INTERNATIONAL COMMISSION FOR Serial. N. 86 (D.c.S) ~, U.~ \".., THE RESTRICTED NORTHWEST ATLANTIC ASHERIES ICNAF Res. Dc.68/9l ANNUAL MEETING - JUNE 1968 Cdend Mesh Selectin Studies f Yellwtail Flunder Limanda ferruginea (Strer); by F. E. Lux Bureau f Cmmercial Fisheries, Bilgical Labratry Wds Hle, Massachusetts INTRODUCTION The yellwtail flunder is the mst imprtant flatfish in New England tter trawl fishery. United States landings in 1966 were 30,000 metric tns. Mst f the catch cmes frm Subarea 5Z and frm grunds just t the west f 52:.. The fish are caught n sandy bttms in depths f 30 t 60 m. When vessels are fishing fr yellwtail this species is the predminant fd fish taken althugh at times small amunts f summer flunder and winter flunder als are caught. Nn-fd species taken, which are discarded by yellwtail draggers, cnsist largely f sculpins, spiny dgfish, and red hake. Abut 75% f the yellwtail are landed at New Bedfrd, Massachusetts. Mst vessels fishing yellwtail frm there are medium tter trawlers f 50 t 85 grss tns. The trawls used, fr the mst part, are fitted with single nyln cdends f 114 mm mesh (internal, stretched measure). This mesh retains large numbers f yellwtail less than 340 g (abut 35 cm), currently the minimum market size. The undersize yellwtail, which frequently are 50% f the catch by weight, are discarded verbard. My ve ry limited infrmatin n survival f discarded fish, btained by placing discards in tanks f running seawater abard cmmercial trawlers, indicates that abut 250/0 survive. Yellwtail f discarded sizes are grwing rapidly (ICNAF Res. Dc. 67/67) and available infrmatin indicates that natural mrtality is relatively lw (ICNAF Res. Dc. 67/28). A savings f fish presumably wuld result if the discards culd be effectively released by increasing cdend mesh size. T btain release infrmatin we therefre examined mesh selectin fr yellwtail abard cmmercial tter trawlers in September 1967. D2
- 2 - " METHODS The wrk was dne abard 2 New Bedfrd tter trawlers. Pair tws were made during a part f the trip, using a cdend f the mesh t be tested n ne vessel and a lined cdend r different size cdend n the ther. The catches frm a series f tws were cmpared (large mesh versus liner) t btain selectin data. The liner was f 38 mm mesh nyln, which retained all sizes f yellwtail present n the grunds. Mesh measurements were made n wet, after use, cdends with an IOES mesh gauge eet at a tensin f 4 kg. Tw rwe f meshes, frm head end t purse ring end f the cdend, were measured, and the mean f these measurements was taken t be the mean mesh size fr the dend. The vessels were die eel pwered medium side trawlers which nrmally fish fr yellwtail. The Catherine and Mary was 56 gl'oss tns, 19.8 m lng and had 180 hrsepwer. The Mrilight was 71 grss tns, 23. 8 m lng and had 0 hrsepwer. Bth vessels used a N, 36 tter trawl with 127 mm mesh nyln twine in net parts frward f the cdend. This is their standard gear. Aside frm very minr variatins, the nets were similarly rigged. Twing speed was the same fr the 2 vessels. This was estimated by the captain f the Mnlight t be 2. 5 t 3 k, depending n current and wind cnditins. I assumed that the 2 vessels had equal fishing pwer, given the' mild weather cnditins prevailing during the experiments. Fr this trip, fr equal amunts f effrt, the cmparative weighuts in metric tns f cmmercial fish caught were as fllws: Catherine and Mary: yellwtail flunder, 12.76, summer flunder, 0.32; Mnlight: yellwtail flunder, 11.05, summer flunder, 0.31. Since the vessels fished within sight f each ther during the trip, even when pair tws were nt being made, they presumably were wrking n the same bdy f fish. We examined selectin characteristics f 2 nyln cdends: ne with a mesh pening f 129 mm made f single, twisted nyln twine 5 mm in diameter, and ne with a mesh pening f 145 mm made f single, braided nyln twine 6 mm in diameter. We made pair tws f each f these nets versus lined nets, and, in additin, we made several pair tws with the 129 mm versus the 145 mm cdend, and with the 129 mm versus a 116 mm cdend, the latter being that which nrmally was used abard the Mnlight. The latter cdend was f single nyln twine (braided) f 6 mm diameter. Chafing gear was used nly n the bttm f the cdend, in the after part where the bag f fish dragged n the sea flr. It was the same n bth vessels and cnsisted f 114 mm mesh nyln tied at the knts with shrt 'lengths f plyprpylene twine which frayed ut in use t frm a prtective mat. This chafing gear is,the kind mst used n New England trawlers, althugh bull hides cntinue t be used by sme fishermen. Ttal catches and length frequency distributins fr each tw were estimated frm samples. We btained a sample f 2 r 3 ne-bushel baskets f unculled yellwtail frm each tw. These fish were measured t the nearest centimeter and separated int baskets f markets (kept catch) and discards, using the cull pint length f 35 em. This gave estimates f the length frequency distributin and the rati, by vlume, f the catch f markets t di.scards fr the tw. The ttal amunt f market yellwtail caught per tw, in bushels, was btained frm the crew wh measure their catch by busheling it. Using the abve rati and the ttal vlume f markets in the tw, we calculated the ttal number f bushels f discards in the catch and added this t the bushels f market fish t get ttal catch. We measured the actual vlume t ttal discards in 5 tws and cmpared this with estimated values t btain a measure f the errr in the abve technique. The actual and estimated percentages fr these tws are given in Table 1. The ttal difference was less than 2%. - 03-
2 3 38 liner 145 3 2 129 145 4 3 129 116 The numbers f large and small yellwtail present n this grund were t few t prvide precise selectin infrmatin at the upper and lwer ends f the selectin curves. This shuld be kept in mind when interpreting the data. Series 1 (mesh size 129 mm). Selectin data fr the 129 mm cdend. based n 6 pair tws. are given in Table 3. Sme market fish near the cull length (35 cm) were released. A selectin curve. drawn by eye t the percentage retentin pints. is shwn in Figure 1. The 50% selectin length f apprximately 29. 5 cm gives a selectin factr f 2. 29. The 25% t 75% selectin range is apprximately 5 cm. _ 04 'lit - 3 - Table 1. Cmpa.risn f actual and estimated prprtins f yellwtail discards in 5 tws. % Discard Yellwtail Ttal Catch Tw ACtual Estimated (Bushels) 1 58 67 26.0 2 65 60 23. 0 3 62 50 26. 0 4 62 65 10. 5 5 69 65 24. 5 --------- - Since twing time varied frm 35 t 90 minutes and was usually nt exactly the same fr each f the vessels in a given pd.; r tw, we adjusted catches t a ne hur tw length. The length frequency distributin fr the measured sample was applied t the estimated ttal number f yellwtail in the catch fr each (ne hur equivalent) tw, t btain the length frequencies f the ttal catch per ne hur tw. In each pair tw series. the length frequency data fr all tws with the ne cdend were cmbined and cmpared with the equivalent data fr the ther cdend. The percentage retentin fr each 2 cm interval was cmputed and selectin curves, based n smthed retentin percentages. were cnstructed. RESULTS Furteen pair tws were made in 4 series (Table 2). All were made n yellwtail grunds 7 t 10 miles nrth and nrthwest f Nantucket Shals lightship (40 0 33'N, 69 28'W). Water depth was 45 t 55 m. Table 2. Number f tws and cdend mesh sizes in each series f yellwtail mesh selectin experiments. Number f Tws Cdend Mesh Size (mm) Series (each vessel) CatIierine and Marl MnligIit 1 6 129 38 liner
Size Categry Market Discard Rati, Market t Discard Series 2 (mesh size 145 mm). Catch - bushels/hur 129 mm 38 mm 3. 9 4. 5 Selectin data fr the 145 mm mesh cdend, based n 3 pair tws, are given in Table 5. A selectin curve, drawn by eye t the percentage retentin pints, is shwn in Figure 2. The 50% selectin length f apprximately 34 cm gives a selectin factr f 2.34. The 250;0 t 75% selectin range is apprximately 8. 5 cm. - D 5 -.87 2.5 6. 8 37-4 - Table 3. Estimated yellwtail length frequency distributins and percent retained fr a 129 mm mesh cdend cmpared with a lined cdend (38 mm mesh) fr 6 ne hur pair tws. Length Interval (cm) 14-15 16-17 18-19 -21 22-23 24-25 26-27 28-29 30-31 32-33 34-35 36-37 38-39 40-41 42-43 44-45 46-47 Ttal Numners Caught 1/ 129 mm 38 mm 0;0 retained 12 10 31 389 1022 1015 1040 635 149 9 8 4340 2 39 259 319 87 29 0 764 1615 1376 916 424 128 22 11 2 6213 Y Smthed by 3's, using gemetric means. 2 8 30 37 62 81 108 125 1 78 66 16 The estimated yellwtail catches fr the 129 mm mesh and the lined cdend, fr the 6 pair tws, are given in Table 4. The 129 mm mesh caught relatively mre market yellwtail than the lined cdend while releasing sme f the undersize fish. The rati f market t discard catch increased frm 37 t 87 percent. Table 4. Cmparisn f yellwtail catch fr the 129 mm mesh and the lined (38 mm) cdend.
- 5 - Table 5. Estimated yellwtail length frequency distributins and percent retained fr a 145 mm mesh cdend cmpared with a lined cdend (38 mm mesh) fr 3 ne hur pair tws. Numbers Cau!l:ht 1/ Leng;th Interval (cm) 145 mm 38 mm 0/0 retained 16-17 0 7 18-19 0 124 4-21 49 313 8 22-23 21 149 16 24-25 11 56 18 26-27 63 299 23 28-29 361 1277 28 30-31 827 2282 35 32-33 748 1786 41 34-35 528 1124 50 36-37 239 362 93 38-39 141 54 111 40-41 34 42 139 42-43 9 7 Ttal 3031 7882 lr-smtlieab,}'3's;-using gemetric means. The 145 mm mesh shwed less effective release f small fish than did the 129 mm gear (Figures 1 and 2). In view f the lw numbers f small fish present, this apparent anmaly prbably resulted frm sampling errr. The estimated yellwtail catches fr the 145 mm mesh and the lined cdend, fr the 3 pair tws, are given in Table 6. The 145 mm mesh caught fewer f bth market and discard fish than the lined cdend. The rati f market t discard catch increased frm 45 t 56 percent. Table 6. Cmparisn f yellwtail catch fr the 145 mm mesh and the lined (38 mm) cdend. Size Categry Catch - bushels/hur 145 mm 38 mm Market Discard Rati, Market t Discard 3.6 6.5.56 7.6 16.7.45 Series 3 (mesh size 129 mm versus 145 mm). Length distributin data fr the 129 and 145 mm mesh cdends, based n 2 pair tws, are given in Table 7. The 145 mm mesh shwed mre effective release f small fish in this series than it did in series 2. - 06 -
- 6 - Table 7. Estimated yellwtail length frequency distributins and percent retained fr a 129 mm mesh cdend cmpared with a 145 mm mesh cdend fr 2 ne hur pair tws. Numbers Caught 0/. Caught Length Interval (cm) 145 mm 129 mm El. 145 mm 16-17 0 11 0 18-19 5 30 17-21 10 70 14 22-23 43 46 24-25 0 27 0 26-27 38 109 35 28-29 197 529 37 30-31 6 1318 47 32-33 696 1417 49 34-35 385 968 40 36-37 273 474 58 38-39 132 127 104 40-41 38 32 119 42-43 29 30 97 44-45 0 8 0 46-47 6 0 Ttal 2449 5193 The estimated yellwtail catches fr these 2 cdends are given in Table 8. The 129 mm mesh caught abut twice the vlume f bth market and discard fish as the 145 mm did, and, hence bth meshes had a catch rati f market t discard f 65 percent. Table 8. Cmparisn f yellwtail catch fr the 129 mm and 145 mm mesh cdends. Size Cs.tegry Catch - bushels /hur 129 mm 145 mm Market Discard Rati, Market t Discard 11.0 17.0.65 5. 3 8.0. 66 Series 4 (mesh size 129 mm versus 116 mm). Length distributin data fr the 129 mm mesh cdend and the 116 mm mesh cdend (standard cmmercial gear n the vessel Mnlight), based n 3 pair tws, are given in Table 9. The 129 mm net released sme numbers f market yellwtail at r just abve the 35 cm cull pint, but few larger nes were released. Substantial amunts f discards were released by the larger mesh. - 07 -
- 7 - Table 9. Estimated yellwtail length frequency distributins and percent retained fr a 129 mm mesh cdend cmpared with a 116 mm mesh cdend fr 3 ne hur pair tws. Length Interval (cm) Numbers Caug:ht 129 mm 116 mm % Caught by 129 mm -21 0 12 0 22-23 2 0 24-25 0 5 0 26-27 10 23 43 28-29 50 4 24 30-31 121 528 23 32-33 247 893 28 34-35 316 830 38 36-37 307 397 77 38-39 95 134 71 40-41 22 30 73 42-43 3 0 44-45 3 3 100 Ttal 1176 3059 The estimated yellwtail catches fr these 2 cdends are given in Table 10. The 129 mm mesh caught, by vlume, 810/0 as much market fish and 26% as much discards as did the 116 mm mesh. The rati f market t discard was very much higher with the 129 mm mesh, 2.12 versus. 68. Table 10. Cmparisn f yellwtail catch fr the 129 mm and 116 rm mesh cdends. Size Categry Catch - bushels/hur 129 mm 116 mm Market Discard Rati, Market t Discard 3. 4 1.6 2.12 4. 2 6. 2.68-08 -
- 8-1 100 80 w z - ~ w a:: ~ Z 60' W U a:: w a. 40 0 1." 1 1 1 1 1 14.5.5 26.5 32.5 38.5 44.5 TOTAL LENGTH (em) Figure 1. Selectin curve fr yellwtail fr a 129 mm nyln cdend. _ 09_
- 9-1401 1 100 Cl W Z - ~ 80 w a:: I- Z lij U a:: 60 lij a.. 40 01 ~ I I I I I 14.5.5 26.5 32.5 38.5 44.5 TOTAL LENGTH (em) Figure 2. Selectin curve fr yellwtail fr a 145 mm nyln cdend. - 010_