Bycatch & discards in soviet purse seine tuna fisheries on fad 163

E-mail: evgeny.romanov@ird.fr

Western Indian Ocean J. Mar. Sci. Vol. 7, No. 2, pp. 163–174, 2008© 2008 WIOMSA

Bycatch and Discards in the Soviet Purse Seine Tuna Fisheries on FAD-associated Schools in the North Equatorial

Area of the Western Indian Ocean

Evgeny V. RomanovIRD, UMR 212 ‘Ecosystèmes Marins Exploites’, Centre de Recherche Halieutique Méditerranéenne et

Tropicale Avenue Jean Monnet – BP 171, 34203 Sète Cedex, France. Phone: +33 (0)4 99573205, Fax +33 (0)4 99573295, e-mail: evgeny.romanov@ird.fr )

Keywords: Indian Ocean, FADs, purse seining, bycatch, non-target species, discarding

Abstract: The bycatches and discards recorded by scientific observers aboard Soviet/Russian/Liberian tuna purse seiners operating in the North Equatorial Area (NEA)(0-10°N, 45-70°E) of the Indian Ocean during the FAD (fish aggregating devices) fishing season (August-November, 1986-1991) are analysed. A total of 108 analysed sets on FAD-associated schools yielded 2,200 metric tons (t) of total catch (21.8 (±4.74) t per positive set). More than 40 fish species and other marine animals were recorded, of which only two species, skipjack and yellowfin tuna, are target species. The average levels of bycatch were 1.923 t per set (non-tuna bycatch 0.915 t), or 96.8 t (46.1 t) per 1,000 t of target species. Principal species in the bycatch were bigeye tuna (0.995 t per set), pelagic oceanic sharks (0.246 t), rainbow runner (0.215 t), triggerfishes (0.199 t), and dolphinfishes (0.169 t). Only one turtle was caught. Estimated discards are equal to 0.891 t per set or 44.9 t per 1,000 t of target species. Potential discards, which included small skipjack, yellowfin, bigeye, and all frigate and bullet tunas, were estimated on the level 0.162 t per set or 8.3 t per 1,000 t of target species. Although the overall level of bycatch and discards in FAD tuna fisheries is low, this fishing mode may pose high risk of over-exploitation to pelagic sharks due to FAD associative behaviour and specific life history traits that make sharks vulnerable to excess fishing pressure.

INTRODucTION

Fishing on tuna aggregations around natural floating objects, anthropogenic flotsam or man-made fish aggregating devices (FADs), is an important part of tuna purse seine fishery activities. This fishing mode currently brings more than 50% of the total tuna catch by purse seine gears (up to 80% in some years) in all the principal purse seine fishing areas worldwide: western central Pacific, western Indian Ocean, eastern tropical Pacific and eastern Atlantic (Fonteneau et al., 2000; Fonteneau, 2008 pers. comm.).

Two species of tropical tuna: skipjack Katsuwonus pelamis (Linnaeus, 1758) and yellowfin Thunnus albacares (Bonnaterre, 1788) are main targets of this fisheries. However, as with most fisheries in the world oceans there are significant bycatch concerns (e.g. Northridge, 1984, 1991a, b; Brothers et al., 1999; Alverson et al., 1994; Wickens, 1995; Hall et al., 2000; Tasker et al., 2000; Kelleher, 2005), and tuna purse seining also yields non-target species, namely teleost and elasmobranch fish, marine mammals, reptiles and molluscs (Joseph, 1994; Bailey et al., 1996; Hall, 1996, 1998; Hall et al., 2000; Romanov, 2002;

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Olson et al., 2006), representing so-called ‘bycatch’ (McCaughran, 1992; Hall et al., 2000; Kelleher, 2005). In the Indian Ocean more than 50 species of fish and other marine animals occur in the catches of tuna purse seines (Romanov, 2002). Bycatch in this fishery may be rather high, and generally depends on the fishing tactics – whether overall fishing effort is directed at different types of surface tuna schools, being either free-swimming, associated with marine mammals, floating logs or man-made FADs, etc. (Romanov, 2002). The FAD fishing technique (purse seine sets around natural logs, anthropogenic flotsam, man-made FAD), which exploit natural aggregating behaviour of the target species to associate temporarily with FAD, were introduced in different purse seine tuna fisheries for several reasons: (i) to improve catch rates; (ii) to minimize fishery expenses, (iii) to comply with regulation, which protect species occasionally caught in other types of associated schools (‘dolphin schools’)1 or even (iv) to develop ‘environmental-friendly’ fisheries. However, FAD fishing produces relatively high bycatch rates, which in all tropical oceans greatly exceeds those of fisheries associated with marine mammals or free-swimming schools (Joseph, 1994; Bailey et al., 1996; Hall, 1996, 1998; Hall et al., 2000; Hall & Williams, 2000; Gaertner et al., 2002; Romanov, 2002). In the western Indian Ocean (WIO) the purse seine FAD fishing technique for tuna was used from the early years of the fishery (Stéquert & Marsac, 1986) and expanded extensively starting from 1995 (Fonteneau et al., 2000). Purse seiners are fishing on FADs in the WIO throughout the year within whole area of fishing operations. However, the principal season of FAD fisheries occurs in the North Equatorial Area (NEA) (from 0-15°N and 45-65°E) during late boreal summer-autumn (August-November) – end of the south-western monsoon and autumn inter-monsoon period. In these months, catches around the FADs represents more than 90% of the total catch obtained during this season (Fonteneau, 2008 pers. comm.).

The overall level of bycatch in the catches of international purse seine fisheries were analyzed earlier by Romanov (2002) based on extrapolations from data on the Soviet/Russian/Liberian purse seine tuna fleet. Some information on bycatch levels and species composition was published also during early years (Stretta et al., 1998; Santana et al., 1998) and was presented for more recent period at the Indian Ocean Tuna Commission (IOTC) meetings (Delgado de Molina et al., 2007). However no comprehensive analyses were published for this particular fishing mode and period when FAD fishing tactics prevail over fishing on other types of schools. There is little published information on the utilization of non-target species caught and on the level of discards from this fishery, especially for the period before 2000. This paper takes into consideration the importance of quantifying bycatches and discards of both target and non-target species in the development of fisheries management based on an ecosystem approach. It is an attempt to present such data in the specific fisheries and time-area strata during the early years of the development of FAD fishing technique in the WIO. Estimates based on information collected by scientific observers for the principal season and area of FAD-associated fisheries (August-November) in the NEA may serve as an important reference point of the historical levels of abundance for non-target species.

MATERIAlS AND METhODS

Analyses are based on data collected by scientific observers aboard Soviet (since 1992 – Russian-flagged or flying ‘flag of convenience’) tuna purse seiners in the WIO, during 1986-1991. Sampling methodology, data processing and the methods of analysis were described in Romanov (2002). Sampling coverage rate was estimated by 5-degree strata as percentage of sampled tuna catch to total tuna catch pooled for all periods of observations. Total tuna catch composition data for this fleet by small-scale strata are available for 1985-1994 from daily radio reports (RR) on fishing activities of the

1Dolphin mortality resulted from fishing on tuna, associated with schools of dolphins in the eastern Central Pacific was major legislative and environmental issue in the 1980s (Joseph, 1994; Hall, 1998).

Bycatch & discards in soviet purse seine tuna fisheries on fad 165

vessel (Romanov, 2002), however, RR contain no information on the catch of non-tuna species. The NEA is defined as 0-10°N, 45-70°E since fishing activity does not expanded beyond 10°N during the period under consideration. A total of 108 purse-seine FAD sets were sampled and 101 positive sets (total catch 2200 t) were analysed in the NEA, which correspond to 87% of total number of sampled sets in the area/strata. The term ‘FAD’ in this paper means any type of floating object used for tuna fishing, including natural logs, palm branches, and anthropogenic flotsam (table desk, freezers, wire rope bobbins, longline floats, etc.), as well as specially constructed fish aggregating devices. Bycatch here is defined as the fraction of the catch that consists of non-target species (including other species of tuna), which are encircled by the purse seine fishing gear and are unable to escape by themselves (Romanov, 2002). Bycatch may be retained on board or discarded (Alverson et al., 1994; Kelleher, 2005). Discards or non-retained catch is a fraction of the sum of target catch and bycatch that is released directly from the purse seine net, or taken on board and subsequently discarded in the ocean. Catches of undersized individuals of the target species, which are not considered as bycatch in this paper, are also often discarded while some species considered here as bycatch, like bigeye tuna Thunus obesus (Lowe, 1839) and albacore tuna Thunnus alalunga (Bonnaterre, 1788) are generally kept onboard and not discarded. Most of the data from logbook and observer records does not include information on discards. Information on discarding practice and level of discards was obtained in 2000 during the survey, based on interviews with five observers and one member of the tuna vessel crew (fish processing officer). Since discarding was discouraged by domestic regulation, interviews were designed as informal conversation in a friendly and cooperative manner. Respondents were asked if they knew anything about (i) discards during their stay onboard fishing vessels, (ii) the source of this information (only personal observations were later considered), (iii) the species discarded/kept, (iv) the level of discards and (v) the incentives for discarding or retaining catch or bycatch species. Estimates of potential discards of small-sized tuna were

calculated from the size composition of the catch. Size composition of the catch for the principal tuna species was extrapolated from tuna size frequencies (fork length, FL) obtained from sampled catch. Length-weight relationships, developed during this study, were used to estimate tuna weight for specimens that we not weighed. The formulae used were:Yellowfin TW=3.111907x10-5xFL2.859513 (n=16240), R2=0.98376, range FL 10.1-189.0 cmSkipjack TW=5.173059x10-6xFL3.339043 (n=6990), R2=0.97152, range FL 4.6-92.0 cmBigeye TW=1.882289x10-5xFL2.980910 (n=1374), R2=0.90199, range FL 54.0-193.0 cm. For albacore, frigate (Auxis thazard) and bullet (Auxis rochei) tunas, which were also recorded as bycatch, no representative size frequency data were recorded. Discard estimates presented here are calculated for the catch and bycatch species taken onboard of fishing vessels. Average values are presented as arithmetic means, plus or minus 95% confidence intervals.

RESulTS AND DIScuSSION

Total catch and sampled catch distributionTotal catches of the Soviet fleet, sampled catch and sampled FAD catch pooled for 1985-1994 by one-degree stratum are presented at the Fig. 1A, B, C. The catches during the FAD fishing season (August-November) are pooled in the same manner and presented in Fig. 1D, E, F, showing the shift in the activity of the vessels toward the North Equatorial Area. The cumulative (August-November) catches in 1985-1994 in the NEA when compared to the overall sampled catch demonstrate a high level of sampling coverage rate. Out of ten NEA 5-degree strata, catches were recorded in nine, numbered here consequently from 1 to 9 (Fig. 2). The total Soviet catch varied considerably within this area, with the highest catch recorded in strata 6, 7, and 8, representing 91% of the total. Total catch by 5-degree square, coverage rate by stratum and the percentage of FAD sets in the sampled catch are shown at the Fig. 2. Areas with highest fishing activity: 6, 7, 8 were sampled at the rate from 6% to 24%. Non-sampled areas: 1, 2, 4, 5 correspond

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Fig. 1. Total catch (A, D), sampled catch (B, E) and sampled FAD catch (C, F) of Soviet purse seiners by one-degree strata in 1986-1991. Left column (A, B, C) is annual values, right column (E, D, F) represents FAD fishing season (August-November)

to areas with minor fishing activity (about 5% of total catch). This suggests that the data sampling adequately represented catch and fishery activity in the area during the season. Species composition. The composition of catches in the FAD sets included 44 species (or higher taxa) (Table 1). Total catch per positive set in the area was 21.8 (±4.74) t. Percentage of principal

species in FAD-associated schools was 70% for skipjack, and 22% for yellowfin (catch per set was 15.2 (±3.95) t and 4.6 (±1.05) t respectively). The majority of FAD sets produced bycatch. Bycatch was not recorded in one case of successive sets during the day on the same FAD. On average, bycatch consisted of 8% of the catch in the area. Total bycatch per positive set was 1.923 (±0.516)

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Bycatch & discards in soviet purse seine tuna fisheries on fad 167

Fig. 2. Purse seine cumulative catches (in August-November 1986-1991) in the North Equatorial Area by 5-degree squares. Pie diagrams represent percentage of sampled catch (red sector) (August-November 1986-1991) relative to the cumulative catch, with upper right corner numbers being sequential numbers of the 5-degree strata, and lower left corner figures the percentage of FAD sets within sampled sets

t and per 1,000 t of target species: 96.8 (±48.0) t (Table 2). Tuna bycatch consisted of bigeye, albacore, frigate and bullet tunas (Table 1, 2). Total tuna bycatch was 1.008 (±0.432) t per positive set and 50.7 (±43.8) t per 1,000 t of target species (Table 2). Non-tuna bycatch was 0.915 (±0.225) t per positive set or 46.1 (±21.8) t per 1,000 t of target species (Table 2). The bulk of the non-tuna bycatch in sets on FAD-associated schools (per positive set / per 1,000 of target species) is made up of carcharhinid (Carcharhinus spp.) and hammerhead (Sphyrna spp.) sharks (0.246 t / 12.4 t), rainbow runner Elagatis bipinnulata (0.215 t / 10.8 t), triggerfish of the genus Canthidermis (0.199 t / 10.0 t), dolphinfish Coryphaena hippurus (0.169 t / 8.5 t), wahoo Acanthocybium solandri (0.041 t / 2.0 t), billfishes of the genera Makaira and Tetrapturus (rarely swordfish Xiphias gladius) (0.023 t / 1.2 t), and mackerel scad Decapterus macarellus (0.011 t / 0.6 t). Capture of a sea turtle (unknown species) was recorded only once (Table 1, 2). Although sea turtle mortality resulting from entanglement in FAD structures is an issue of major concern during recent

years (Anon., 2007a) it seems that bycatch of sea turtles was low during the period of this study. Similar species assemblages in the commercial catches around FAD were reported earlier for all tropical oceans (Arenas et al., 1999; Bailey et al., 1996; Romanov, 2002; Goujon, 2004). Fishery-independent underwater survey of drifting FAD-associated communities in the equatorial Indian Ocean (Taquet et al., 2007) reported 32 species/genera. Divers’ observations demonstrate high similarity with observer data analysed here at the generic level, while only 14 species appeared to be the same for both studies. A number of factors, such as identification bias (both for observers and divers), sampling area and period, gear selectivity and coverage area limitations during underwater observations may result in such discrepancies. Fisheries observer data give better representation of circumnatant species (see Parin & Fedoryako, 1999; Taquet et al., 2007), which usually stay at a distance from FADs (large sharks and rays, billfish, tuna and tuna-like species) and mesopelagic migrants entering epipelagic waters (Taquet et al., 2007). Divers by comparison, collect more precise

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Fig. 3. Length frequencies of skipjack tuna K. pelamis (SKJ), yellowfin tuna T. albacares (YFT), and bigeye tuna T. obesus (BET), from the Soviet purse seine catches in the North Equatorial Area. Data extrapolated to sampled catch; boxed outlines of sizes assumed to be potential discards

information on small species of intra- or extranatant fish communities (Taquet et al., 2007), which often escape through the purse seine net wall. However both approaches demonstrate lower biomass and high diversity of non-target species in comparison with the principal commercial species in this fishery. Tuna size frequencies. The caught tuna show a range of sizes, from juveniles to adult: 29-77 cm for skipjack (average weight 3.02 kg), 30-151 cm for yellowfin (average weight 3.60 kg) and 31-142 cm for bigeye (average weight 2.97 kg) (Fig. 3). As observed in all purse seine fisheries, the size composition of yellowfin and bigeye tuna caught in FAD sets are highly skewed toward small fish (<100 cm) (Fig. 3), while purse seiners fishing other types of tuna schools, such as free-swimming or whale-associated, usually target large mature fish (> 100 cm FL) (Romanov, 2001; Anon., 2007b).

Small, non marketable-sized tuna, both of target and non-target species occurring in the catch, are a potential source of discards from these fisheries. Discards. Interviews with observers and crew members confirmed anecdotal information that, at least up until 1991, Soviet vessels generally did not discard small-sized yellowfin, skipjack, kawakawa, frigate and bullet tunas, but retained them for sale as ‘small mixed’ tuna. The fate of other bycatch species, retained onboard or discarded, is based on a number of complex criteria related either to crew consumption preferences, or the possibility and profitability of selling these species at local markets: generally at Port Victoria (Seychelles); Singapore (Singapore) or Bangkok (Thailand). The description of preferences for utilization or discarding of the other species is given in the Table 3. Data available suggest that elasmobranch species (except fins of shark) as well as small species of

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Bycatch & discards in soviet purse seine tuna fisheries on fad 169

Table 1. Species composition of tuna purse-seine FAD catches in the North Equatorial Area of the western Indian Ocean

Family, species Common names Occurrence

PISCESDasyatidae ?*Dasyatis spp.** Stingrays +MyliobatidaeManta birostris (Walbaum, 1792) Giant manta +Mobula spp. Devil rays. +CarcharhinidaeCarcharhinus falciformis (Müller & Henle, 1839) Silky shark +C. longimanus (Poey, 1861) Oceanic whitetip shark +?C. obscurus (LeSueur, 1818) Dusky shark +?Carcharhinus spp. Requiem sharks ?SphyrnidaeSphyrna lewini (Griffith & Smith, 1834) Scalloped hammerhead +Sphyrna spp. Hammerhead sharks. +Belonidae sp. Needlefishes +LampidaeLampris guttatus (Brünnich, 1788) Opah +SphyraenidaeSphyraena barracuda (Edwards, 1771) Great barracuda +Sphyraena spp. Barracudas +CarangidaeCaranx spp. Jack, trevally +Decapterus macarellus Cuvier, 1833 Mackerel scad +Decapterus spp. Scads +Elagatis bipinnulata (Quoy & Gaimard, 1824) Rainbow runner +Seriola spp. Amberjack +Naucrates ductor (Linnaeus, 1758) Pilotfish +CoryphaenidaeCoryphaena hippurus Linnaeus, 1758 Common dolphinfish +Coryphaena spp. Dolphinfishes +KyphosidaeKyphosus cinerascens (Forsskål, 1775) Blue seachub +EcheneidaeEcheneis naucrates (Linnaeus, 1758) Live sharksucker +GempylidaeRuvettus pretiosus Cocco, 1833 Oilfish +EphippididaePlatax spp. Batfish +ScombridaeAcanthocybium solandri (Cuvier, 1832) Wahoo +Auxis rochei (Risso, 1810) Bullet tuna +Auxis thazard (Lacépède, 1800) Frigate tuna +Katsuwonus pelamis (Linnaeus, 1758) Skipjack tuna +Scomberomorus commerson (Lacépède, 1800) Narrow-barred Spanish mackerel +Scomberomorus spp. Seerfishes +Thunnus alalunga (Bonnaterre, 1788) Albacore +Thunnus albacares (Bonnaterre, 1788) Yellowfin tuna +Thunnus obesus (Lowe, 1839) Bigeye tuna +

Contd. on next page

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Family, species Common names Occurrence

IstiophoridaeMakaira indica (Cuvier, 1832) Black marlin +M. mazara (Jordan et Snyder, 1901) Indo-Pacific blue marlin +Makaira spp. Marlins +Tetrapturus audax (Philippi, 1887) Striped marlin +XiphiidaeXiphias gladius (Linnaeus, 1758) Swordfish +BalistidaeCanthidermis maculata (Bloch, 1786) Spotted oceanic triggerfish +MonacanthidaeAluterus monoceros (Linnaeus, 1758) Unicorn leatherjacket +Aluterus spp. Filefishes +DiodontidaeDiodon spp. Porcupinefishes +?CHELONIDEA Sea turtles +

Number of species (taxa) 44

*The sign (?) denotes doubtful, in the author’s opinion, species identification by observer.** Misidentification of the pelagic stingray, Pteroplatytrygon violacea

Contd. from previos page

Table 2. Estimates of the bycatch (t) of various species (groups) of marine animals in FAD-associated school sets. Groups are shown in bold

Species, a group of species Per positive set Per 1,000 t of target species

Bigeye (T. obesus) 0.995±0.433 50.1±33.2Albacore (T. alalunga) 0.011±0.010 0.6±0.6Frigate and bullet tunas (A. thazard, A. rochei) 0.001±0.001 0.1±0.1 Tuna bycatch 1.008±0.432 50.7±43.8 Shark bycatch 0.246±0.056 12.4±5.6 Billfish bycatch 0.023±0.011 1.2±0.8Wahoo (A. solandri) 0.041±0.023 2.0±1.6Rainbow runner (E. bipinnulata) 0.215±0.105 10.8±7.8Dolphinfish (C. hippurus) 0.169±0.076 8.5±5.7Barracuda (S. barracuda) 0.003±0.003 0.1±0.2 Triggerfishes (Balistidae, Monacanthidae) 0.199±0.067 10.0±5.6Mackerel scad (D. macarellus) 0.011±0.011 0.6±0.7 Mantas, mobulas (Mobulidae) 0.002±0.004 0.1±0.2 Sea turtles + 0.1±0.1 Other bycatch 0.005±0.003 0.3±0.2 Non-tuna bycatch total 0.915±0.225 46.1±21.8Total bycatch 1.923±0.516 96.8±48.0

flotsam community, rainbow runner and barracuda are always discarded. Wahoo and dolphinfish are often discarded while the billfish bycatch is always retained onboard. There was an unexpected preference of the crew to retain blue sea chub,

Kyphosus cinerascens, for their own consumption, however, this species as well as billfishes were a minor part of overall bycatch. These data show that during FAD fishery a major part of non-tuna bycatch was discarded.

Bycatch & discards in soviet purse seine tuna fisheries on fad 171

Based on data presented in the Table 3, a formula for discard calculation is derived as follows:Discards = total bycatch - (tuna bycatch + billfishes bycatch + weight of sharks fins + bycatch of blue sea chub + bycatch of irregularly retained species). Due to insufficient data, no attempts were made to estimate the weight of retained sharks fins, nor the weight of the discarded carcasses without fins. The weight of blue sea chub retained by crews was also not assessed. Therefore, total discards were assessed as:Discards = total bycatch - (tuna bycatch + billfishes bycatch)

Based on this formula, estimated discard per positive set is equal to 0.891 t, or 44.9 t per 1,000 t of target species (Table 4). All the respondents stated that Soviet vessels did not discard small tunas, but it is assumed that small species, such as frigate and bullet tunas, and small specimens (less than 1 kg) of bigeye, and the two target species, potentially could be discarded due to non-marketable size, as commonly practiced among other fleets in this fishery (Hall, 1998; Hall & Williams, 2000). Applying this assumption and the size frequencies of three tuna species in the catches, potential discards of target species and tuna bycatch were estimated.

Table 3. Utilization/discarding of bycatch by Soviet purse seine vessel crews

Group of species Utilization or discarding practice

Sharks Fins for selling, carcass always discardedMantas, mobulas Not utilized, always discardedBillfishes For crew consumption, never discardedWahoo Rarely utilized, mostly discardedDolphinfish Rarely utilized, mostly discardedRainbow runner Not utilized, always discardedBarracuda Not utilized, always discardedFlotsam community (generally triggerfishes, Blue sea chub (K. cinerascens) often used formackerel scad, other small fishes) crew consumption, other species always discarded Sea turtles Not utilized, always discarded

Table 4. Estimates of nominal discards, potential discards and total discards in Soviet purse seine FAD tuna fisheries in North Equatorial Area (t)

Per set Per 1,000 t of target species

Group of species Nominal Potential Total % Nominal Potential Total % discards discards discards discards discards discards

Skipjack - 0.136 0.136 12.9 - 6.9 6.9 13.0Yellowfin - 0.023 0.023 2.2 - 1.2 1.2 2.3Bigeye - 0.002 0.002 0.2 - 0.1 0.1 0.2Frigate and bullet tunas - 0.001 0.001 0.1 - 0.1 0.1 0.2Sharks 0.246 - 0.246 23.4 12.4 - 12.4 23.3Mantas, mobulas 0.002 - 0.002 0.2 0.1 - 0.1 0.2Wahoo 0.041 - 0.041 3.9 2.0 - 2.0 3.8Rainbow runner 0.215 - 0.215 20.4 10.8 - 10.8 20.3Dolphinfish 0.169 - 0.169 16.0 8.5 - 8.5 16.0Barracuda 0.003 - 0.003 0.3 0.1 - 0.1 0.2Flotsam community .215 - 0.215 20.4 10.9 - 10.9 20.5Sea turtles + - + + 0.1 - 0.1 0.2Total 0.891 0.162 1.053 100.0 44.9 8.3 53.2 100.0

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A length-weight relationship shows that yellowfin of 37 cm FL and less had weight less than 1 kg. For skipjack and bigeye, specimens of 38 cm FL weights are below 1 kg. No reliable size frequencies were obtained for albacore and therefore discards were not estimated. It should be noted however that albacore is usually kept on board and that as albacore make up only 0.05% of the total sampled catch in the area the potential discards are very low. All frigate and bullet tunas are considered as potential discards due to their small size and low market demand. Skipjack of 38 cm FL and smaller, make up 0.89% of this species’ catch in weight while for yellowfin and bigeye tuna, these proportion are 0.50% and 0.15% respectively, thus 0.136 t of skipjack, 0.023 t of yellowfin, 0.002 t of bigeye, and 0.001 t of frigate and bullet tunas are discarded per positive set. From estimates of potential discards and their relative values per set, indices were calculated, which allowed an assessment of the overall level of target species discarded from the fishery. Volume of discards per 1,000 t of target species is the most commonly used measure for such estimates (Hall, 1998; Romanov, 2002). In addition to the variables discussed above, affecting discarding practice (namely size, state of fish, crew preferences, etc.), which were assumed to be constant, the volume of discards for both target species will be related to the total catch and the relative percentage in the catch. During this study, the skipjack:yellowfin ratio in the sampled catch is 3.292, i.e. every 1,000 t of target species catch produced 767.2 t of skipjack and 232.8 t yellowfin. Based on this relative catch composition and discarding rate, potential discards per 1,000 t of target species were calculated as 6.9 t of skipjack and 1.2 t yellowfin tuna. Potential discard rates of other tuna were estimated as equal to 0.1 t of bigeye and 0.1 t of aggregated frigate and bullet tunas (Table 4). The remainder of the estimated total discards other species, are equal to 0.162 t per set or 8.3 t per 1,000 t of target species (Table 4). The total level of discards, both nominal and potential, from the FAD fishery during its high season is equal to 5.1% of total catch. By comparison, the potential discards from sets on other types of schools equal 0.5% of the catch for free schools and 1.8% for whale-associated

schools (Romanov, 2001). Consequently, the FAD fishing practice has a higher potential impact on species affected. However, the estimated bycatch and discards are relatively low in comparison with other types of fisheries in the world’s oceans. The average level of discard exceeds 60% in shrimp trawl fisheries and is 20-30% of the catch for tuna longline, dredge and trap/pot fisheries (Kelleher, 2005). Analysis of world fisheries demonstrate that the upper level of discards recorded usually exceeds the 50% limit in shrimp trawling, demersal and midwater fish trawling, trap/pot, dredge, demersal longlining and gillnetting (Kelleher, 2005). Estimates of discards in the tuna purse seine fisheries lie within 0.4-10% (Kelleher, 2005) which is similar to the present study. Despite relatively low level of bycatch and discards in the pelagic tuna purse seine fisheries there is still insufficient information on the impact of these fishing operations. In particular the impact of FAD fishing on vulnerable species such as turtles and cartilaginous fish, which mortality was documented both here and in other Indian Ocean studies (Chanrachkij & Loog-om, 2007; Delgado de Molina et al., 2007). The mortality of protected and/or vulnerable species caused by wide adoption of FAD fishing techniques should be closely monitored through observer programs currently in place with major Indian Ocean purse seine fleets. The FAD fishing practice may pose a high risk of over-exploitation of pelagic sharks due to their associative behaviour and specific life history traits which make them more vulnerable. Although sea turtle bycatch was low during early years of this fishery, recent developments suggest a high risk for endangered turtles related mostly with FAD structures such as ghost fishing (Chanrachkij & Loog-om, 2007; Delgado de Molina et al., 2007). FAD fishing mortality should be carefully assessed as to its risk to such populations and management recommendations and mitigation measures to eliminate or reduce risks should be put in place. Acknowledgements: I express my gratitude to all observers involved in sampling and all the vessel crew for the assistance to observers in sampling. Thanks to Alain Fonteneau (IRD) for his idea and suggestion to develop this analysis for IOTC working party on tropical tunas; this

Bycatch & discards in soviet purse seine tuna fisheries on fad 173

paper is direct result of his request. The author is grateful to V. V. Gerasimchuk, L. K. Pshenichnov, B. G. Trotsenko (YugNIRO) for useful comments during development of the early version of this manuscript. Comments of anonymous reviewer improved clarity of the manuscript and gives also important food for thoughts on non-uniform bycatch terminology used in various studies.

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