Bycatch in the tuna purse-seine fisheries of the western Indian Ocean

Bycatch in the tuna purse-seine fisheries of the western Indian Ocean – Statistical Data Included

Evgeny V. Romanov

Abstract–Bycatch taken by the tuna purse-seine fishery from the Indian Ocean pelagic ecosystem was estimated from data collected by scientific observers aboard Soviet purse seiners in the western Indian Ocean (WIO) during 1986-92. A total of 494 sets on free-swimming schools, whale-shark-associated schools, whale-associated schools, and log-associated schools were analyzed. More than 40 fish species and other marine animals were recorded. Among them only two species, yellowfin and skipjack tunas, were target species. Average levels of bycatch were 0.518 metric tons (t) per set, and 27.1 t per 1000 t of target species. The total annual purse-seine catch of yellowfin and skipjack tunas by principal fishing nations in the WIO during 1985-94 was 118,000-277,000 t. Nonrecorded annual bycatch for this period was estimated at 944-2270 t of pelagic oceanic sharks, 720-1877 t of rainbow runners, 705-1836 t of dolphinfishes, 507-1322 t of triggerfishes, 113-294 t of wahoo, 104-251 t of billfishes, 53-112 t of mobulas and mantas, 35-89 t of mackerel scad, 9-24 t of barracudas, and 67-174 t of other fishes. In addition, turtle bycatch and whale mortalities may have occurred. Because the bycatches were not recorded by some purse-seine vessels, it was not possible to assess the full impact of the fisheries on the pelagic ecosystem of the Indian Ocean. The first step to solving this problem is for the Indian Ocean Tuna Commission to establish a program in which scientific observers are placed on board tuna purse-seine and longline vessels fishing in the WIO.


One of the most important requirements of the UN Convention on the Law of the Sea of 1982, which determines strategies for exploitation of marine living resources (Article 119, b), is to take into account the impact of fisheries on “… species associated with or dependent upon harvested species with a view to maintaining or restoring populations of such associated or dependent species above levels at which their reproduction may become seriously threatened …” (United Nations, 1983). Estimating the magnitude of bycatch is one of the first steps to determine the impact of fisheries on associated species.

Tuna purse-seine fisheries probably apply the most intensive direct human impact on the tropical epipelagic ecosystems in all oceans. Because of the worldwide scale of purse-seine fisheries, an assessment of their impact on associated and dependent species is essential.

Two tunas, yellowfin Thunnus albacares (Bonnaterre, 1788) and skipjack Katsuwonus pelamis (Linnaeus, 1758), are the target species of most purse-seine fisheries. In this study bycatch is defined as the fraction of the catch that consists of nontarget species (including other species of tuna) that are encircled by the fishing gear and are unable to escape by themselves. Bycatch of associated and nonassociated species during purse-seine fishing for tropical tunas may be rather high, and generally depends on fishing tactics.

The species composition of bycatch in purse-seine fisheries depends on the structure, behavior, and spatial organization of surface multispecies aggregations. Schools of different tuna species and other pelagic fishes, marine mammals, and other marine animals have aggregated distributions. From our observations and in the opinion of other researchers (Au and Perryman, 1985; Au and Pitman, 1986; Au, 1991; Cort, 1992), marine birds are also an integral component of the majority of these multispecies groups.

The tunas, as a rule, prevail by biomass and abundance in such groups. Tuna schools are traditionally classified by the visually distinctive part of the group or by whether they associate with floating objects or marine mammals (Scott, 1969; Petit and Stretta, 1989). “Free-swimming schools” may include associations between different species of tuna. For each type of school, its various components occur in different ratios.

Some epipelagic species that occur in the purse-seine bycatches are not members of multispecies aggregations. They, instead, may comprise members of the flotsam community or are tuna forage. Several associated components, such as whales and birds, usually escape or avoid the nets and do not become bycatch. Therefore, the composition of the catch often does not represent the actual species composition of the multispecies associations.

Assessments of bycatches have been made for the eastern Pacific Ocean purse-seine tuna fishery (Joseph, 1994; Garcia and Hall, 1995; Hall, 1996, 1998; Anonymous, 1997, 1998, 1999), where the bycatch problem attracted attention because of dolphin mortality during sets on dolphin-associated tuna schools. The economic, political, and ecological implications of this problem produced wide international attention (Charat-Levy, 1991; Joseph, 1991, 1994; Hall, 1998). Bycatch estimates for the western Pacific purse-seine tuna fisheries have been published also (Bailey et al., 1996).

In the western Indian Ocean (WIO), tuna-dolphin associations are well known in coastal pelagic zones, e.g. Gulf of Aden (Demidov (1)) and Sri-Lanka (de Silva and Boniface (2)). They are often used in small-scale troll and pole-and-line fisheries for locating yellowfin tuna. In offshore regions of the WIO tuna-dolphin associations are rare, purse seining for them is not practiced, and there is no dolphin bycatch problem. Perhaps for this reason, the magnitude of bycatch in the WIO is unknown, except for recent information on species composition (Santana et al., 1998). Bycatches are not recorded for tuna seiners operating in the WIO, except bycatches of nontarget tuna species. This paper represents a first attempt to estimate catches of associated species by tuna purse seiners in the WIO, based on scarce information collected by scientific observers.

Materials and methods

Bycatch assessments were based on data collected by YugNIRO scientific observers aboard Soviet (since 1992–Russian) tuna purse seiners in the WIO, during 1987, and 1990-91. The vessels were the “Rodina” type. (3) In addition, observer data collected in the same area aboard sisterships by AtlantNIRO (4) and “Zaprybpromrazvedka” (5) during 1986-90 and data by TINRO (6) and TURNIF (7) during 1990 and 1992 were used. The fishing vessels all used purse seines

of 1800 m in length, 250-280 m in depth, and 90-100 mm mesh size in the bunt.

The principal goal of the observer sampling program was an estimation of the species composition of catches in this fisheries, biological analysis of the principal species, and estimates of the length and weight compositions of these principal species in the catches. The observers were placed on board opportunistically (i.e. if a vessel had a free sleeping bed and if there was available funding), without a sampling scheme and without preference to any vessel type. Thus, the sampling could be considered as random. Two other types of Soviet fishing vessels, “Tibiya” (8) and “Kauri,” (9) which took part in the Indian Ocean fisheries during 1985-87 and since 1991 (under the Liberian flag), were not sampled. In this study coverage rate was estimated as percentage of sampled catch to total catch.

The observers recorded the results of each set. The type of school, according to Scott (1969) and Petit and Stretta (1989), of each set was recorded. I considered sets for which an observer recorded catch in any quantity as positive sets. The average bycatch level was estimated for all positive sets.

For the positive sets, species composition, total weights, and numbers of each species in the catch were recorded. In the vessels of the “Rodina” type, the retained catch was frozen and stored separately. The retained catch was weighed after freezing while being moved to the ship’s holds. In nine cases, the weight of some of the catch was estimated by the ship masters because the holds were overloaded and some catch was stored in the freezers till landing. Therfore estimates of retained catch are presented in this study as frozen weights rather than wet weights. The bycatch was estimated as wet weight. Only bycatch taken on board was sampled. The sets when bycatch was not taken onboard but discarded alive (usually with negligible target species catch) and malfunction sets, which do not produce any catch, were not analyzed in this study. Large species, sharks and billfishes generally, were weighed and counted. The weights of specimens heavier than 200 kg (i.e. Mobulidae) were estimated. When the bycatch was more than 200-300 kg, species composition and weight were estimated by using representative samples.

Sometimes the observer recorded the bycatch in numbers. In these rare cases, the total weights of the fishes were estimated from the average weights of these species in previous catches.

The observers had free access to every fish in the catch. Nevertheless, some observers had difficulties identifying some billfishes, sharks, and Mobulidae species. Therefore, I pooled the records with doubtful species identification into these three groups for my analysis. These are marked by “?” in the tables.

The data were grouped and analyzed by free-swimming schools (including associations between schools of different species of tuna) and associated schools. The latter included whale-associated schools and log-associated schools (associated with floating objects).

Schools caught in the area of seamounts and shoals–at the peaks of the Equator Seamount and at Saya-de-Malha bank–were considered free-swimming schools. Some observers did not record the type of floating objects that were set on; therefore the sets on natural floating objects (50% to 90% of the log sets sampled) and on fish aggregation devices (FADs) (10-50%) were grouped. Several log sets were made in areas with surface evidence of water masses or current interactions (rips). A set that could not be clearly identified as to set type was made in such an area and was treated as a log set because of the species composition of the catch and the occurrence of small scattered debris in the rips.

Because tuna purse-seine fishing in the WIO is clearly seasonal (monsoons governing fishing techniques and operations), the data were analyzed by season. I followed Romanov’s (1982) seasonal divisions, in accordance with long-term average seasonal variations in the monsoon atmospheric circulation for the WIO. The winter season (northeastern monsoon) lasts from December to March, the spring intermonsoon period falls during April and May, the summer (southwestern monsoon) lasts from June to August, and the autumn intermonsoon period lasts from September through November. The wind regime determines the onset and duration of the hydrological seasons, which do not quite coincide with seasons of atmospheric circulation owing to a considerable time lag of the processes occurring in the ocean. However, the wind regime is instrumental in determining the tactics of purse seining for tuna; therefore I used seasonal strata based on atmospheric rather than on hydrological processes.

The spatial and temporal distribution of catch and effort for the Soviet tuna purse-seine fishery in the Indian Ocean was determined from data in the YugNIRO database, a collection of daily radio reports from vessels fishing in the area from 1983 until the mid-1990s. (10) The catches reported by the author’s estimates varied by 96-99% during 1985-91, decreasing to 71% in 1992. This study did not take into account reflagging of some Soviet (from 1992–Russian) vessels with the Liberian flag, and the vessels’ nationality was defined in this study by the location of their shipowners. Analysis of fleet activity and extrapolations of results were made on the assumption that the operations and procedures on vessels that did not carry observers did not differ from the operations and procedures on vessels with an observer aboard; similarly it was assumed that the species composition of the catch from these vessels did not differ.

Some of the bycatch was retained on board the fishing vessels. Unused bycatch was discarded in the ocean. The observers usually did not record the levels of discards, and it was not possible to assess quantitatively the discards of tuna and associated species.

Average values are presented as arithmetic means, plus or minus 95% confidence intervals for estimated values. Estimates of unrecorded bycatches for all fishes, except tunas, are provided in numbers and metric tons per positive set and per 1000 t of target species.


Primary data and adequacy of samples

A total of 494 purse-seine sets were sampled and 377 positive sets were analyzed. The total catch in the sets that were sampled amounted to 7713 t. The distribution of sets sampled by years, seasons, and the types of schools is given in Tables 1 and 2. The catch sampled by type of school is presented in Table 3.

The observer coverage rate varied from 0% (no observers at sea) to 75% and averaged 14% during 1986-92. During the periods when observers were on board, the coverage rate averaged 30% and varied from 5% to 75%. The spatial distribution of sampled sets agreed quite well with the distribution of the total fishing effort of the Soviet fleet in the WIO (Fig. 1). Sampled sets were distributed throughout the region of the principal international tuna purse-seine fishing activity in the WIO (Ardill (11)). Thus, I believe that the observers’ data are representative of the catch by the Soviet purse-seine fleet in terms of sample size and geographical distribution of the sets.


The data series available for analysis was a combination of samples different in size, obtained in different years and seasons (Tables 1-3). Therefore the results of all analyses may have been subject to interannual variability (it is impossible to evaluate the effect of interannual variability from the data available), and the estimated average annual figures may have been subject to seasonal variability as well, on account of the unequal size of samples taken in different seasons.

Because the daily radio reports, which formed the basis of the database, did not include information on sets by school type, it was not possible to directly extrapolate the observers’ data to the Soviet fleet separately by school types. However, I believe that the observers’ data correctly reflect the seasonal ratio of sets on different school types.

To assess the possibility of using annual averages of the available samples and extrapolating them to the whole catch of the Soviet fleet, the author compared the seasonal magnitudes of total fishing effort (fishing days, sets) and catches reported by the Soviet fishing fleet (from the database) with the seasonal magnitudes of effort (number of sets) and catch (t) sampled by the observers (Fig. 2, A and B). The seasonal distribution of the sampled catch followed the same pattern as the long-term average seasonal distribution of the catch by the fleet (Fig. 2B). The effort did not fully agree with seasonal increase in the fishing effort of Soviet vessels during the autumn season (Fig. 2A), which may have resulted in a slight underestimation of average annual values of bycatch from log-associated schools. I did not attempt, however, to take this factor into consideration.


The average CPUE (e.g. total catch per set) in the purseseine tuna fisheries varies greatly by type of vessel. I did not find a strong correlation between the bycatch per set and the total catch, or catch of target species in the same set. Level of bycatch generally depends on the type of association and the total fishing effort directed at this type of association.

Species composition and catch by school types

A total of 50 species (or higher taxa) of fishes and other marine animals were recorded in the catch of the fishing vessels (Table 4).

Free-swimming schools

Free-swimming schools are the predominant type of surface schools in the WIO. Such schools occurred in the area all year round (Table 2, Fig. 2, A and B). Soviet purse seiners set on free-swimming schools generally south of the equator (including Mozambique Channel)(Fig. 1B). Yellowfin, skipjack, and bigeye (Thunnus obesus) tunas were the principal components of free-swimming schools, comprising 80%, 15%, and 4%, respectively (Table 5). Monospecific (nonassociated) tuna schools, consisting completely of yellowfin or skipjack tuna, were found to occur in 47% of free-school sets. Multispecies free schools were observed in 53% of all free-school sets and generally consisted of two target species and bycatch. Bycatch occurred in 45% of the free-school sets, and nontuna bycatch in 22%.

A total of 19 species (or higher taxa of fishes) were observed in catches on free schools (Table 4). Some species were considered to be tuna prey (e.g. Exocoetidae) and some to be accidental bycatch (e.g. Gempylus serpens, Canthidermis maculatus, Diodon spp.). Nontuna bycatch in this type of association was on average 0.060 [+ or -] 0.031 t per positive set (Fig. 3) and 3.403 [+ or -] 2.770 t per 1000 t of target species (Fig. 4). The bulk of this bycatch was sharks of the genus Carcharhinus (0.023 t/1.296 t), rays of the Mobulidae family (0.020 t/1.128 t), marlins of the genus Makaira, and sailfish (Istiophorus platypterus) (0.016 t/0.895 t) (Tables 4 and 6). In the present study, bycatches are presented in parentheses as t per positive set/t per 1000 t of target species.


Whale-shark–associated schools

Two schools associated with whale sharks were sampled only in the winter season (Table 2, Fig. 2, A and B) south of the equator (Fig. 1C). In these sets, the bycatch consisted of the shark itself and a small quantity of albacore (Thunnus alalunga) (Table 4). This small sample size prohibited reliable bycatch estimates for whale-shark–associated schools and inferences of the species compositions of such associations.

Whale-associated schools

In the observers’ logbooks, among the 45 sets on whale-associated tuna schools, 13 sets were made on schools associated with sei whales (Balaenoptera borealis Lesson, 1828) and one set on a school associated with a fin whale (B. physalus (Linnaeus, 1758)). (12) The remaining sets were made on unidentified baleen whales. According to verbal reports by some observers, (13) tuna schools associated with Bryde’s whale (B. edeni Anderson, 1878), Minke whales (B. acutorostrata Lacepede, 1804), and pygmy blue whales (B. musculus brevicauda Linnaeus, 1758) were also observed in the WIO. From personal observations and those of the observers, sperm whales (Physeter catodon Linnaeus, 1758) were found often in the areas of the tuna purseseine fishery; tunas, on the other hand, were not observed to associate with sperm whales. According to observations made during setting and searching operations, whales associated with tunas generally were found in groups of up to 8 individuals, more often in groups of 2-3 whales.

Whale-associated schools were most often observed from January to April. A whale-associated school was observed in July north of the equator (Table 2, Figs. 1C, 2, A and B). Schools of this type were distributed mainly south of the equator at latitudes 4-9 [degrees] S. Skipjack, yellowfin, and bigeye tunas dominated in whale-associated schools–59%, 32%, and 6%, respectively (Table 5). The percentage of each species in different sets varied greatly: 0-100% for skipjack, 0-100% for yellowfin, and 0-74% for bigeye tuna. Associations consisting of one tuna species and a whale were encountered in eight cases (22%). Bycatch in whale-associated schools was found in 68% of the sets, and nontuna bycatch in 43% of the sets.

During sets on whale-associated schools, the fishermen keep the whale(s) inside the purse seine as long as possible. Whales often remain in the net until the end of pursing and then escape from the purse seine by either diving under the purse line, by ramming through the net wall, or by sinking the corkline (a rare occurrence).

Observers registered a single case of entanglement in the net and subsequent death of a young sei whale about 10 m in length and about 12 t in weight. The dead animal was taken up on the vessel’s deck, released from the purse seine, and discarded into the ocean. It is not possible to assess the frequency and probability of whale mortality by the purse-seine fishery in the WIO.

There were 17 species (or groups) of marine animals identified in the catches of whale-associated schools (Table 4). Salps, ctenophores, and batfish (Platax spp.) were considered accidental bycatch, whereas long-finned fathead (Cubiceps pauciradiatus) was a prey item of both tunas and whales. Nontuna bycatch in this type of association averaged 0.306 [+ or -] 0.344 t for a positive set or 10.891 [+ or -] 15.787 t per 1000 t of target species (Figs. 3 and 4). Sharks of the genus Carcharhinus and Isurus made up the bulk of the bycatch in whale-associated school sets (0.289 t/10.302 t) (Tables 4 and 6).

Log-associated schools

Log-associated schools are one of the predominant school types found in the WIO all year round (Table 2, Fig. 2, A and B). Sets on log-associated schools were made throughout the sampling area as far south as 15 [degrees] S (Fig. 1D). In log-associated schools the bulk of the catch were skipjack, yellowfin, and bigeye tunas–67%, 24%, and 3%, respectively (Table 5). Log-associated schools in all cases consisted of several fish species. Bycatch was found in 93% of the sets, and nontuna bycatch in 87%. The absence of bycatch was rare, observed only during successive sets on the same floating object.

The species composition associated with floating objects was the most diverse of any set type and included 45 species (or higher taxa of fishes) (Table 4). Nontuna bycatch was at its highest in log-associated sets, as much as 0.780 [+ or -] 0.144 t per positive set or 41.337 [+ or -] 14.281 t per 1000 t of target species (Figs. 3 and 4). The bulk of the bycatch in sets on log-associated schools was made up of rainbow runner, Elagatis bipinnulata (0.195 t/10.314 t), common dolphin-fish, Coryphaena hippurus (0.191 t/10.098 t), triggerfish of the genus Canthidermis (0.137 t/7.277 t), sharks of the genus Carcharhinus (0.175 t/9.288 t), wahoo, Acanthocybium solandri (0.031 t/1.621 t), billfishes of the genera Makaira and Tetrapturus (0.019 t/1.008 t), and mackerel scad, Decapterus macarellus (0.0093 t/0.491 kg). One capture of a sea turtle (unknown species) was recorded (Tables 4 and 6).

All types of schools

Considering all school types in the aggregate, skipjack, yellowfin, and bigeye tuna prevailed in the catch –51%, 42%, and 4% by weight, respectively (Table 5). Albacore represented a mere 0.2%, frigate tuna 0.9%, and kawakawa, Euthynnus affinis, less than 0.1%. Nontuna bycatch accounted for less than 3% of the catch.

On the average, there was 0.518 [+ or -] 0.099 t of nontuna bycatch caught per positive set, or 27.127 [+ or -] 8.869 t per 1000 t of target species (Fig. 3). Bycatch levels by species (groups) are given in Table 6.


The lowest fish bycatch in the WIO tuna purse-seine fishery was taken from free schools (mainly carcharhinid sharks and Mobulidae rays) (Figs. 3 and 4, Tables 4 and 6). Bycatch of fishes was highest and most diverse from catches on log-associated schools. Rainbow runner, common dolphinfish, triggerfish, carcharhinid sharks, wahoo, billfishes, and mackerel scad were predominant. Whale-associated schools were characterized by an intermediate level of bycatch (mainly carcharhinid and lamnid sharks) (Figs. 3 and 4, Tables 4 and 6).

It is interesting to compare the bycatch rates obtained in this study with those published for other regions. The principal bycatch fishes in the Pacific (Bailey et al., 1996; Hall, 1996, 1998; Anonymous, 1997) are the same as those presented here. Bycatch levels are known to vary considerably by year, area, fleet (Bailey et al., 1996; Hall, 1996; Anon., 1997), and school type; this variability hampered direct comparisons of the results from the present study with those from published data. However, for the purpose of comparison, I pooled my estimates by groups in accordance with the published data (Bailey et al., 1996; Hall, 1996, 1998; Anonymous, 1997). Bycatch levels per set and per 1000 t of target species for various regions of the Pacific and my estimates for the Indian Ocean are on the same order of magnitude for most groups in similar types of associations (Figs. 5 and 6).


I also attempted to estimate the unrecorded bycatch by the purse-seine fleets of the principal fishing nations of the WIO by a comparison of fishing tactics. The Soviet fleet in the WIO made an equal proportion of sets on free-swimming schools and on log-associated schools during the year (Table 2). Seasonally they switched effort from sets on free-swimming schools to those on log-associated schools (Fig. 7, A and B). The fishing practices of French and Spanish tuna seiners showed similar seasonality until the mid-1990s (Anonymous; (14,15,16) Pianet; (17,18) Moron (19)).


The fishing tactics of the Japanese (Hallier (20) Okamoto and Miyabe (21)) and Mauritian (Norungee et al.; (22) Norungee and Lim Shung (23)) purse-seine fleets differed considerably from that described above. Japanese and Mauritian vessels made sets on log-associated schools all year round, with single instances of sets on other schools types.

Only two school types (log schools and free schools) have been described by Hallier; (20) Hallier; (24) Parajua Aranda; (25) Anonymous; (14,15,16) Pianet; (17,18) Hastings and Domingue; (26) and Moron (19) for the tuna purse-seine fishery in the Indian Ocean. Free schools in these analyses included all types of associations with marine animals. The proportion of sets of the French fleet on other types of schools and on resulting catches is not known. Cort (1992) presented such data for Spanish vessels, based on fishing logbooks. Therefore, I used the observers data of the Seychelles Fishing Authority (SFA) (Cort, 1992) for the vessels of France, Spain, Japan, and USSR to assess these values in the WIO. The percentage of sets on whale-associated schools varied from 1.7% to 8.8% in 1986-90, the percentage among positive sets was from 1.2% to 9.1%, and the catch from such schools was 1.6% to 7.8% (cited from Cort, 1992). These values are slightly lower than the observer data I report in the present study (9%, 10%, and 14%, respectively), which is explained by the fact that the SFA data included Japanese vessels known to fish on log-associated schools only. Nevertheless, the SFA values and those from our observers were on the same order of magnitude. Proceeding from this, I estimated the ratio of sets on various school types and the magnitude and species composition of bycatch by the French and Spanish vessels. These values were close to those for the Soviet fleet employing similar fishing tactics. (27)

Thus, the average bycatch estimates presented in this study can be extrapolated for this period to the total WIO purse-seine catch of principal fishing nations targeting all types of schools. (28) Estimates of bycatch from log-associated schools, I believe, can be extended, with some caution, to the pooled purse-seine catch of Japan and Mauritius in the WIO.

The annual purse-seine catches of yellowfin and skipjack tunas by fleets targeting all types of schools (France, Spain, and USSR) (29) in the WIO ranged between 115,000 and 242,000 t in 1985-94 (Anonymous (30)). Japanese and Mauritian catches varied from 3000 to about 51,000 t. Based on these values, the estimated bycatch was 3257 to 8246 t of various fishes during the same period (Table 7). These fishes could serve as food for the coastal countries of the area. Estimated bycatch in numbers is presented in Table 8.

Turtle bycatch and whale mortality in purse seines are also possible in the WIO, but the probability of the latter is very low. No instances of whale mortality have been recorded earlier for tuna purse-seine fisheries in other areas (Northridge, 1984, 1991a, 1991b; Medina-Gaertner and Gaertner, 1991; Santana et al., 1991; Cort, 1992; Cayre et al., 1993; Bailey et al., 1996). No avian mortality by the Soviet tuna purse-seine fishery has been noted by observers. A similar fact was reported for the western Pacific (Bailey et al., 1996).

Target fishing for rainbow runner, dolphinfish, triggerfishes, wahoo, mackerel scad, and barracuda is not conducted in the WIO, and these fish are taken only as bycatch. Their bycatch levels, estimated in this study, do not seem to endanger the populations of these species.

Estimated bycatch of billfishes (104-251 t annually) was less than 1% of the total catch for these species (14,000-33,000 t during 1985-94) in the WIO (Anonymous (30)). The bycatch by the purse-seine fishery was unlikely to substantially affect the billfish stocks.

Many pelagic sharks are taken as bycatch by the longline, trawl, coastal driftnet, and other fisheries, but are not recorded. The total shark catch by all fisheries may be considerable. Many shark species are characterized by low abundance, low fecundity, long life span, and consequently, by high vulnerability to overfishing. Underestimation of the removal through fisheries of a number of pelagic shark species, and the impact of the fisheries on their populations, may lead to a reduction in their abundance to critical levels, diminishing the biodiversity of the pelagic ecosystem of the Indian Ocean.

Some part of the bycatch is released into the ocean alive, although subsequent survival rates are unknown. The lack of bycatch and discard records and estimates of survival rates of discarded animals prevents assessment of the impact of the fishery on the Indian Ocean pelagic ecosystem.

Fishing tactics in the WIO have changed considerably by all principal purse seine fleets toward the extensive use of FADs in recent years (generally from 1995). The majority of Japanese vessels have left the area and have moved to the eastern Indian Ocean. Therefore estimates presented here for total WIO purse-seine fisheries are applicable for a limited time span only (pre-1995). Recent development of the WIO fisheries warrants further investigation of bycatches through extensive observer sampling by time–area strata.

Establishing a scientific program by the Indian Ocean Tuna Commission to monitor the principal tuna fisheries in the region, by placing international scientific observers on purse-seine and longline vessels, might be the first step toward a more accurate assessment of the impact of bycatches on the epipelagic ecosystem of the Indian Ocean. This program might also lead to developing technical and management measures to reduce the bycatches or to use them.

The solution to the bycatch problem should take two directions: 1) an effort to reduce or eliminate bycatches of undesired species; or 2) to use bycatch animals to make them target species. The former involves developing gear modifications or changes in fishing tactics. The latter involves management regulation of the fishery so that bycatch species are treated in the same way as other target species.

Table 1

Numbers of sets sampled by year. Positive sets are sets

in which an observer registered catch in any quantity.

1986 1987 1988 1989

Total number of sets 115 102 30 41

Number of positive sets 68 62 28 41

Percentage of sets with catch 59% 63% 93% 100%

1990 1991 1992 Total

Total number of sets 113 54 39 494

Number of positive sets 92 53 33 377

Percentage of sets with catch 81% 98% 85% 76%

Table 2

Numbers of sets sampled by season and type of school.



Type of school Winter Spring Summer Autumn positive

Free-swimming 136 35 27 8 206/121

Whale-shark-associated 2 0 0 0 2/2

Whale-associated 23 21 1 0 45/37

Log-associated 46 50 80 65 241/217

Total 207 106 108 73 494/377

Table 3

Sampled catch (metric tons) by season and type of school.


Type of school Winter Spring Summer Autumn Total

Free-swimming 1884 249 73 24 2230

Whale-shark-associated 28 0 0 0 28

Whale-associated 584 467 4 0 1055

Log-associated 925 785 1156 1534 4400

Total 3421 1501 1213 1558 7713

Table 4

Species composition of tuna purse-seine catches in the western

Indian Ocean. “?” denotes doubtful, in the author’s opinion,

species identification by observer.

School type

Free- Whale- Log-

Family and species swimming associated associated



Dasyatis spp. + +


Manta birostris +? +? +

(Donndorff, 1798)

Mobula spp. + + +


Rhincodon typus Smith, 1828 (1)


Isurus oxyrinchus Rafinesque, +


Isurus spp. ?


Carcharhinus falciformis + + +

(Bibron, 1839)

C. longimanus (Poey, 1861) + + +

?C. obscurus (LeSueur, 1818) +? +?

Carcharhinus spp. ? ?


Sphyrna lewini +

(Griffith & Smith, 1834)

Sphyrna spp. +

Exocoetidae sp. +

Belonidae sp. +

Tylosurus crocodilus +

(Peron & LeSueur, 1821)


Lampris guttatus +

(Brunnich, 1788)


Sphyraena barracuda +

(Walbaum, 1792)

Sphyraena spp. +


Caranx spp. +

Decapterus macarellus Cuvier, +


Decapterus spp. +

Elagatis bipinnulata + +

(Quoy & Gaimard, 1824)

Seriola spp. + +

Naucrates ductor +

(Linnaeus, 1758)


Coryphaena hippurus Linnaeus, + +


Coryphaena spp. +


Kyphosus cinerascens +

(Forsskal, 1775)


Gempylus serpens Cuvier, 1829 +

Ruvettus pretiosus Cocco, 1829 +


Platax spp. + +


Scomberomorus commerson +

(Lacepede, 1800)

Scomberomorus spp. +


Acanthocybium solandri +

(Cuvier, 1831)

Auxis rochei (Risso, 1810) +

Auxis thazard (Lacepede, 1800) + + +

Euthynnus affinis +

(Cantor, 1849)

Katsuwonus pelamis + + +

(Linnaeus, 1758)

Thunnus alalunga + +

(Bonnaterre, 1788)

Thunnus albacares + + +

(Bonnaterre, 1788)

Thunnus obesus (Lowe, 1839) + + +


Istiophorus platypterus +

(Shaw & Nodder, 1792)

Makaira indica (Cuvier, 1832) + +

M. mazara (Jordan et Snyder, + +


Makaira spp. + +

Tetrapturus audax +

(Philippi, 1887)


Xiphias gladius +

(Linnaeus, 1758)


Cubiceps pauciradiatus Gunter, +



Canthidermis maculatus + +

(Bloch, 1786)


Aluterus monoceros +

(Linnaeus, 1758)

Aluterus spp. +


Diodon spp. + +?



Balaenoptera borealis Lesson, +


Salpae +

Ctenophora +

Chelonidea +

Number of species (taxa) 19 17 45

(1) Recorded in whale-shark-associated schools.

Table 5

Average tuna catch per positive set (t) by “Rodina”-type Soviet

vessels in the western Indian Ocean (total and by species). YFT =

yellowfin tuna, SKJ = skipjack tuna, BET = bigeye tuna, ALB =

albacore, FRI = frigate tuna, KAW = kawakawa. + = catch was

<0.001 t.


Type of school Total YFT

Free-swimming 18.4 [+ or -] 5.2 14.7 [+ or -] 4.9

Whale-associated 31.0 [+ or -] 9.3 9.8 [+ or -] 4.3

Log-associated 20.6 [+ or -] 3.2 4.9 [+ or -] 0.9

Total 20.6 [+ or -] 2.7 8.6 [+ or -] 1.8


Type of school SKJ BET

Free-swimming 2.8 [+ or -] 1.7 0.8 [+ or -] 1.0

Whale-associated 18.3 [+ or -] 8.5 2.0 [+ or -] 2.4

Log-associated 13.9 [+ or -] 2.7 0.6 [+ or -] 0.2

Total 10.5 [+ or -] 1.9 0.8 [+ or -] 0.4


Type of school ALB FRI

Free-swimming 0.03 [+ or -] 0.03 0.05 [+ or -] 0.06

Whale-associated — 0.2 [+ or -] 0.2

Log-associated 0.04 [+ or -] 0.04 0.3 [+ or -] 0.3

Total 0.03 [+ or -] 0.03 0.2 [+ or -] 0.2


Type of school KAW

Free-swimming —

Whale-associated —

Log-associated 0.001 [+ or -] 0.001

Total +

Table 6

Estimates of the bycatch (t) of various species (groups) of marine

animals by school type. The numerator is the average values per

a positive set, the denominator is the average values per 1000 t

of target species. + = catch was <0.001 t.

School type (1)

Species or group of species Free-swimming

Billfishes (Istiophoridae, Xiphiidae) 0.016/0.895

Wahoo (A. solandri) —

Sharks (Lamnidae, Carcharhinidae, Sphyrnidae) 0.023/1.296

Rainbow runner (E. bipinnulata) 0.001/0.054

Dolphinfishes (C. hippurus) +/0.027

Barracuda (S. barracuda) —

Triggerfishes (C. maculatus, Aluterus spp.) +/+

Mackerel scad (D. macarellus) —

Mantas, mobulas (Mobulidae) 0.020/1.128

Sea turtles —

Other bycatch +/0.002

Total { For positive set 0.060 [+ or -] 0.031

{ For 1000 t of target species 3.403 [+ or -] 2.770

School type (1)

Species or group of species Whale-associated

Billfishes (Istiophoridae, Xiphiidae) 0.006/0.218

Wahoo (A. solandri) —

Sharks (Lamnidae, Carcharhinidae, Sphyrnidae) 0.289/10.302

Rainbow runner (E. bipinnulata) —

Dolphinfishes (C. hippurus) 0.001/0.051

Barracuda (S. barracuda) —

Triggerfishes (C. maculatus, Aluterus spp.) —

Mackerel scad (D. macarellus) —

Mantas, mobulas (Mobulidae) 0.009/0.318

Sea turtles —

Other bycatch +/0.003

Total { For positive set 0.306 [+ or -] 0.344

{ For 1000 t of target species 10.891 [+ or -] 15.787

School type (1)

Species or group of species Log-associated

Billfishes (Istiophoridae, Xiphiidae) 0.019/1.008

Wahoo (A. solandri) 0.031/1.621

Sharks (Lamnidae, Carcharhinidae, Sphyrnidae) 0.175/9.288

Rainbow runner (E. bipinnulata) 0.195/10.314

Dolphinfishes (C. hippurus) 0.191/10.098

Barracuda (S. barracuda) 0.002/0.132

Triggerfishes (C. maculatus, Aluterus spp.) 0.137/7.277

Mackerel scad (D. macarellus) 0.0093/0.491

Mantas, mobulas (Mobulidae) 0.002/0.126

Sea turtles +/0.025

Other bycatch 0.018/0.958

Total { For positive set 0.780 [+ or -] 0.144

{ For 1000 t of target species 41.337 [+ or -] 14.281

School type (1)

Species or group of species All types of schools

Billfishes (Istiophoridae, Xiphiidae) 0.017/0.880

Wahoo (A. solandri) 0.018/0.934

Sharks (Lamnidae, Carcharhinidae, Sphyrnidae) 0.151/7.938

Rainbow runner (E. bipinnulata) 0.114/5.962

Dolphinfishes (C. hippurus) 0.111/5.836

Barracuda (S. barracuda) 0.001/0.076

Triggerfishes (C. maculatus, Aluterus spp.) 0.080/4.195

Mackerel scad (D. macarellus) 0.005/0.283

Mantas, mobulas (Mobulidae) 0.009/0.455

Sea turtles +/0.014

Other bycatch 0.011/0.553

Total { For positive set 0.518 [+ or -] 0.099

{ For 1000 t of target species 27.127 [+ or -] 8.869

(1) Because of the small sample size, estimates of bycatch for

whale-shark-associated schools are not presented in the Table.

Table 7

Bycatch estimates in tons in the western Indian Ocean purse-seine

fisheries during 1985-94. MIX = fleets targeted all types of

schools (France, Spain, USSR), LOG = fleets targeted log-associated

schools (Japan and Mauritius).

Species, a group

of species 1985 1986 1987 1988 1989

MIX 913 1047 1257 1674 1622

Pelagic oceanic sharks LOG 31 30 61 81 108

Total 944 1077 1318 1755 1730

MIX 686 786 944 1257 1218

Rainbow runners LOG 34 33 68 90 120

Total 720 819 1012 1347 1338

MIX 671 770 925 1231 1193

Dolphinfishes LOG 34 33 67 88 117

Total 705 803 992 1319 1310

MIX 483 554 665 885 857

Triggerfishes LOG 24 24 48 64 84

Total 507 578 713 949 941

MIX 108 123 148 197 191

Wahoo LOG 5 5 11 14 19

Total 113 128 159 211 210

MIX 101 116 139 185 180

Billfishes LOG 3 3 7 9 12

Total 104 119 146 194 192

MIX 52 60 72 96 93

Mobulas and mantas LOG <1 <1 1 1 1

Total 53 60 73 97 94

MIX 33 37 45 60 58

Mackerel scad LOG 2 2 3 4 6

Total 35 39 48 64 64

MIX 9 10 12 16 16

Barracudas LOG <1 <1 1 1 1

Total 9 11 13 17 17

MIX 64 73 88 117 113

Other fishes LOG 3 3 6 8 11

Total 67 76 94 125 124

MIX 3120 3576 4295 5718 5541

Total nontuna bycatch LOG 137 134 273 360 479

Total 3257 3710 4568 6078 6020

Species, a group

of species 1990 1991 1992 1993 1994

MIX 1503 1471 1793 1796 1925

Pelagic oceanic sharks LOG 180 278 477 451 143

Total 1683 1749 2270 2247 2068

MIX 1129 1105 1347 1349 1446

Rainbow runners LOG 199 309 530 500 159

Total 1328 1414 1877 1849 1605

MIX 1105 1082 1318 1320 1415

Dolphinfishes LOG 195 303 518 490 156

Total 1300 1385 1836 1810 1571

MIX 794 778 948 949 1017

Triggerfishes LOG 141 218 374 353 113

Total 935 996 1322 1302 1130

MIX 177 173 211 211 227

Wahoo LOG 31 49 83 79 25

Total 208 222 294 290 252

MIX 167 163 199 199 213

Billfishes LOG 20 30 52 49 16

Total 187 193 251 248 229

MIX 86 84 103 103 110

Mobulas and mantas LOG 2 4 6 6 2

Total 88 88 109 109 112

MIX 54 53 64 64 69

Mackerel scad LOG 10 15 25 24 8

Total 64 68 89 88 77

MIX 14 14 17 17 19

Barracudas LOG 3 4 7 6 2

Total 17 18 24 23 21

MIX 105 102 125 125 134

Other fishes LOG 18 29 49 47 15

Total 123 131 174 172 149

MIX 5134 5025 6125 6135 6574

Total nontuna bycatch LOG 799 1239 2121 2004 638

Total 5933 6264 8246 8139 7212

Table 8

Bycatch estimates in numbers in the western Indian Ocean purse-seine

fisheries during 1985-94. Codes are same as table 7. MIX = fleets

targeted all types of schools (France, Spain, USSR); LOG = fleets

targeted log-associated schools (Japan and Mauritius).

Species, a group

of species 1985 1986 1987

MIX 45,600 52,273 62,780

Pelagic oceanic sharks LOG 2161 2100 4280

Total 47,761 54,373 67,060

MIX 162,457 186,232 223,664

Rainbow runners LOG 8112 7883 16,065

Total 170,569 194,115 239,729

MIX 107,711 123,473 148,291

Dolphinfishes LOG 5373 5221 10,641

Total 113,084 128,694 158,932

MIX 621,823 712,823 856,096

Triggerfishes LOG 31,215 30,334 61,820

Total 653,038 743,156 917,916

MIX 17,444 19,996 24,016

Wahoo LOG 876 851 1734

Total 18,320 20,847 25,750

MIX 750 859 1032

Billfishes LOG 26 25 51

Total 776 884 1083

MIX 250 286 344

Mobulas and mantas LOG 3 2 5

Total 253 288 349

MIX 45,134 51,739 62,138

Mackerel scad LOG 2266 2202 4487

Total 47,340 53,941 66,625

MIX 1350 1547 1858

Barracudas LOG 68 66 134

Total 1418 1613 1992

Species, a group

of species 1988 1989 1990

MIX 83,581 80,993 75,052

Pelagic oceanic sharks LOG 5653 7531 12,546

Total 89,234 88,524 87,598

MIX 297,770 288,550 267,386

Rainbow runners LOG 21,218 28,267 47,090

Total 318,988 316,817 314,476

MIX 197,424 191,312 177,280

Dolphinfishes LOG 14,053 18,723 31,190

Total 211,477 210,035 208,470

MIX 1,139,747 1,104,458 1,023,450

Triggerfishes LOG 81,646 108,774 181,205

Total 1,221,393 1,213,232 1,204,655

MIX 31,973 30,983 28,710

Wahoo LOG 2290 3051 5083

Total 34,263 34,034 33,793

MIX 1374 1332 1234

Billfishes LOG 68 90 151

Total 1442 1422 1385

MIX 458 444 411

Mobulas and mantas LOG 7 9 15

Total 465 453 426

MIX 82,726 80,164 74,285

Mackerel scad LOG 5926 7895 13,153

Total 88,652 88,059 87,438

MIX 2474 2397 2221

Barracudas LOG 177 236 393

Total 2651 2633 2614

Species, a group

of species 1991 1992

MIX 73,455 89,529

Pelagic oceanic sharks LOG 19,456 33,320

Total 92,911 122,849

MIX 261,694 318,961

Rainbow runners LOG 73,029 125,065

Total 334,723 444,026

MIX 173,505 211,474

Dolphinfishes LOG 48,370 82,835

Total 221,875 294,309

MIX 1,001,661 1,220,857

Triggerfishes LOG 281,018 481,252

Total 1,282,679 1,702,109

MIX 28,099 34,248

Wahoo LOG 7883 13,501

Total 35,982 47,749

MIX 1208 1472

Billfishes LOG 233 400

Total 1441 1872

MIX 403 491

Mobulas and mantas LOG 23 39

Total 426 530

MIX 72,703 88,613

Mackerel scad LOG 20,398 34,931

Total 93,101 123,544

MIX 2174 2650

Barracudas LOG 610 1044

Total 2784 3694

Species, a group

of species 1993 1994

MIX 89,676 96,094

Pelagic oceanic sharks LOG 31,488 10,030

Total 121,164 106,124

MIX 319,485 342,351

Rainbow runners LOG 118,190 37,646

Total 437,675 379,997

MIX 211,821 226,982

Dolphinfishes LOG 78,282 24,934

Total 290,103 251,916

MIX 1,222,862 1,310,387

Triggerfishes LOG 454,799 144,863

Total 1,677,661 1,455,250

MIX 34,304 36,760

Wahoo LOG 12,759 4064

Total 47,063 40,824

MIX 1474 1580

Billfishes LOG 378 120

Total 1852 1700

MIX 491 527

Mobulas and mantas LOG 37 12

Total 528 539

MIX 88,758 95,111

Mackerel scad LOG 33,011 10,515

Total 121,769 105,626

MIX 2654 2844

Barracudas LOG 987 314

Total 3641 3158


I am grateful to AtlantNIRO scientists V. F. Bashmakov, G. A. Budylenko, V. Z. Gaikov, M. E. Grudtsev, to TINRO scientist K. A. Karyakin for their data made available to the author and to V. F. Bashmakov and G. A. Budylenko for their personal sampling efforts. I sincerely thank masters of the KUTF tuna seiners A. G. Burlyko, V. N. Volvach, A. A. Kiryanov for their assistance rendered to observers in sampling.

The author is grateful to V. F. Demidov, N. N. Kukharev, M. A. Pinchukov, L. K. Pshenichnov, S. T. Rebik, B. G. Trotsenko for useful discussions when preparing the manuscript and to two anonymous reviewers for their comments and suggestions.

The author wishes to thank I. V. Charova for translating the paper into English. Revisions and an edition of the paper by R. J. Olson (I-ATTC) and his corrections of English were extremely valuable.

(1) Demidov, V. F. 1998. Personal commun. Southern Scientific Research Institute of Marine Fisheries and Oceanography (YugNIRO), 2, Sverdlov St., 98300, Kerch, Crimea Ukraine.

(2) de Silva, J., and B. Boniface. 1991. The study of the handline fishery on the west coast of Sri Lanka with special reference to the use of dolphin for locating yellowfin tuna (Thunnus albacares). In Indo-Pacific Tuna Development and Management Programme (IPTP) Coll. Vol. Work. Doc TWS/90/18., Vol. 4, p. 314-324. Food and Agriculture Organization of the United Nations (FAO), Viale delle Terme di Caracalla, 00100, Rome, Italy.

(3) Length overall: 85 m; GRT (gross tonnage): 2634; carrying capacity: ~1600 [m.sup.3].

(4) AtlantNIRO–The Atlantic Scientific Research Institute of Marine Fisheries and Oceanography, 5 Dmitry Donskoi St., 236000 Kaliningrad, Russia.

(5) The Department of Searching and Scientific Research Fleet of the Western Basin “Zaprybpromrazvedka,” [5.sup.a] Dmitry Donskoi St., 236000 Kaliningrad, Russia.

(6) TINRO–The Pacific Scientific Research Institute of Marine Fisheries and Oceanography, 1 Shevchenko Alley, 690600 Vladivostok, Russia.

(7) TURNIF–The Pacific Department of Fish Searching and Scientific Research Fleet, 2 Pervogo Maya St., 690600 Vladivostok, Russia.

(8) Length overall: 55.5 m, GRT: 736, carrying capacity: ~361 [m.sup.3].

(9) Length overall: 79.8 m, GRT: 2100, carrying capacity: ~1200 [m.sup.3].

(10) Daily information on fishing activity of these vessels in the Indian Ocean in 1983-84 and since 1995 is not available.

(11) Ardill, J.D. 1995. Atlas of industrial tuna fisheries in the Indian Ocean (IPTP/95/AT/3). IPTP, Colombo, Sri Lanka, 138 p. FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(12) Species identification could be erroneous.

(13) Bashmakov, V. F. 1990. Personal commun. Atlantic Scientific Research Institute of Marine Fisheries and Oceanography (AtlantNIRO), 5 Dmitry Donskoy St., Kaliningrad, 236000, Russia.

(14) Anonymous. 1992. Report of the workshop on stock assessment of yellowfin tuna in the Indian Ocean, Colombo, Sri Lanka, 7-12 October 1991, 90 p. [IPTP/91/GEN/20.] FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(15) Anonymous. 1994a. Report of the expert consultation on Indian Ocean tunas, 5th session, Mahe, Seychelles, 4-8 October 1993, 32 p. [IPTP/94/GEN/22.] FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(16) Anonymous. 1994b. National report of Spain. In Proceedings of the expert consultation on Indian Ocean tunas, 4-8 October, 1993 (J. D. Ardill, ed.), p. 44-47. IPTP Coll. Vol. 8., TWS/93/1/14. FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(17) Pianet, R. 1994a. Purse seine fishery trends in the western Indian Ocean from data collected in Victoria (Seychelles), 1984-1992. In Proceedings of the expert consultation on Indian Ocean tunas, 4-8 October, 1993 (J. D. Ardill, ed.), p. 41-44. IPTP Coll. Vol. 8., TWS/93/1/13. FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(18) Pianet, R. 1994b. National report of France. In Proceedings of the expert consultation on Indian Ocean tunas, 4-8 October, 1993 (J.D. Ardill, ed.),p. 48-52. IPTP Coll. Vol. 8, TWS/93/1/16. FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(19) Moron, J. 1996. National report of Spain. In Proceedings of the expert consultation on Indian Ocean tunas, 6th session, Colombo, Sri Lanka, 25-29 September, 1995 (A. A. Anagnuzzi, K. A. Stobberup, N. J. Webb, eds.), p. 63-69. IPTP Coll. Vol. 9. FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(20) Hallier, J.-P. 1991. Tuna fishing on log associated schools in the Western Indian Ocean: an aggregation behaviour. In IPTP Coll. Vol. Work. Doc, Vol. 4, p. 325-342 [TWS/90/66.] FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(21) Okamoto, H., and N. Miyabe. 1996. Review of Japanese tuna fisheries in the Indian Ocean. In Proceedings of the expert consultation on Indian Ocean tunas, 6th session, Colombo, Sri Lanka, 25-29 September, 1995 (A. A. Anagnuzzi, K. A. Stobberup, N.J. Webb, eds.),p. 15-21. IPTP Coll. Vol. 9. FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(22) Norungee, D., A. Venkatasami, and C. Lim Shung. 1994. Catch and landing statistics of the Mauritian tuna fisheries (1987-1992) and an analysis of the skipjack tuna catch of the Mauritian purse seine fishery (1987-1993). In Proceedings of the expert consultation on Indian Ocean tunas, 5th session, Mahe, Seychelles, 4-8 October, 1993 (J. D. Ardill, ed.), p. 266-273. IPTP Coll. Vol. 8, TWS/93/4/5. FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(23) Norungee, D., and C. Lim Shung. 1996. Analysis of the purse seine fishery of Mauritius, 1990-1994, and comparison of catch rate and species composition of catches of Mauritian purse seiners to those of French fleet. In Proceedings of the expert consultation on Indian Ocean tunas, 6th session, Colombo, Sri Lanka, 25-29 September, 1995 (A. A. Anagnuzzi, K. A. Stobberup, N. J. Webb, eds.), p. 15-21. IPTP Coll. Vol. 9. FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(24) Hallier, J.-P. 1994. Purse seine fishery on floating objects: What kind of fishing effort? What kind of abundance indices? In Proceedings of the expert consultation on Indian Ocean tunas, 5th session, Mahe, Seichelles, 4-8 October, 1993 (J. D. Ardill, ed.), p. 192-198. IPTP Coll. Vol. 8., TWS/93/2/25. FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(25) Parajua Aranda, J.I. 1991. Spanish status report of yellowfin tuna fishery 1984-1990. In IPTP Coll. Vol. Work. Doc., Vol. 6, TWS/91/13, p. 99-130. FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(26) Hastings, R. E., and G. Domingue. 1996. Recent trends in the Seychelles industrial fishery. In Proceedings of the expert consultation on Indian Ocean tunas, 6th session, Colombo, Sri Lanka, 25-29 September, 1995 (A. A. Anagnuzzi, K. A. Stobberup, N. J. Webb, eds.), p. 97-109. IPTP Coll. Vol. 9. FAO, Viale delle Terme di Caracalla, 00100, Rome, Italy.

(27) Data from logbooks (Cort, 1992) show a lower proportion of sets and of catches on whale-associated schools for Spanish vessels, but in the author’s view a comparison of data collected in the same way (by observers) is preferable.

(28) France and Spain (along with catch from the vessels from these two countries flying “flags of convenience” [Panama, Cote d’Ivoire, and recently Belize] and applying the same fishing tactics), and USSR (recently Russia or Liberia).

(29) Including vessels flying flags of convenience.

(30) Anonymous. 1998. Indian Ocean tuna fisheries data summary, 1986-1996. Indian Ocean Tuna Commission (IOTC) data summary 18, 180 p. IOTC, P.O. Box 1011, Victoria, Seychelles.

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Evgeny V. Romanov Southern Scientific Research Institute of Marine Fisheries and Oceanography (YugNIRO) 2, Sverdlov St. 98300, Kerch, Crimea, Ukraine E-mail address:

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