Netherlands Journal of Sea Research Tel + 32 ( ? 18 Fax (0) ,S 26 (1): (1990)

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1 I z w o Netherlands Journal of Sea Research Tel + 32 ( ? 18 Fax (0) ,S 26 (1): (1990) ON THE SURVIVAL OF PLAICE AND SOLE DISCARDS IN THE OTTER-TRAWL AND BEAM-TRAWL FISHERIES IN THE NORTH SEA F.A. VAN BEEK, P.l. VAN LE E U W E N and A.D. RIJNSD O R P Netherlands Institute for Fishery Investigations, RO. Box 68, 1970 AB IJmuiden, The Netherlands ABSTRACT This paper sum m arizes th e results of survival exp erim en ts w ith plaice and sole discards caught by com m ercial beam -trawl vessels and a research vessel fishing w ith an otter traw l, and w ith soles escaped through th e m eshes in covered-cod-end experim ents. In the com m ercial beam -traw l fish ery th e survival of both plaice and sole discards w as estim ated to be less than 10%. T h e survival o f soles escaped through th e m eshes was estim ated at 6 0 %. The survival of discards was negatively affected by the catching process. During th e catching process m ortality w as caused through th e action of the tickler chains and th e injuries im posed during the stay in th e net. Under p resent-day com m ercial practice the processing o f th e catch on deck hardly affects the survival of discards. 1. INTRO DUCTIO N Trawl fisheries for flatfish are generally m ixed fish e r ies w ith a variety of target species. In the N orth Sea th e fishery for flatfish is m ainly carried out w ith beam traw ls, w h ich are used in m ore than 75 % of the total international landings of sole (S olea solea L.), plaice (P leuronectes platessa L.), turbot (S cophthalm us m axim u s L.) and brill (S cophthalm us rho m bu s L). B eam -traw l fishe ry is characterized by a substantial bycatch of various species of fish and invertebrates and a considerable catch of dead m aterial such as stones, fossil shells, sand, wood and garbage. A decreasing part of the total international catch is taken by otter trawls. Both kinds of gear use tickle r chains in front of the ground rope to drive the flatfish from th e sea bottom so that they can be caught by th e net. Sole fishery in the North S ea has to be conducted w ith a legal m inim um mesh size of 80 or 90 mm. At these m esh sizes the 50% retention length of sole is 26 o r 30 cm, respectively (R ijn s d o r p et al., 1981; van B eek e t al., 1982) and a substantial part of the undersized sole ( < 2 4 cm) can escape through the m eshes. In other com m ercial flatfish species, such as plaice, turbo t and brill, which have a m uch lower retention length than sole, the num bers escaping through the m eshes are negligible. C onsequently discard rates in plaice are m uch higher than in sole ( d e V e e n & R o d e n b u r g, 1971; d e V e e n e t a i, 1975). Dab (Lim anda lim anda L.), a com m ercially less attractive flatfish species, is also discarded in great quantities; turbot and brill, however, are discarded in low quantities as these species do not recruit to the fishing grounds before reaching the m inim um landing size (d e V een et al., 1975). To assess the possible effects of technical m anagem ent m easures such as m esh size regulations, closed areas etc., to im prove th e exploitation of plaice and sole, know ledge on the survival of plaice and sole discards is param ount. As on ly in sole substantial num bers escape through th e m eshes also the survival of these soles is im portant to evaluate th e effect of an increase in mesh size. In th e past research on the survival of discards focussed on plaice (G a r s t a n g, 1905; B o r l e y, 1909; d e V e e n et a i, 1975; K e l l e, 1976, 1977). Inform ation on the survival of soles escaping through the m eshes is lacking. G a r s t a n g (1905) concluded that the via bility of plaice discards was good directly after catching, but that high m ortality occurred during th e ir stay on deck. B o r l e y (1909) showed experim entally that the survival of plaice discards was m ainly affected by the haul duration, the presence of M edusae and the cond itions on deck. Hot sunshine especially caused a substantial m ortality. T he survival of plaice discards in the otter-trawl fishe ry was estim ated to be below 10% in m ost cases. In the beam -traw l fishery the condition of plaice discards was better and in short hauls under favourable conditions 50 to 75 % of the catch survived (B o r l e y, 1909). The survival of both plaice and sole discards in the shrim p fishery, em ploying a light beam traw l w ithout tic k le r chains but w ith rollers attached to the ground rope, was estim ated at about 50-60%, but was reduced to 20 % in plaice and 30% in sole when shaking sieves were used to sort the catch (K e l l e, 1976). The survival of discards in m odern flatfish fishery is expected to have changed with the introduction of heavy beam trawlers at the early 1960 s. In com parison with the otter- and beam -trawl fishery in the be

2 152 F.A. VAN BEEK, P.L. VAN LEEUWEN & A.D. RIJNSDORP ginn ing of the century, the fishing speed had increased to about 5 knots (9 km -h -1) in the late 1970 s and to about 7 knots (13 km -h-1 ) at present, and the w eight of the gear, including a large num ber of tic k le r chains, increased to about 1O103 kg at present. A lso the tim e that discards are exposed on deck w as reduced by th e introduction of conveyer belts in the early 1980 s to im prove the w orking conditions of the crew. T he present paper sum m arizes the results of the survival experim ents w ith plaice and sole discards carried out between 1972 and 1982 on board of com m ercial beam traw l vessels and a research vessel In th e N orth Sea. A dditional experim ents were carried out to estim ate the survival of soles that escaped through the m eshes. sea water in 2. M ETHO DS S urvival experim ents w ith plaice and sole discards were carried out on board com m ercial beam trawl vessels operating under norm al com m ercial conditions. A dditional experim ents were carried out with plaice caught w ith an otter trawl on R.V. Tridens. Discards were sorted from the catch and the ir condition was classified according the scale given in Table 1. From each condition class a random sam ple was taken and placed in a plastic tank (size: 40x60 cm and 12 cm high) with a m axim um of 15 fish per tank. Stacks of about 10 tanks alm ost filled with seawater were placed in a wooden fram e and supplied with continuous flow of fresh seawater (Fig. 1). The tanks were placed on de ck in a position w here rolling and pitching of the ship were m inim al. On board R.V. Tridens the stacks of tanks were placed in a large closed basin of seawater, with continuous flow. If conditions perm itted, the experim ents were checked every 12 hours and the dead fish was recorded and rem oved. The experim ent was term inated when all fish had died or at the end of the cruise. In a num ber of cases th e experim ent had to be stopped prem a turely due to the w eather conditions. TABLE 1 Classification of the condition of flatfish according to the damage. A: Fish lively, no visible signs of loss of scales or mucus. B: Fish less lively, some scratches and some scales missing, mucus layer affected up to 20%, some small red spots on the blind side. C: Fish lethargic, several scratches and some areas without scales, mucus layer affected up to 50%, several red spots on the blind side. D: Fish lethargic, head reddish, many scratches and areas without scales, mucus layer affected for more than 50%, blind side with many red spots and haemorrhages. water outlet " = Fig. 1. Experimental set-up used to estimate the survival of flatfish discards. S urvival experim ents were initiated w ith discards from catches that varied in the gear used (otter or beam traw l), the num ber of chains in front of the net, haul duration (between 15 and 120 m in), fishing speed, total volum e of the catch and th e engine power of the vessel. A lso sea conditions (wind force) and w ater tem perature were considered. The technical inform ation of the experim ents is sum m arized in Table 2. The length range of the discards used in the survival experim ents was 20 to 30 cm in plaice and 20 to 28 cm in sole. In addition, the effect of the increasing processing speed on the survival of plaice discards was exam ined by com paring discard m ortality of plaice processed by the tradition al m ethod and of plaice discards processed w ith a sorting device. By this m ethod the catch is dum ped in a water filled container w hich supplies a conveyer belt along w hich the catch is sorted and processed. D iscards and other unwanted m aterial stay on the conveyer belt and are transported back to sea im m ediately. In th e sorting device used in our experim ents, the catch was kept w et w ith sprinklers throughout their stay on the conveyer belt. The overall survival of the discards (S ) at experim ental tim e i was calculated from the proportion (Pj) and the survival (S,j) m easured in each condition class (j = 1.. 4) of the total discard catch according to: s i = FY s j

3 PLAICE AND SOLE DISCARDS 153 TABLE 2 Technical information on the survival experiments. Exp. numb. horse power haul duration total catch x 40kg n u m h e r depth water air weather cod-end cover of chains (m) temp. temp. cond. date plaice: otter trawl (fishing speed: 3.5 knots) Nov Feb plaice: beam trawl (fishing speed: knots) SW 1-4 Nov-Dee SW SW SW W 1-2 May-Jun S Var 2 Sep Var Dec plaice: beam trawl, deck-processing with conveyer belt (fishing speed: knots) SW 1-4 Nov-Dee SW SW SW Oct Dec sole: beam trawl (fishing speed: knots) 28) S 1-2 Aug ) S Var 2 Aug Var Var W 2-3 Aug-Sep W W W W W 2-3 )1 Irish Sea T he survival of soles escaped through the m eshes was estim ated in conjunction with m esh selection expe rim ents on com m ercial beam traw lers using the covered cod-end technique (R ijn s d o r p e t a i, 1981; v a n Beek et al., 1982). T he survival experim ents w ere set up in pairs, experim ent A with soles from the cod-end and experim ent B w ith soles tha t escaped through the m eshes of the cod-end and retained in the cod-end cover. The length range of the fish w as 18 to 25 cm. T he fish w ere not classified in condition categories. F urther experim ental procedures w ere identical to those applied in plaice.

4 154 F.A. VAN BEEK, P.L. VAN LEEUWEN & A.D. RIJNSDORP 3. RESULTS 3.1. DISCARDS The frequency of occurrence of the different con dition classes of th e discards in the catch is show n in Fig. 2. T he condition of plaice discards was poorer in the beam -traw l experim ents than in the otter traw l. In both plaice and sole the condition decreased with increasing haul duration. Survival of the 4 condition classes (Fig. 3) corresponded to the subjective cla s sification of con dition, but the survival curves of the sam e condition class differed between otter and beam traw l and between diffe re nt haul durations. O bviously not all injuries that may cause m ortality w ere visible. p laice o tte r traw l In each experim ent th e discard survival was estim ated from the m ortality rate in each condition class and the proportion of th is condition class in the total catch. In general the m ortality rate w as highest in the firs t 24 or 48 h and levelled o ff afterw ards (Tables 3 and 4). How ever, in som e experim ents substantial m ortality started only a fter 24 h. T he overall survival w a s estim ated as the survival after an experim ental tim e of 84 h. A fte r this tim e m ortality ap parently ceased or was very low (Fig. 4). In th e fe w experim ents stopped prem aturely after 60 or 72 h, the percentage o f surviving discards was extra polated assum ing a s im ila r m ortality rate between h and h as in the other experim ents. O verall discard survival was estim ated to be betw een 0 and 50 % in plaice and betw een 4 and 40% in sole. In plaice the survival w as low er in beam -traw l discard s than in otter-traw l discards (M ann-w hitney, p < ). D iscard survival in the beam traw l was 100 p la ic e o tte r tr a w l to tn 60 - cond A cond B cond C cond D plaice beam traw l p la ic e b e a m tr a w l 100 = so le beam tra w l so le beam tra w l to haul d uration Fig. 2. Frequency distributions of the condition (A, B, C or D) of plaice discards in otter- and beam-trawl catches of 60- and 120-min haul duration and of sole discards in beamtrawl catches of 60- and 120-min haul duration. Damage increases from condition A to D (compare Table 1) h o u r s m a in te n a n c e Fig. 3. Survival (%) of the different condition classes (A-D) in plaice and sole discards during maintenance in the holding tanks.

5 PLAICE AND SOLE DISCARDS 155 TABLE 3 Plaice: survival in percentage of discards (18-27 cm) and the number of fish at the start of each experiment (N). The bold figures are extrapolated values estimated under the assumption that the mortality between 60 and 72 and between 72 and 84 hours was equal to the average mortality at this time in the other experiments. The figures in italics are interpolations by eye between the last observation at 36 hours and the final at 96 hours. The experiment number corresponds to the information given in Table 2. Experiment Number Otter trawl: 20 min N Time interval (hours) after start of experiment Otter trawl: 60 min Otter trawl: min Beam trawl: 60 min Beam trawl: 120 min Beam trawl: 60 min and deck-processing with a conveyer belt Beam trawl: : 120 min and deck-processing with a conveyer belt

6 156 F.A. VAN BEEK, P.L. VAN LEEUWEN & A.D. RIJNSDORP TABLE 4 Sole: survival expressed in percentage of discards (20-28 cm). N indicates and the number of soles at the start of each experiment. The bold figures are extrapolated values estimated under the assumption that the mortality between 72 and 84 hours was equal to the average mortality at this time in the other experiments. The experiment number corresponds to the information given in Table 2. A: soles collected from the cod-end Experiment Number N Beam trawl: min Time interval (hours) after start of experiment _ Beam trawl: min _ Beam trawl: 120 min B: soles escaped through the meshes and collected in the cod-end cover -. t, Time interval (hours) after start of experiment Exoeriment N Number Beam trawl: min Beam trawl: min 31 i Beam trawl: 120 min

7 PLAICE AND SOLE DISCARDS p la ic e beam tra w l i p la ic e d is c a rd s i * i * i» i 1 i» i «i ' i i sole beam tra w l sole d is c a rd s i i Ï I -I 1 I 1 I 1 I 1 I 1 I 1 I 1 I hours m aintenance haul d u ra tio n (m in) Fig. 4. Survival (%) of plaice and sole discards from 2-hour Fig. 5. Survival (%) in plaice and sole discards after 84 hauls in the beam trawl experiments during maintenance. hours in experimental holding tanks in relation with haul The vertical bars indicate the standard deviations. duration (min.). h ig h e r in hauls of 60 m in than in hauls of 120 min (Fig. 5), the relation being significant in sole (S pearm an r s = , n = 11, p < ) and m arg ina lly s ig n ific a n t in plaice (Spearm an rs = , n = 19, < p < ). In otter traw l discards of plaice the opposite w as observed, but the num ber of discards of 60 m in hauls w as rather sm all. W ater tem pe rature had a slight but sig nificant effect on the survival of plaice discards in the 60-m in-beam -traw l hauls (S pearm an rs = , n = 8, p < ) and a m argin ally s ig n ific a n t in the 120-m in hauls (S pearm an r s = , n = 11, < p < ). S urvival in creased at low er w ater tem peratures. The m ethod of processing the catch on board did not affect the survival of plaice discards in 120-m in hauls in a pairw ise com parison of the 3 experim ents. In the 3 pairw ise exp erim e nts of 60-m in haul duration, no difference occurred between conveyer belt and traditional processing in one experim ent (14.0 vs 15.0) but a sub stantia lly higher survival occurred w ith conveyerbelt processing in the two o th e r experim ents ( and ). No s ig n ifica n t effect of the m axim um w ind force on the survival of plaice discards could be dem onstrated in 60- or 120-m in hauls (S pearm an rs = , n = 8, p ; rs = , n = 11, p ) SURVIVAL OF SOLES ESCAPED THROUGH THE MESHES A num ber of experim ents were carried out to estim ate the survival of soles escaped throu gh the m eshes and collected in the cod-end cover. The survival of soles originating from the cover w as slightly but sig n ifica n tly higher than in those from the codend (W ilcoxon s signed ranks test, P < ). The

8 158 F.A. VAN BEEK, P.L. VAN LEEUWEN & A.D. RIJNSDORP results, given in Table 4 and Fig. 6, show that survival decreased with increasing haul duration and th a t substantial survival occurred at short hauls of 15 m in. As the decreased survival at longer haul durations may be caused by the prolonged stay of the escaped soles in the cod-end cover, the extrapolated survival at a haul duration of 0 min is about 60% and this figure w ould give a credible estim ate of the survival of sole that escape through the meshes. ra 80 i haul d u ra tio n (m in) Fig. 6. Survival (%) (after 84 hours) of sole escaped through the meshes and collected in a cod-end cover of a beam trawl in relation to haul duration. The extrapolated survival at a haul duration of 0 min is estimated at 60%. 4. DISCUSSIO N Several factors m ay contrib u te to the m ortality of discards durin g catching and sorting on board. The experim ents presented in th is paper allow a first evaluation o f the main factors influencing the discard m ortality in m odern beam trawl fishery. A lthough the m aintenance conditions of the fish in the tanks on board w ere care fu lly m onitored and kept as optim al as possible, they m ay have caused som e additional m ortality. Fish can get sea sick and stressed; according to K e l l e (1977) stress is a m ajor cause of m ortality. T herefore, our experim ents m ay have overestim ated the true m ortality of plaice and sole discards. A lthough th e additional m ortality resulting from m aintenance can be investigated in experim ents initiated w ith fish not affected by the fishing gear, our experim ental set-up did not allow this. T w o observations indicate that th e additional m ortality is relatively low. First, no sig nificant effect of rolling and pitching of the ship in poor weather conditions could be dem onstrated for plaice. Second, in the otter-traw l experim ents, plaice of condition class A (no visible dam age) showed 100% survival; in the beam -traw l experim ents the survival of plaice and sole of condition A w as above 70% (Fig. 3). R epeated inspection of experim ental fish durin g the exp erim e nt show ed that fish w hich were recorded to be undam aged at the start of the experim ent showed signs of dam age later on in the experim ents. To evaluate the catching process itself, survival experim ents w ere initiated w ith discard s w hich diffe re d in the num ber o f tic k le r chains, total catch and haul duration. Because the num ber of experim ents was relatively sm all, on ly a qualitative evaluation is possible. In both sole and plaice discards the survival w as negatively correlated w ith haul duration, indica ting that m ortality factors w orking in the codend dom inated over the effects of the tic k le r chains. The tic k le r chains will affect the survival of discards irre spective o f haul duration. A m axim um estim ate of the contribution of the tic k le r chains to the discard m ortality of sole is given by the extrapolated survival of th e discard s and of soles w hich escaped through the m eshes at 0 m in, about 50 % and 60 % respectively. A com parison w ith the survival of about 25% in 60-m in hauls and 10% in 120-m in hauls shows th a t besides th e possible effect of the tickle r chains also duration of the stay In the cod-end substantially con tribu te d to the m ortality. In our experim ents haul duration and the total w eight of the catch w ere correlated and there fore it is not possible to disentangle the contribution of haul duration and total catch w eight. T he m ain injuries of discards which could be observed m acroscopically were the loss of scales and m ucus, and haem orrhages. T he latter only appeared as da rk patches on the blind side of the body several days after catch ing. It is assum ed that these injuries w ere m ainly caused by the scraping and pressing of the various objects in the cod-end such as starfish, stones, shells, sand and pieces of wood. The nature and degree of the injuries w ill depend on th e com position of species and objects in the catch. T his com position varied betw een the experim ents but was not recorded. However, it is likely that the variation in com position of the catch contributed to the observed variation in survival in com parable experim ents. T he survival of discards classified as either condition A or B (not o r only slightly dam aged fish) varied greatly. T his variability is certainly influenced by the condition classification being subjective, but also by experim ental conditions. In particular, tem perature and direct sunlig ht are thought to play an im portant role (K e l l e, 1976). Also, invisible haem orrhages in the brain can develop from oxygen deficiency. K e lle (1977) show ed that exhausted plaice prevented from bre athing because they w ere com pressed in the codend developed bleeding in the brain w hich was, in m ost cases, lethal. O ur experim ents do not allow a

9 PLAICE AND SOLE DISCARDS 159 fu rth e r analysis of these factors. O th er factors that m ay con trib u te to the m ortality of discards are depre ssuriza tion (F e a t h e r s & K n a b l e, 1983) and m u scu la r fatig ue (P a r k e r et al., 1959; B e a m is h, 1966). T he range of w ater depth of the m ain fishing gro u n d s in the southern North Sea is m, so discard s w ill exp erience a fall in hydrostatic pressure of ~ 1 to 5 bar. A s plaice and sole do not have a swim blad der the effect of depressurization w ill be relatively sm all in com parison with e.g. gadoids. A s these factors will g e nerally cause a quick death (P a r k e r et al., 1959; B e a m is h, 1966; F e a t h e r s & K n a b l e, 1983) the ir effect w ill be included in th e m ortality estim ates of our experim ents. T he higher survival of plaice discard s in the ottertraw l experim ents and the reported high v ita lity of p laice in th e historical investigations (G a r s t a n g, 1905) m ight in dica te that fishing speed is probably an im p ortan t factor w hich am plifies the m echanical dam age in flicted during the stay in the cod-end. O nce on deck the discards are sub je cte d to m echanical dam age from sorting, and to tem perature differences, sun and desiccation. The influence of these factors on the survival can be substantial (B o r l e y, 1909; K e l l e, 1976) but is not considered in our experim ents. In som e of the plaice experim ents th e catch w as not dum ped on deck and sorted by hand, but w as processed m ore q u ickly on a conveyer belt w here the catch was kept wet. M ost m odern beam -traw lers are equipped w ith such a device. C om parison of the survival of plaice discards processed by hand and w ith the conveyer belt show ed a higher survival in the latter at haul du ra tions of 60 m in. O ther things being equal, this in dicates that th e processing on deck can actually affect the m ortality of discards and that the qu ick processing under con tinu ou s supply o f w ater gives a s lig h tly b e tte r survival. The negative effect on the survival of discard s of a long handling tim e on board w as also observed by G a r s t a n g (1905), B o r l e y (1909) and N e il s o n e t al. (1989). However, in m odern beam - traw l fishery w ith haul durations o f 120 m in the processing of the catch on deck does not m easurably affe ct the survival of plaice discards as the c o n d i tio n of plaice discard s is already ba dly reduced durin g the catch ing process itself. This contrasts w ith th e shrim p fishery, w here the survival rate of about 50 % in plaice and sole discards is sub stantia l ly reduced when the catch is processed with a shaking sieve (K e l l e, 1976). Rotating sieves and specia lly-d esig ned shaking sieves w ere show n to im prove the survival rate of plaice discards (B o d d e k e, 1989). In pra ctice the haul duration of com m e rcial beam traw lers is in th e ord er of 120 m in unless the ground is very rough and a high am ount of bycatch necessitates a reduction in haul duration (d e V een e t al., 1975). T herefore, the survival of both plaice and sole discards can be estim ated at 10%. T his percentage is a m axim um estim ate, because it is likely that in the survival experim ents not all fish died w ithin the experim ental period of 84 hours on w hich th e above survival estim ate is based. The survivors are possibly the fish captured in the last m inutes of the haul. The survival experim ents w ith soles collected from the cod-end cover show ed a decreasing survival rate w ith increasing haul duration. The extrapolated survival at a haul duration of 0 m in was estim ated at about 60%. The m ortality of soles escaping through the net m ay be caused by the injures inflicted by the catch ing process (tickler chains, duratio n of stay in th e net) and during escape through the meshes. Sole can escape through relatively narrow m eshes by bending in the dorso-ventral axis and squeezing through the mesh. W hen th e net is hauled in, m any soles are seen to be caught halfw ay throu gh the m eshes ( stekers ). Visual inspection of such stekers shows som e loss of scales. Inspection of the experim ental sole that died also show loss of scales and haem orrhages. The estim ated survival of about 60% therefore seem s a reasonable, but necessarily c rude estim ate. It im plies that th e gain of an increase in m esh size will be low er than generally calculated assum ing that all soles th a t escape throu gh the m eshes survive. 5. REFEREN CES B e a m is h, F.W.H., Muscular fatigue and mortality in haddock, Melanogrammus aeglefinus, caught by otter trawl. J. Fish. Res. Bd Canada 2 3 : B e e k, Results of mesh selection experiments on sole and plaice with commercial beam trawler vessels in the North Sea in ICES 1982/B:17:1-20(mimeo). B o d d e k e, R., Management of the brown shrimp (Crangon crangon) stock in Dutch coastal waters. In: J.F. C a d d y. Marine Invertebrate Fisheries. Wiley & Sons, New York: B o r l e y, J.O., Report on the vitality of trawl-caught plaice. Mar. Biol. Ass. Report II, Part II, F e a t h e r s, M.G. & A.E. K n a b l e, Effects of depressurization upon largemouth bass. N. Am. J. Fish. Management 2: G a r s t a n g, W., Vorläufiger Bericht über die Naturgeschichte der Scholle auf grund der Untersuchungen der Kommision B in der Zeit bis zum 30 Juni Rapp. Proc. Verb. Cons. int. Explor. Mer 3, Appendix H: K e l l e, W., Sterblichkeit untermasziger Plattfische im Beifang der Garnelenfischerei. Meeresforschung 2 5 : , Verletzungen an untermassigen Plattfischen in der Garnelenfischerei. Arch. Fisch. Wiss. 28: N e i l s o n, J.D., K.G. W a iw o o d & S.J. S m i t h, Survival of Atlantic halibut (Hippoglossus hippoglossus)

10 160 F.A. VAN BEEK, P.L. VAN LEEUWEN & A.D. RIJNSDORP caught by longllne and otter trawl gear. Can. J. Fish. Aquat. Sei. 46: P a r k e r, R.P., E.C. B l a c k & P.A. La r k i n, Fatigue and mortality in troll-caught pacific salmon (Oncorhynchus). J. Fish. Res. Bd Canada 16: R i j n s d o r p, A.D., F.A. v a n B e e k & P.I. v a n L e e u w e n, Results of mesh selection experiments on sole with commercial beam trawler vessels in North Sea and Irish Sea in 1979 and ICES 1981/B:31: 1-19 (mimeo). V e e n, J.F. d e & W.F. R o d e n b u r g, Discarding in the Dutch sole fisheries in 1969 and ICES C.M /B: 11: 1-4 (mimeo). V e e n. J.F. d e, P.H.M. H u w a e & M.S.S. L a v a l e y e, On discarding in the sole fishery and preliminary observations on survival rates of discarded plaice and sole in ICES C.M. 1975/ F: 28: 1-11 (mimeo).