Report of the Inter-Benchmark Workshop on Sole in Division IIIa and Subdivisions (Skagerrak and Kattegat, Western Baltic Sea)

Transcription

1 ICES IBPSOLKAT REPORT 2015 ICES ADVISORY COMMITTEE ICES CM 2015/ACOM:57 REF. ACOM, WGBFAS; WGNSSK Report of the Inter-Benchmark Workshop on Sole in Division IIIa and Subdivisions (Skagerrak and Kattegat, Western Baltic Sea) 1 July 31 October 2015 By correspondence

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 of the Inter-Benchmark Workshop on Sole in Division IIIa and Subdivisions (Skagerrak and Kattegat, Western Baltic Sea), 1 July 31 October 2015, By correspondence. 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 IBPSOLKAT REPORT 2015 i Contents Executive Summary Introduction IBPSolKat Terms of Reference List of participants The benchmark process Issue description Input to stock assessment Commercial trawler tuning fleet (WD1) Private logbook tuning fleets Survey index (WD2) Recruitment index from survey tuning series (WD3) Stock assessment F settings in assessment (WD5) Proposed revised assessment (WD4) Short-term forecast Reference Point estimation (WD6) External Reviewer s report References Annex 1: Stock Annex table Annex 2: List of working documents... 32

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5 ICES IBPSOLKAT REPORT Executive Summary The assessment of sole in Division IIIa and Subdivisions has for several years been considered uncertain. Poor sampling coverage, a short fishery-independent tuning series, retrospective pattern of estimated fishing mortality (F), and the knowledge of spatial changes in selectivity due to management measures implemented, are some of the main contributors to the uncertainty. The present inter-benchmark was conducted July 1 October 31 by correspondence. The inter-benchmark was chaired by Johan Lövgren, Sweden and the external reviewer was Owen Hamel, USA. A number of web conferences were conducted to continually discuss and agree on the progress. The issues examined were in accordance with the ToR for the benchmark (see Section 1). The main conclusions form the inter-benchmark included: 1 ) Different approaches using the commercial tuning fleets were addressed during the inter-benchmark; however it was concluded that the commercial tuning fleet should be removed from the assessment due to changes in selectivity that the IBP was not able to quantify. 2 ) Sole survey. A survey series to establish a time-series of catch and effort data independent of the commercial fishery targeting sole in Kattegat and Skagerrak was initiated in A GAM model was used to statistically take into account the reduced surveyed area and the spatial distribution in the ALK appliance. The internal consistency (Pearson correlation, r) between all age classes improved by using the GAM. It was concluded that the sole Survey with the GAM model was to be included in the Assessment. 3 ) The justification of using private logbooks in the assessment was elaborated in the inter-benchmark. The inclusion of the private logbooks increased the stability and improved the SSB and Fishing Mortality retrospective patterns. It was decided to continue to include the private logbooks in the assessment. 4 ) Selectivity and F. The F and FMSY are calculated on the basis of average F of age 4 8 sole which is in accordance with the lower selectivity of age 2and 3 fish and the numbers-at-age in the population. 5 ) Reference points: The inter-benchmark procedure resulted in a revised perception of SSB and F development over time and consequently reference points were redefined. The Eqsim software (WKMSYREF3) was used to explore S/R relationships and to redefine both PA and MSY reference points for F and biomass.

6 2 ICES IBPSOLKAT REPORT Introduction The assessment of sole in Division IIIa and Subdivisions has for several years been considered uncertain. Poor sampling coverage, poorly defined growth from age readings, a short fishery-independent tuning series, retrospective behaviour of estimated fishing mortality (F), and the knowledge of spatial changes in selectivity due to management measures implemented, are some of the main contributors to the uncertainty. Associated to mandatory implementation of selective devices (i.e. exit windows such as SELTRA and SELTRA like devices) in the trawl gear used for sole among other species (implemented to protect cod) since 2011, the Danish fishermen have experienced a decrease in catch efficiency for sole. A request was therefore sent to ICES in 2014 to quantify the loss in catch efficiency and its effect on the assessment and advice for the sole stock. Since no gear experiments have been conducted with SELTRA and its effect in catching sole, ICES was not able to quantify an eventual loss in catch efficiency. At the Baltic Fisheries Assessment Working Group (WGBFAS) in April 2015 it was again recognized that especially the commercial trawler tuning fleet biased the estimation procedure for Spawning Stock Biomass (SSB) and F. Assuming that the higher estimated F from the trawler tuning series was mainly due to the implemented gear changes and management regulations in recent five years, especially the SELTRA trawl (i.e. lower catchability), it was suggested to remove the commercial tuning fleet as a biomass indicator from the assessment. However, it was decided that the uncertainty and especially the bias in the assessment should be further examined, and therefore an inter-benchmark protocol was set up. 1.1 IBPSolKat Terms of Reference An Inter-Benchmark Workshop (IBPSOLKAT) on Sole in Divisions IIIa and Subdivisions (Skagerrak and Kattegat, Western Baltic Sea), chaired by Johan Lövgren (Sweden) and with Owen Hamel, US as external reviewer, will be established and will meet by correspondence from August 1 to October 15, 2015, to: 1 ) Evaluate the present analytical assessment method of sole with emphasis on: 1.1 ) Review quality and use of the commercial trawler cpue time-series used in the assessment with emphasis on the effect of current spatial coverage limitations (Kattegat only), changes in quota regulation and selectivity and gear changes in recent years with resulting changes in catchability. 1.2 ) Review survey design and estimation of survey index for the assessment. 1.3 ) Improve assessment model settings: ) Investigate the FBAR range and the age groups used for estimating F-at-age; ) Investigate selection pattern assumptions; ) Review options for catch projections, especially assumptions on future recruitment and F in the intermediate year; ) Investigate the inclusion of additional biological parameters (i.e. stock mean weight and maturity).

7 ICES IBPSOLKAT REPORT ) Advice on assessment method to be used in update assessment of sole in Division IIIa and Subdivision ) Propose / revise reference points using the guidelines and process outlined in WKMSYREF3 (i.e. using EqSim/PlotMSY); 4 ) Update the stock annex as appropriate. IBPSOLKAT will report by 1 November 2015 for the attention of the Advisory Committee. 1.2 List of participants name address phone/fax Jesper Boje DTU Aqua jbo@aqua.dtu.dk Section for Fisheries Advice Charlottenlund Slot Jægersborg Alle Charlottenlund Denmark Ruth Fernandez ICES Secretariat ruth.fernandez@ices.dk H.C. Andersens Blvd Copenhagen V Denmark Owen Hamel NOAA Fisheries +1 owen.hamel@noaa.gov Invited Expert 1315 East West Highway Silver Spring MD US Johan Lövgren Chair Institute of Marine Research Turisgatan Lysekil Sweden johan.lovgren@slu.se

8 4 ICES IBPSOLKAT REPORT The benchmark process The present inter-benchmark (IBPSolKat) was conducted between July 1 and October 31 by correspondence. A number of web conferences were conducted to continually discuss and agree on the progress. The issues examined were in accordance with the ToRs for the benchmark (see Annex WD1). Although the benchmark included few participants (one chair, one reviewer, one assessor and one supporting ICES staff) the process went satisfactorily, but would probably have gained if more experts were attending directly. The present attendance is considered a minimum attendance for such benchmarks. It should be noted that several experts conducted work for the benchmark behind the scenes and thereby contributed to the scientific weight of the input to the benchmark.

9 ICES IBPSOLKAT REPORT Issue description The ToRs for the benchmark were broken down into the items specified below. Inter-benchmark protocol on sole in Divisions IIIa and Subdivisions (Skagerrak and Kattegat, Western Baltic Sea). Items 1 ) Commercial tuning fleet: 1.1 ) Representativeness of the current tuning fleet; assess the spatial coverage in relation to the total fishery. 1.2 ) Assess changes in fleet behaviour, management restrictions and the use of selective devices in trawl; review of historic changes in management and gear regulations in Skagerrak and Kattegat. 1.3 ) Final evaluation of the utilization of the tuning fleet as an adequate biomass index for the stock. 2 ) Sole survey: 2.1 ) Assessment of the index obtained from the survey; examination of index estimation (i.e. by a GAM model to take account of spatial differences in growth and in survey coverage). 2.2 ) ALK from the survey to support harbour samplings. 2.3 ) Recruitment index from the survey (age 1) to support assumptions for the short-term forecast (STF). 3 ) Use of private logbooks. 4 ) Assessment: 4.1 ) Justification for use of constant F for ages 6+ and examination of the FBAR basis (ages 4 8). 4.2 ) Examination of the SAM (State space Assessment Model) model options for selectivity. 5 ) Reference points: 5.1 ) Recalculation of FMSY and precautionary reference points in case of changed perception of stock and fishery.

10 6 ICES IBPSOLKAT REPORT Input to stock assessment 4.1 Commercial trawler tuning fleet (WD1) The currently used commercial trawl fishery tuning fleet has been examined in working document 1 at this benchmark: An analysis of utilization of logbook information from the Danish trawl fishery as indicative of stock biomass of sole in Division IIIa and Subdivisions A short summary of the main conclusions from the document is given below. Overall quota and gear regulations have changed substantially over time in Danish waters, which likely have affected the behaviour and proportions of the main Danish fleet segments. The behavioural effects are difficult to quantify. The restrictive sole quotas prior to 2006 have most likely resulted in misreporting that is difficult to accommodate for in logbook analyses. Also the gear restrictions in 2003/2004 have resulted in changes in the proportions of fleet segments. Further, the magnitude of these issues might also differ between management areas/divisions. In example the cease of the mm trawler fleet segment in 2004 is evident and since this segment was part of the tuning fleet, the further series from 2004 until present is considered biased (Figure 1). A detailed description of the changes in regulations in the past two decades is provided in WD 1. The main part of the fishery for sole have traditionally been in Kattegat (IIIaS), however in recent years also the Belts (SD 22 24) and Skagerrak (IIIaN) have increased their contribution. The distribution of catches from the commercial trawler tuning fleet as used since 2005 (trawlers in Kattegat, lengths m, mesh sizes mm in April August) is provided in Figure 2 along with the distribution of the total fishery. The main area covered by this fleet is obviously the northern Kattegat (northwest of the island of Læsø) while for some years (2012) fishing spots in the mid or southern Kattegat are not covered. In addition, considering the entire fishery area for sole in IIIa and 22 24, the tuning fleet only represents a small part of this area. The implementation of exit windows (SELTRA and SELTRA like devices) in 2011 is known to have affected the catch efficiency in trawl as fisherman perceives that more sole escape from the gear. However, the effect is not quantified for sole. Experiments on plaice have calculated the loss of plaice at approximately 20%, but sole is behaving different from plaice in the trawl and results are therefore not directly applicable. ICES evaluated the likely effect of SELTRA gears (ICES, 2015b), but concluded in its advice that at present the effect is not quantifiable (ICES, 2015c). In the stock assessment (SAM model) the commercial trawl tuning fleet has for some years been observed to deviate from the remaining tuning indices and heavily affect especially the estimates of fishing mortality. Figure 3 illustrates the effect of leaving out the commercial trawl tuning fleet; F decreases significantly in the past decade, most likely due to the implementation of selective devices that reduces catch efficiency. Residuals of this fleet from the SAM assessment conducted in April 2015 (ICES, 2015a) (Figure 4) clearly shows that older fish are less abundant in this fleet than assumed by the model (negative residuals for ages 7 and 8 since 2010). In addition to the issues above some generic issues obviously ague against commercial tuning fleets. First, as the ALK data are also used in the catch matrix the weight of these data is too high in the assessment and the fit between catch matrix and commercial fleet overly good. Second, commercial fishery cpue is usually higher and more stable

11 ICES IBPSOLKAT REPORT than fishery-independent cpue since they are able to target hot spots where fish aggregate even if the stock size is decreasing. Third, even if there not is a rations system in place, there are many incentives to not report the accurate catches, and black landings, highgrading or discarding will seriously affect the estimate of cpue. Based on the above the benchmark concluded to discard the use of the commercial trawl tuning fleet in the assessment. 4.2 Private logbook tuning fleets Due to the lack of unbiased catch rate indices, DTU Aqua, in collaboration with the Danish Fishermen Organisation, in 2004 established a database with data from private logbooks for calibration of the sole assessment. The private logbooks cover the period from 1987 to 2008 and provides information on effort (number of hours trawling / number of nets) catches (kg by major species or species group) and location (name of fishing ground) from seven trawlers and three gillnetters. The current private logbooks used in the assessment are 1) Private logbooks from gillnetters, ages 2 8, , April October and 2) Private logbooks from trawlers, ages 2 8, , October December. During the benchmark assessment in 2010, the consistency and reliability of private logbooks collected annually were discussed. Whereas WKFLAT (ICES, 2010) considered the historical data likely to be unbiased the group was concerned that given a future situation with incentive to misreport catches, also the private logbooks might be compromised. As, furthermore, the number of vessels participating in the voluntary logbook programme has declined over time, in 2010 WKFLAT concluded that the tuning series should be maintained in the assessment without being updated in future. Given the recent development in the fishery with changes in gear use and in management regulations the two logbook series were considered for use in the sole assessment. Since the implementation of selective devices was introduced in 2010 after the cease of the two series (2007 and 2008, respectively) these series are not considered biased due to this change in fishery pattern. Further in contradiction to the commercial trawl tuning series, these private logbooks cover fishing grounds in the entire Kattegat. An exploratory SAM run was conducted without these two logbook series, i.e. only the survey as assessment calibration. Without the private logbook series no calibration information is available prior to 2004 when the survey started. The perception of F changes significantly over time and the assessment shows a retrospective behaviour (Figure 5). For these reasons the benchmark decided to keep the two logbook indices in the assessment. 4.3 Survey index (WD2) A survey targeting sole in Kattegat and Skagerrak was initiated in 2004 in order to establish a time-series of catch and effort data independent of the commercial fishery. The survey has over time undergone changes and reductions in area coverage (120 hauls to 70 hauls) but overall covers Kattegat (IIIaS) and a minor part of Skagerrak (IIIaN) considered the main stock distribution area of sole. Previously the biomass index from the survey has been calculated as a swept-area estimate based on a stratification scheme.

12 8 ICES IBPSOLKAT REPORT 2015 In order to obtain indices that utilizes and accommodates spatial information and also incorporates the spatial changes that have occurred in the survey, additional cpue indices are derived from smooth spatially varying age length keys using the methodology described in Berg and Kristensen (2012). The method avoids ad hoc borrowing of samples from neighbouring samples, when certain age groups are missing, and it provides an objective fill-in procedure for missing length groups also. This is possible because the probability of age-given length is modelled using smooth functions of the length of a fish and the spatial coordinates where the haul was taken, rather than relying on some specific stratification of length and space. Due to the rather high sampling level of sole from the survey a limited gain might be obtained from this approach, as age information exist already for most fish. Eight different methods were examined among which six use GAM models (Table 1). The index estimates derived from the methods were rather similar, but the methods using continuation logits for deriving ALK in general gave higher correlation between adjacent age groups (following cohorts) especially for the older age groups. One method in particular (Spatial Continuation logit reduced method) gave highest correlation between the youngest age groups (age 1 vs. age 2) (method 6 in Table 1). Since the benchmark also aimed to include age group 1 into the assessment for forecast purposes (previously only ages 2 9 are included form survey) this method was preferred. Exploratory SAM runs comparing the different estimated indices gave similar results, however, the Spatial Continuation logit reduced method (method 6) gave slightly better retrospective behaviour of SSB and F. The benchmark therefore decided to use the GAM estimated index from the Spatial Continuation logit reduced method. On the issue list prior to the benchmark was raised the possibility to use ALK from the survey to support port samplings from the fishery. However, since all fish from the port samplings are not only length measured but also aged, there is no fish to apply an ALK. The few and scattered landings of sole in Danish ports means that the extra effort of ageing all sampled fish is insignificant. 4.4 Recruitment index from survey tuning series (WD3) The Danish sole survey (Fisherman-DTU Aqua survey) have been used for calibration of the sole assessment since The survey covers the major distribution area of sole in IIIa, i.e. both adult and juvenile grounds have been covered since The age range in the catches from the survey is 1 9+ although the majority is between ages 2 and 4. The assessment of the sole stock has until now only covered ages 2 8+ due to lack of catches of ages <2 years. However, the data on ages 1 from the survey are considered valuable information that can be utilized to estimate stock numbers to improve the quality of the short-term forecast. Historic distribution of age groups within the surveyed area has been examined and age 1 seems to be consistently distributed within the centre of the surveyed area, and is thus considered to be appropriately covered spatially. Figure 6 shows the age 1 indices from the survey calculated by means of the different methods explored under Section 4.3. Previously, a deterministic forecast procedure have been adopted for this stock (MFDP) using the median estimates from the SAM assessment. Recruitment (age 2) was fixed in this forecast and equal to the recent average (geomean ). With the incorporation of age 1 from the survey in the present assessment, it is now suggested to allow SAM to perform a catch forecast based on the estimated stock numbers (ages 1 9). The forecast procedure in SAM includes

13 ICES IBPSOLKAT REPORT sampling from the five recent years ( ) in line with the present average procedure from an S/R recruitment function (Random-walk). Figure 7 shows the estimated historic recruitment and the estimated numbers used in forecasts. Age 1 residuals from the SAM run is comparable to the remaining age groups and no particular blocks are present (Figure 8).

14 10 ICES IBPSOLKAT REPORT Stock assessment Since 2010 a stochastics assessment model, State space Assessment Model (SAM) has been used to assess the stock status of sole. 5.1 F settings in assessment (WD5) A simple examination of the catch matrix using a separable VPA approach provides the best fit to a dome-shaped selection curve with a right leg that flattens to a constant selectivity larger than 0 (Figure 9). The SAM model estimates fishing mortality from random walk and do thus not assume a stable exploitation pattern (as separable models). In the previous stock assessments the estimation procedure for ages 6 9 have been combined, i.e. fishing mortality is set equal for ages Given the assumed domeshaped selectivity with a relatively constant right wing it is proposed to keep the present SAM settings and estimate constant and coupled F for ages 6 9. SAM modelling in addition provides an option for either independent estimation of F- at-ages or correlated estimation. It is assumed that F-at-age is correlated to some degree estimated by the model. Therefore it was decided to use correlated random walks for the estimation of fishing mortalities. All proposed settings for the SAM model is provided in Table 1 of WD Proposed revised assessment (WD4) Implementation of gear devices to protect cod in the mixed fishery for cod, plaice, Nephrops and sole, have also changed the catchability for sole. This gradual change in selectivity have been noticed from fleet sensitivity analyses in the stock assessment for sole, and led to the present inter-benchmark process. Decisions taken by the present benchmark have resulted in changed tuning series to the assessment as follows: a ) Fisherman-DTU Aqua survey, ages 1 9, , 2014, October November. b ) Private logbooks from gillnetters, ages 2 8, , April October. c ) Private logbooks from trawlers, ages 2 8, , October December. A SAM assessment was conducted with the above changes. Figure 10 provides normalized fleet residuals of the assessment model. No major blocks of either negative or positive residuals are present, however, a clear discrepancy are visible for the older age groups in in catch matrix and for the survey. Especially for ages 7 and 8 the survey has a higher catchability than for the catch matrix. This contradiction obviously exist between the survey and the catch matrix (and also formerly the commercial trawl tuning series) and is likely due to the selective devices introduced in recent years in the fishery. Estimated spawning stock biomass, fishing mortality and recruitment from the revised assessment is outlined in Figure 11. This revised assessment is compared to the previous assessment that was basis for advice in June (ICES, 2015d) in Figure 12. Stock perception has changed significantly for the recent decade according to the suggested changes; SSB is now estimated higher and F is estimated lower. Retrospective plots of this revised assessment shows improvement compared to previous assessment in spring 2015 (Figure 13). Previously estimation of SSB and F

15 ICES IBPSOLKAT REPORT showed retrospective behaviour with underestimation of F and a slight overestimation of SSB. The present benchmark changes have led to improvement in this respect, with insignificant retro behaviour. More diagnostics from the final suggested run are available in the WD 4.

16 12 ICES IBPSOLKAT REPORT Short-term forecast The forecast procedure until now has been to assess the stock by a stochastic assessment model and use the outcome of this (medians) as an input to a deterministic forecast. The benchmark group considered this practice inappropriate and suggested utilizing the uncertainty calculated by SAM into the forecast scenario. An example of a forecast scenario by SAM is provided in Figure 14. Although it is recognized that uncertainty boundaries in forecast is not yet utilized by ICES, it is considered important to carry the uncertainty from the assessment into the forecast. Compared to the deterministic approach (MFDP3a software) the stochastic forecast (medians) are conservative and generally leads to lower catch advice. This is likely due to the incorporated uncertainty in the forecast estimation.

17 ICES IBPSOLKAT REPORT Reference Point estimation (WD6) The revised assessment (Section 5.2) has changed the perception of stock and fishery. Therefore new reference points have to be defined accordingly. WKMSYREF3 (ICES, 2014) gave guidelines for MSY reference point estimation among others and provided software as well (Eqsim). Data from the suggested revised SAM assessment (WD4) were used for the estimations by means of Eqsim. Settings were as follow: DATA AND PARAMETERS SETTING COMMENTS SSB-recruitment data Reduced dataseries reflection recent poor recruitment regime ( ) Based on consistent higher productivity and on evaluation by WKMSYREF3 Additional analyses conducted with full time-series Mean weights and proportion Short period to reflect recent regime mature Exploitation pattern Short period to reflect recent fishery Assessment error in the advisory year.cv of F Autocorrelation in assessment error of the advisory year, Fphi 0.25 Based on evaluation of F forecasted (intermediate year) and realized F estimated next year for the period Based on evaluation of F forecasted (intermediate year) and realized F estimated next year for the period Based on the time-series trend for the S/R covering all year back to 1984 it was decided to follow previous recommendation from WKMSYREF3 that the early part, , should be considered a high productivity regime that no longer is relevant to MSY considerations (Figure 15), and therefore these years were discarded from the further S/R analyses by Eqsim. Three different models, Ricker, B&H and segmented regression were applied to the truncated time-series, Segmented regression was weighted highest (0.54). However, the breakpoint for this regression is considered to be at a far too high SSB, approximately at 2700 t. The breakpoint is assumed a candidate for Blim given the SR characteristics; in accordance with the ICES guidelines for reference point estimation the SR relation is assumed type 5, i.e. a distinct plateau and a wide range of SSB, and therefore Bloss is considered a candidate for Blim. The SSB estimates has produced a substantive high recruitment given their low biomass. The two estimates are therefore considered outliers and are suggested to be rejected for Blim and Bloss considerations. Given that Bloss is 1853 t (in 1992) Blim is suggested equal to Bloss. The segmented regression fitted with the breakpoint SSB=1853 t is provided in Figure 16. Assuming σ=0.20 (actual sigma was 0.15 but considered unrealistically low) Bpa is estimated to 2600 t by the formula Blim e 1.645σ. MSY Btrigger is proposed equal to Bpa. The proposed new reference points are summarized in Table 2. Eqsim simulations with a truncated SR time-series ( ) and segmented regression with a breakpoint at 1853 t (Blim), give the scenarios provided in Figure 17 a d and

18 14 ICES IBPSOLKAT REPORT 2015 Table 3. The estimated range of FMSY is , however the upper range is constrained by the FP.05 at Figure 17c shows that catches at F05 (FP.05-5% risk to Blim without Btrigger) is close to MSY (564 t) and that the estimated FMSY ( without the 5% risk) is close to the point where catches are expected to decrease substantially if F is increased. The historical average values, F (0.35) are higher than the estimated FMSY ( ), yield (751 t) is higher than MSY (564 t), and SSB (2620 t) is within the BMSY range ( t). This comparison with historical values shows that the estimated MSY and BMSY is within historic range and thus considered likely estimates. The interbenchmark approved these new defined reference points.

19 ICES IBPSOLKAT REPORT External Reviewer s report External Reviewer s Report - IBPSolKat 2015 Reviewer: Owen Hamel - DRAFT October 29, ) Issues addressed 1.1 ) Tuning Fleets ) Commercial cpue index Due to a number of changes in regulations and gear over time in the Danish commercial fishery, the commercial cpue was excluded from the assessment model for this stock. These changes in the fishery over time, along with changes in the spatial extent of fishing, cause either the recent approach of limiting the data used for the cpue index, or the expanding of the data to other segments of the fleet, to be problematic. Moreover, expanding the data reintroduces the issue of possible substantial misreporting in the early 2000s. Given all of this and the presence of alternative cpue indices for an early period and an index from a current and ongoing survey, the use of the commercial cpue index in any form is not advised ) Private logbooks Two private logbook-based indices that have been used in the assessment, and which represent the period form or 2008, were considered. These logbook indices, in contrast to the current cpue index discussed above, represent more consistent gear and fishery over the period which they cover, and no concern of substantial misreporting exists, and thus these indices are therefore considered appropriate to use ) Sole survey Cpue indices from the survey for sole in Skagerrak and Kattegat ( , 2014 ongoing) were developed using a number of alternative approaches to develop age length keys, in some cases being spatially varying. Given the potential spatial complexity of the stock, a spatial model for estimating age-at-length appears advantageous. The spatial continuation logit model applied on the dataset representing consistent sampling ( Spatial CL and reduced ) had the highest correlation between age 1 in one year and age 2 in the following year, and was generally comparable to other models for older ages, in addition to having the lowest log-likelihood value (though not significant). While the choice of method made only a marginal difference in the current assessment, this method appears to have the potential to provide better predictions of incoming year classes. An age-1 recruitment index from the survey is not included in the assessment. This provides some indication of quite recent recruitment (although the CV for age 1 is greater than that for the other ages). 1.2 ) Selectivity and F Selectivity of the fishery appears to have changed over time (and is allowed to change freely from year to year), with a drop in the relative F of the age 5+ sole over the last decade. This is possibly due to changes in gear and spatial extent of the fishery. Due to data availability, selectivity of ages 6+ are assumed equal.

20 16 ICES IBPSOLKAT REPORT 2015 F and FMSY are calculated on the basis of the average F of age 4 8 sole. This is reasonable given the lower selectivity of age 2 and 3 fish and the numbers-at-age in the population. There is some concern that the change in relative selectivity of 5 and 6+ fish in recent years may have changed the relative impact of the same F4 8, and therefore the view of historical fishing rates. However, this does not impact the validity of the measure for current management given that it is developed based upon recent years selectivities. 2 ) Use of final assessment as a basis for advice The final assessment incorporates the indices, length and age information, and F measures discussed above, including age composition information from the commercial fleet ( ), the survey ( , 2014) and the private logbooks ( or ). Retrospective analysis indicates consistent information over time, with no concerning retrospective patterns. The updated reference points reflect the information available to develop precautionary management, and the resulting management advice reflects the information in the assessment. 3 ) Recommendations for future work 3.1 ) Investigate spatial structure of stock at-age, size and over time. 3.2 ) Investigate measures of fishing intensity or impact other than average F over some range of ages. These can include Spawning Potential Ratio, and potentially other measure of fishing impact being developed. Given the change in relative F for the different age classes in recent years, a measure(s) that reflect the impact of the fishing and which are consistent over this changing relative F could be advantageous for decision-making. 3.3 ) Develop additional graphs for diagnostics (such as fits to each survey or cpue index, including ghost indices which are not included in fitting the model).

21 ICES IBPSOLKAT REPORT Table 1. Survey index estimation. Correlation (Pearson) between log(cpue) at-age in year y at-age a, and in year y+1 and age a+1, by method. METHOD #/AGES AGE1 AGE2 AGE2 AGE3 AGE3 AGE4 AGE4 AGE5 AGE5 AGE6 AGE6 AGE7 AGE7 AGE8 AGE8 AGE9 1) Empirical ) Empirical reduced 3) Continuation logit (CL) 4) CL and reduced ) Spatial CL ) Spatial CL and reduced 7) CL and spatial model 8) Spatial CL and spatial model 9) Old indices NA NA Table 2. Proposed new reference points for sole in Division IIIa and Subdivisions Reference point Value Technical basis MSY Btrigger 2600 t. Bpa FMSY 0.23 Equilibrium scenarios w. stochastic recruitment short time-series , constrained by Fpa Blim 1850 t. Bloss from 1992, low productivity regime Bpa 2600 t Blim e 1.645σ, σ=0.20 FLIM Median from equilibrium scenarios prob(ssb<blim)<50% w. stochastic recruitment Fpa 0.23 Flim e σ, σ = 0.18

22 18 ICES IBPSOLKAT REPORT 2015 Table 3. Summary of reference points from Eqsim (segmented regression) and associated catches and biomasses. Reference point FMSY without Btrigger FMSY lower without Btrigger FMSY upper without Btrigger New FP.05 (5% risk to Blim without Btrigger) FMSY upper precautionary without Btrigger) FMSY with Btrigger FMSY lower with Btrigger FMSY upper with Btrigger FP.05 (5% risk to Blim with Btrigger) MSY (at precautionary FMSY) 564 t Median SSB at FMSY 2198 t Median SSB lower (median at FMSY upper) 1803 t Median SSB upper (median at FMSY lower) 2763 t Figure 1. Effort by the two segments in the present trawler tuning fleet; mm mesh sizes and mm mesh sizes.

23 ICES IBPSOLKAT REPORT Figure 2. Density distribution of sole catches from the tuning fleet (trawlers m, mm mesh, April August) in based on logbook and VMS data (blue areas) and the distribution of total catches of sole (red yellow areas).

24 20 ICES IBPSOLKAT REPORT 2015 Figure 3. Sensitivity plot of fishing mortality from SAM assessment. Each curve represents the omission of one tuning fleet. Figure 4. Normalized residuals for the commercial trawler fleet from SAM assessment as conducted in April (WGBFAS 2015).

25 ICES IBPSOLKAT REPORT Figure 5. SAM run with only the survey as a tuning fleet. Left: development of fishing mortality compared to the WGBFAS run in April 2015 (grey curve) and left is the retrospective behaviour of F. Figure 6. Age 1 indices from the survey estimated by nine different methods (see Table 1).

26 22 ICES IBPSOLKAT REPORT Figure 7. Estimated recruitment (age 1) from SAM using the age 1 indices from the survey. The forecast years are shown as blue bars. Figure 8. Survey residuals by age and year from the SAM assessment after inclusion of age 1.

27 ICES IBPSOLKAT REPORT Figure 9. Examination of the catch matrix. Residuals and selection pattern from a separable VPA on ages 2 to 10 with terminal F of 0.1 at age 5 (ref. age) and terminal S of 0.5.

28 24 ICES IBPSOLKAT REPORT 2015 Figure 10. Normalized residuals from a proposed final setting SAM run.

29 ICES IBPSOLKAT REPORT Figure 11. Stock assessment summary: SSB, F and recruitment from SAM.

30 26 ICES IBPSOLKAT REPORT 2015 Figure 12. Comparison of revised assessment (SSB and F) according to present inter-benchmark and the assessment from spring Figure 13. Comparison of retrospective plots for present revised assessment (upper) and the assessment that was basis for the June 2015 advice (lower).

31 ICES IBPSOLKAT REPORT Figure 14. A forecast scenario from the SAM model assuming TAC constraint in 2015 and status quo F in S/R Sole IIIa and Rage SSB Figure 15. Stock trajectory for SSB and recruitment (age 1).

32 28 ICES IBPSOLKAT REPORT 2015 Figure 16. Eqsim results applying a segmented regression method for the truncated stock recruitment time-series ( ) forcing the breakpoint though 1853 t.

33 ICES IBPSOLKAT REPORT Figure 17. Eqsim results, truncated time-series , method segmented regression forcing breakpoint through SSB=1853 t, without Btrigger in simualtions Panels a c: historic values (dots), median (solid black curve) and 90% intervals (dotted black curve) of recruits, SSB and landings for exploitation at an F range from 0 to 0.8. Panel c also show mean landings (red solid curve). Panel d shows the probability of SSB<Blim (red curve), SSB<BPA (green curve) and the cumulative distribution of FMSY based on yield as landings (brown curve). The F corresponding to p(ssb<blim)>5% is denoted F05 and shown as vertical line on panel a c.

34 30 ICES IBPSOLKAT REPORT References Berg, C.W. and Kristensen, K Spatial age length key modelling using continuation ratio logits. Fisheries Research, 129: ICES Report of the Benchmark Workshop on Flatfish (WKFLAT). ICES CM 2010/ACOM:37. ICES Report of the Joint ICES-MYFISH Workshop to consider the basis for FMSY ranges for all stocks (WKMSYREF3), November 2014, Charlottenlund, Denmark. ICES CM 2014/ACOM: pp. ICES. 2015a. Report of the Baltic Fisheries Assessment Working Group (WGBFAS). ICES CM 2015/ACOM:10. ICES. 2015b. 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. ICES CM 2015 / ACOM:45. ICES. 2015c. EU request on Sole in Division IIIa and Subdivisions SELTRA trawl. ICES Advice ICES. 2015d. ICES Advisory Committee. Advice June 2015.

35 ICES IBPSOLKAT REPORT Annex 1: Stock Annex table The table below provides an overview of the stock annex updated by IBPSolKat. Stock annexes for other stocks are available on the ICES website Library under the Publication Type Stock Annexes. Use the search facility to find a particular stock annex, refining your search in the left-hand column to include the year, ecoregion, species, and acronym of the relevant ICES expert group. Stock ID Stock name Last updated Link sol-kask Sole (Solea solea) in Division IIIa and Subdivisions (Skagerrak and Kattegat, Western Baltic Sea) 3 November 2015 Sole IIIa

36 32 ICES IBPSOLKAT REPORT 2015 Annex 2: List of working documents Boje, J., M. Storr-Paulsen, M. Vinther, J. Egekvist and K. Häkonsson An analysis of utilization of logbook information from the Danish trawl fishery as indicative of stock biomass of sole in Division IIIa and Subdivisions Working doc 1, IBPSolKat Vinther, M. and J. Boje Sole survey indices. Working doc 2, IBPSolKat Boje, J Recruitment indices from the Danish sole survey. Working doc 3, IBPSolKat Boje, J. and A. Nielsen A revised assessment of sole in Division IIIa and Subdivisions Working doc 4, IBPSolKat Boje, J A note on F estimation in the Sole IIIa, assessment Working doc 5, IBPSolKat Boje, J. and M. Vinther Revision of reference points for sole in Division IIIa and Subdivisions Working doc 6, IBPSolKat 2015.