trawls has not been quantified for sole. Experiments conducted on plaice and catch efficiency by SELTRA trawls estimated the loss to be 21% (Krag et al., 2015). Although plaice and sole may behave differently in the catching process in SELTRA gears, plaice is considered the most likely species to compare with sole. The assumption made is a reduction in effort by 20%. This may be a fair assumption, but it is quite uncertain and the material to support it is limited. The report describes that several versions of the SELTRA trawl are used, some of which may be associated with less escapement. It is not clear how the actual fishing practise conforms with the conditions under which the reduction in efficiency was measured. Therefore, a reliable quantification of the increase in CPUE cannot be made. An expert judgement of the increase will just be a qualified guess. Some increase is likely, but probably not very large. The answer to the second question is that the effect on the assessment is almost proportional to the increase in CPUE, with regard to the SSB in This is as expected, because the CPUE is the only recent fishery independent information in the assessment, and should have a dominating impact on the estimate of the abundance in the last year. The assessment itself is quite uncertain, with a strong retrospective pattern which remains despite the change in recent CPUE. The CPUE residuals are strong with strong year effects, and hardly change by the change of CPUE. Hence, the adjustment of CPUE is minor compared to the noise in those data. Moreover, the fit to the data gives no guidance to the choice of CPUE option. It seems clear, however, that both the SSB and the recruitment are going down and that the stock is not in a good state. Conclusion. The report is clearly written and addresses the terms of reference adequately as far as possible. The revised assessment is comparable to the previous one by WGBAFS and is in accordance with the stock annex. It seems likely that the recent gear regulation has reduced the catchability for the commercial sole fishery, but the magnitude of this reduction is rather uncertain. The assumed value is based on experiments with plaice, but it is uncertain how relevant that is for sole, and to what extent the experimental conditions conform with the behaviour of the sole fishery. The effect of the revised CPUE on the assessment is as expected, but there is no indication in the assessment diagnostics that one option is preferable over the other. The assessment as such is quite uncertain with strong retrospective deviations, and the assumed change in CPUE seems minor compared to the noise in those data. It is also noted that the stock is not in a good state. Altogether, the justification for revising the TAC upwards is not very strong.