ICES ADHOC REPORT 2015

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 22-24 By J. Boje, M. Vinther, and L. Krag DTU Aqua, Denmark

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 info@ices.dk Recommended format for purposes of citation: ICES. 2015. 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:45. 19 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. 2015 International Council for the Exploration of the Sea

ICES ADHOC REPORT 2015 i Contents 1 EC Request... 2 2 Summary... 2 3 Background and assumptions... 2 4 Input to assessment... 3 5 Assessment... 4 6 Short-term projection... 12 7 Quality considerations... 15 8 References... 15 Annex 1 Technical Minutes... 16

2 ICES ADHOC REPORT 2015 1 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 1994. 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 2010. 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%.

ICES ADHOC REPORT 2015 3 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 2010. 4 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 2010 2013 (grey shaded). Official logbooks TR All combined 1994 2013 1 1 0.25 0.66 3 9 3599 11208 14943 7856 3762 914 750 218 3605 6392 20 034 15 748 5874 2974 828 531 4414 7584 4370 6933 4502 2429 1211 761 3510 1235 3449 3557 4579 2426 1389 619 4185 2465 3649 4914 3154 3157 3401 2171 5192 14 720 2639 5082 3415 2728 1875 1524 6994 10 637 19 579 2081 3314 3959 2964 3130 6269 8419 9986 6209 1952 2956 1005 1520 6444 11 200 5295 4445 8895 2291 2456 3186 6091 15 602 17 717 6983 8115 3535 1299 1457 5267 12 361 16 276 7507 6008 3721 1743 1250 3732 5946 14 773 8052 1594 447 886 146 4162 3968 12 025 11 457 7179 1172 753 139 4042 4355 5020 6246 4843 2629 1212 696 4930 2546 2959 1870 7031 3033 2633 1382 5145 10 533 7512 6512 2401 1910 691 749 4758 8763 6663 2874 2128 749 1307 637 3984 6160 8880 2795 762 608 87 617 6284 10 012 18 858 8547 5276 728 2426 728 6451 6839 7225 4871 2957 641 212 1208

4 ICES ADHOC REPORT 2015 5 Assessment The present assessment is conducted according to stock annex for sole in Division IIIa and Subdivisions 22-24 (ICES, 2014a). The State-space Assessment Model (SAM) is used and input data, diagnostics and results from this assessment is available at stockassessment (www.stockassessment.org). 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. 1037 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).

ICES ADHOC REPORT 2015 5 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 1 1 2 0.63 0.44 0.90 2 1 3 0.19 0.13 0.28 3 1 4 0.19 0.13 0.28 4 1 5 0.19 0.13 0.28 5 1 6 0.19 0.13 0.28 6 1 7 0.19 0.13 0.28 7 1 8 0.19 0.13 0.28 8 1 9 0.19 0.13 0.28 9 2 2 0.54 0.43 0.68 10 2 3 0.54 0.43 0.68 11 2 4 0.54 0.43 0.68 12 2 5 0.54 0.43 0.68 13 2 6 0.54 0.43 0.68 14 2 7 0.54 0.43 0.68 15 2 8 0.54 0.43 0.68 16 3 3 0.35 0.29 0.41 17 3 4 0.35 0.29 0.41 18 3 5 0.35 0.29 0.41 19 3 6 0.35 0.29 0.41 20 3 7 0.35 0.29 0.41 21 3 8 0.35 0.29 0.41 22 4 2 0.54 0.35 0.84 23 4 3 0.26 0.21 0.33 24 4 4 0.26 0.21 0.33 25 4 5 0.26 0.21 0.33 26 4 6 0.26 0.21 0.33 27 4 7 0.26 0.21 0.33 28 4 8 0.26 0.21 0.33 29 5 2 0.47 0.31 0.70 30 5 3 0.47 0.40 0.56 31 5 4 0.47 0.40 0.56 32 5 5 0.47 0.40 0.56 33 5 6 0.47 0.40 0.56

6 ICES ADHOC REPORT 2015 Table 3. Sole IIIa. Fishing mortality-at-age (age 6-9 assumed constant). YEAR\AGE 2 3 4 5 6+ 1984 0.087 0.392 0.461 0.385 0.388 1985 0.092 0.359 0.416 0.380 0.333 1986 0.094 0.348 0.429 0.404 0.368 1987 0.098 0.335 0.432 0.422 0.442 1988 0.101 0.334 0.435 0.415 0.409 1989 0.103 0.330 0.442 0.434 0.443 1990 0.101 0.324 0.449 0.441 0.422 1991 0.096 0.307 0.432 0.444 0.522 1992 0.092 0.299 0.423 0.460 0.618 1993 0.089 0.302 0.427 0.482 0.637 1994 0.086 0.292 0.412 0.474 0.563 1995 0.085 0.301 0.406 0.459 0.584 1996 0.085 0.308 0.397 0.430 0.506 1997 0.081 0.280 0.376 0.412 0.469 1998 0.079 0.253 0.359 0.407 0.440 1999 0.076 0.239 0.342 0.390 0.396 2000 0.073 0.233 0.335 0.379 0.365 2001 0.071 0.219 0.301 0.366 0.305 2002 0.070 0.210 0.285 0.363 0.359 2003 0.067 0.191 0.272 0.347 0.349 2004 0.069 0.209 0.295 0.360 0.393 2005 0.072 0.234 0.313 0.362 0.438 2006 0.074 0.244 0.330 0.382 0.416 2007 0.078 0.261 0.357 0.403 0.388 2008 0.079 0.264 0.380 0.419 0.427 2009 0.079 0.262 0.392 0.411 0.315 2010 0.075 0.249 0.380 0.401 0.319 2011 0.070 0.229 0.363 0.359 0.279 2012 0.065 0.191 0.319 0.310 0.327 2013 0.064 0.191 0.321 0.305 0.356

ICES ADHOC REPORT 2015 7 Table 4. Sole IIIa. Stock number-at-age from SAM. YEAR\AGE 2 3 4 5 6 7 8 9+ 1984 2617 1812 587 377 104 92 128 523 1985 4478 2242 743 212 219 71 35 288 1986 3970 4357 1886 622 134 130 92 221 1987 3920 3330 3019 1205 435 149 88 238 1988 3991 3714 2634 1644 459 140 75 186 1989 5451 2330 2471 1681 1110 231 85 148 1990 7322 4627 1718 1603 963 577 97 108 1991 6658 5429 2858 1019 910 715 456 178 1992 7608 5112 3049 1447 614 445 371 471 1993 5490 6737 3501 2036 864 269 266 340 1994 2689 4725 4595 2012 1088 317 130 180 1995 3199 2471 4170 3332 1400 789 252 239 1996 2199 3274 1593 2130 1345 655 357 467 1997 1236 1087 1519 1179 1498 1026 665 659 1998 3130 750 736 910 706 750 684 822 1999 3443 4242 561 735 604 512 467 805 2000 2491 2324 2493 367 455 406 412 984 2001 3579 2039 1600 1253 284 408 170 931 2002 5801 3786 1204 1306 1326 313 331 1267 2003 3135 3621 2672 1110 1242 675 121 786 2004 3534 3393 3166 1596 828 612 321 518 2005 2923 4398 4080 2177 792 291 263 317 2006 2452 2033 3312 2516 1564 472 226 355 2007 2605 1751 1310 1819 949 581 279 364 2008 2983 1464 1104 809 1216 591 498 520 2009 2882 2889 1285 873 494 448 157 397 2010 2387 2668 1533 608 462 224 331 391 2011 1884 2422 1870 816 264 177 66 503 2012 1313 1295 1416 814 461 134 205 585 2013 1118 1369 1077 802 491 201 69 373 2013* 949 1024 707 535 311 127 280 *Estimated by simple forward projection of 2012 stock

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 2010-2013 YEAR RECRUITS LOW HIGH TSB LOW HIGH SSB LOW HIGH F48 LOW HIGH 1984 2617 1378 4972 1417 1063 1888 938 727 1209 0.402 0.298 0.543 1985 4478 2586 7753 1785 1312 2429 1005 768 1316 0.359 0.263 0.490 1986 3970 2380 6621 2586 2013 3322 1930 1496 2491 0.387 0.297 0.504 1987 3920 2471 6218 2658 2162 3268 2031 1650 2499 0.436 0.342 0.556 1988 3991 2520 6321 2693 2199 3299 2058 1682 2519 0.415 0.326 0.529 1989 5451 3443 8631 3012 2440 3718 2052 1695 2485 0.441 0.351 0.554 1990 7322 4543 11 803 3963 3172 4951 2645 2173 3218 0.431 0.343 0.541 1991 6658 4220 10 504 4286 3488 5267 3128 2568 3809 0.488 0.394 0.605 1992 7608 4680 12 367 5515 4466 6810 3894 3227 4700 0.547 0.433 0.691 1993 5490 3464 8699 4809 3942 5868 3832 3141 4676 0.564 0.445 0.715 1994 2689 1673 4322 4218 3561 4996 3750 3160 4450 0.515 0.420 0.632 1995 3199 2047 5001 4090 3467 4824 3491 2959 4119 0.524 0.423 0.648 1996 2199 1408 3435 3373 2868 3968 2987 2537 3517 0.469 0.383 0.576 1997 1236 729 2095 2718 2304 3207 2474 2093 2924 0.439 0.357 0.540 1998 3130 2001 4895 2236 1868 2676 1732 1456 2060 0.417 0.337 0.516 1999 3443 2192 5408 2861 2367 3458 2304 1915 2771 0.384 0.310 0.476 2000 2491 1579 3932 2614 2176 3141 2194 1825 2637 0.362 0.292 0.448 2001 3579 2306 5553 2637 2176 3195 1978 1652 2369 0.316 0.248 0.404 2002 5801 3455 9740 3641 2930 4524 2643 2181 3202 0.345 0.276 0.430 2003 3135 1969 4991 3460 2888 4145 2914 2430 3495 0.333 0.261 0.425 2004 3534 2318 5389 3801 3181 4541 3084 2587 3676 0.367 0.292 0.461 2005 2923 1933 4421 4098 3426 4902 3537 2947 4245 0.398 0.317 0.500 2006 2452 1622 3706 3468 2912 4130 2976 2488 3558 0.392 0.313 0.491 2007 2605 1714 3960 2613 2193 3112 2063 1739 2447 0.385 0.307 0.483 2008 2983 1917 4643 2328 1910 2838 1687 1400 2033 0.416 0.322 0.538 2009 2882 1742 4768 2590 2092 3207 1982 1620 2426 0.350 0.274 0.446 2010 2387 1453 3923 2399 1914 3006 1783 1444 2201 0.347 0.271 0.445 2011 1884 1138 3118 2269 1805 2853 1778 1423 2222 0.312 0.238 0.409 2012 1313 695 2477 2040 1607 2590 1666 1330 2087 0.322 0.243 0.428 2013 1118 434 2880 1489 1108 2002 1222 950 1571 0.339 0.248 0.462

ICES ADHOC REPORT 2015 9 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).

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 2010. 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.

ICES ADHOC REPORT 2015 11 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 2010 2013 for the trawl tuning fleet.

12 ICES ADHOC REPORT 2015 6 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 2016. 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 2014. 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 211 205 FMSY SSB2015/MSY Btrigger 0.256 0.249 FMSY *SSB2015/MSY Btrigger 0.20 1590 +29% -42% 0.23 1.77 22% -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 2015. c) Landings 2015 relative to TAC 2014. 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.

ICES ADHOC REPORT 2015 13 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: 4-8 2014 Age N M Mat PF PM SWt Sel CWt 2 2566 0.1 0 0 0 0.262 0.066 0.262 3 949 0.1 1 0 0 0.258 0.204 0.258 4 1024 0.1 1 0 0 0.295 0.334 0.295 5 707 0.1 1 0 0 0.319 0.325 0.319 6 535 0.1 1 0 0 0.379 0.321 0.379 7 311 0.1 1 0 0 0.396 0.321 0.396 8 127 0.1 1 0 0 0.314 0.321 0.314 9 280 0.1 1 0 0 0.331 0.321 0.331 2015 Age N M Mat PF PM SWt Sel CWt 2 2566 0.1 0 0 0 0.262 0.066 0.262 3. 0.1 1 0 0 0.258 0.204 0.258 4. 0.1 1 0 0 0.295 0.334 0.295 5. 0.1 1 0 0 0.319 0.325 0.319 6. 0.1 1 0 0 0.379 0.321 0.379 7. 0.1 1 0 0 0.396 0.321 0.396 8. 0.1 1 0 0 0.314 0.321 0.314 9. 0.1 1 0 0 0.331 0.321 0.331 2016 Age N M Mat PF PM SWt Sel CWt 2 2566 0.1 0 0 0 0.262 0.066 0.262 3. 0.1 1 0 0 0.258 0.204 0.258 4. 0.1 1 0 0 0.295 0.334 0.295 5. 0.1 1 0 0 0.319 0.325 0.319 6. 0.1 1 0 0 0.379 0.321 0.379 7. 0.1 1 0 0 0.396 0.321 0.396 8. 0.1 1 0 0 0.314 0.321 0.314 9. 0.1 1 0 0 0.331 0.321 0.331 Input units are thousands and kg - output in tonnes

14 ICES ADHOC REPORT 2015 Table 7. Sole IIIa. Management options table for short-term prediction. Basis: F(2014) = Fsq = mean F(11-13) = 0.3242; 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 0.256 0.249 FMSY *SSB2015/MSY Btrigger 0.23 1765 22% -30% Precautionary approach 0.016 0.015 TAC(Fpa) *0.024 0.01 2000 38% -96% Zero catch 0.000 0.000 F=0 0.00 2016 39% -100% 0.205 0.199 0.309 0.300 Status quo 0.381 0.370 50% Fsq (F2014*0.5) -15% TAC(F2014*0.76) F2014*0.99 (FMSY) 0.16 1815 26% -44% 0.25 1713 18% -15% 0.32 1642 14% 5% 0.365 0.354 No change TAC (F2014*1.1) 0.30 1658 15% 0% 0.385 0.374 Fsq (F2014) 0.32 1638 13% 6% 0.418 0.406 +15% TAC(F2014*1.08) 0.36 1606 11% 15% Weights in 000 tonnes. 1) SSB 2014 relative to SSB 2013. 2) Catches 2015 relative to TAC 2014. 3) Catches are calculated based on landings and 3% discard rate (in weight), the same as 2013 discard rate.

ICES ADHOC REPORT 2015 15 7 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 22 24 (Skagerrak, Kattegat, and the Belts). Report of the ICES Advisory Committee, 2014. ICES Advice 2014. Section 6.3.26. ICES. 2014b. Report of the Baltic Fisheries Assessment Working Group. ICES 2014 CM/ACOM:10. Krag, LA, Herrmann, B., Feekings, J., Karlsen, JD., 2015. Escape panels in trawls a consistent management tool? Paper under submission to ICES JMS. Madsen, N., Frandsen, R., Holst, R., and Krag, L. A. 2010.Development of new concepts for escape windows to minimize cod catches in Norway lobster fisheries. Fisheries Research, 103: 25 29. Madsen, N., and Valentinsson, D. 2010. 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: 2042 2050.

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 22-24 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 2010 2013 at all ages has been increased by 20%. The main results are properly documented. The report states that the full output is available at www.stockassessment.org. 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 2010 2013, 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.

ICES ADHOC REPORT 2015 17 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 2014. 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.