BULLETIN OF MARINE SCIENCE, 71(1): 209–221, 2002

PELAGIC OF THE ARABIAN SEA WITH AN EMPHASIS ON STHENOTEUTHIS OUALANIENSIS

Michael V. Chesalin and German V. Zuyev

ABSTRACT Taxonomic composition, vertical and horizontal distribution, and the biology of cepha- lopods were studied during the 30th cruise of the Soviet research vessel PROFESSOR VODYANITSKY in the Arabian Sea from February to April 1990. Small planktonic cephalo- pods were collected by midwater trawl down to 500 m depth, while large nektonic spe- cies were captured by jiggers and hand nets during night drift stations from the surface to 120 m depth. Nineteen species of 13 families were recorded. Mainly early life stages and small-sized planktonic oceanic species were present in trawl samples, with Sthenoteuthis oualaniensis, Abralia marisarabica, Abraliopsis lineata, Ancistrocheirus lesueurii, Onychoteuthis banksii, sicula, and reinhardti prevailing. The nektonic S. oualaniensis, O. banksii, Thysanoteuthis rhombus and pelagic octo- puses Tremoctopus violaceus and Argonauta argo were recorded on the surface. Among all cephalopods, S. oualaniensis absolutely dominated numbers and biomass in the epipelagial of the Arabian Sea. Juveniles (3–10 cm ML) and middle-sized specimens (10–30 cm ML) were observed at night, hunting on lantern fishes and flying fishes around the drifting vessel. The giant females (30–62 cm ML) were caught mainly close to the thermocline. The mean biomass of S. oualaniensis on studied area was estimated as 4.5 t km-2, and the most productive sites were identified.

The Arabian Sea is one of the richest regions for cephalopods in the Indian Ocean (Aravindakshan and Sakthivel, 1973), but fishing activity in the open sea is still low, due to insufficient studies of the resource. Chun (1910) provided the most detailed study on Indian Ocean cephalopods. Belyaev (1962) pointed out the important role of cephalo- pods in the ecosystem of the Arabian Sea, based on the distribution of beaks in the sedi- ments. Filippova (1968) and Silas (1968) presented some data about composi- tion and biology of cephalopods in the Indian Ocean and the Arabian Sea. Zuyev (1971) provided detailed descriptions of distribution and biology of ten neritic cephalopods of the suborder Myopsina and of one oceanic species (Sthenoteuthis oualaniensis) in the northwestern Indian Ocean, including the Arabian Sea. Aravindakshan and Sakthivel (1973) reported on the location of nurseries in the Indian Ocean. Nesis (1974) de- scribed the cephalopod larvae from the western Arabian Sea. In addition, several publica- tions considered descriptions and biological features of several species (Nesis, 1970; Okutani, 1983; Tsuchiya, 1991; Tsuchiya et al., 1991). More recently, comprehensive studies on the fauna and distribution of the adult and early life stages of pelagic cephalo- pods in the Arabian Sea were published by Piatkowski and Welsch (1991) and Piatkowski et al. (1993). For many years, special attention has been focused on the study of the purpleback flying S. oualaniensis (Lesson, 1830). It has local commercial significance in the Arabian Sea and the Indian Ocean (Aravindakshan and Sakthivel, 1973; Nesis, 1977; Okutani and Tung, 1978; Silas et al., 1982; Roper et al., 1984; Raje and Savaria, 1987), but its resources are poorly exploited. The most comprehensive earlier information on S. oualaniensis from the tropical zone of the Indian Ocean was presented by Zuyev (1967, 1971), Zuyev and Nesis (1971), Zuyev et al. (1985). The most detailed information on S.

209 210 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002 oualaniensis in the Arabian Sea was published by Chesalin (1993) and Nesis (1993). However, the faunal composition of cephalopods of the Arabian Sea as well as many features of the distribution, biology, productivity, life cycle of S. oualaniensis, are still poorly known, so that any new information has important scientific significance.

MATERIALS AND METHODS

Cephalopods were sampled outside the 200 nautical mile (nmi) exclusive economic zones of Oman and Pakistan from February to April 1990 during the 30th cruise of RV PROFESSOR VODYANITSKY. The sampling was conducted on a grid of stations between 14o and 21o30'N, and 59o and 65o30'E (Fig. 1). The early stages and macroplanktonic adult cephalopods (generally <3 cm ML) were taken from macrozooplankton samples collected by an enlarged Isaacs Kidd midwater trawl with a small- cellular netting insert of 500 µm mesh size and a mouth opening of approximately 3 m2. Towing speed varied between 2.8 and 3.5 kts. The duration of tows was 30 min and the volume of water filtered ranged between 6680 and 7945 m3. Trawling was carried out obliquely from 100 m to the surface, as well as horizontal tows in the maximum of the sound-scattering layer (SSL) at night and in the maximum of the deep-scattering layer (DSL) in daytime. In total of 24 samples were ob- tained, i.e., 10 night samples from oblique hauls (0–100 m), eight night samples in the epipelagic SSL (20–120 m), and six daytime samples in the mesopelagic DSL (225–475 m). Samples were stored in 3–4% buffered formalin. Cephalopods were sorted from the samples in the laboratory, identified according to the guide of Nesis (1988), counted, and measured. The abun- dance of the various species and taxonomic groups was expressed as number per trawl (n tr−1). The horizontal distribution of some cephalopod paralarvae from the night hauls in the top 120 m are given as number per cubic meter (n m−3). During night three drift light stations were conducted to count and catch nektonic squids. The distance between stations varied from 5 to 30 nmi. In total, 146 drift light stations were performed

Figure 1. Map showing the sampling grid for cephalopods on the 30th cruise of RV PROFESSOR VODYANITSKY, February to April 1990. Crosses, night drift stations; triangles, trawl samples. CHESALIN AND ZUYEV: PELAGIC CEPHALOPODS OF THE ARABIAN SEA 211 in the open Arabian Sea, with a total duration of 345 h. The micronektonic juveniles (generally 3– 10 cm ML) and adult nektonic squids (10–33 cm ML) were caught at the surface from the leeward side of the ship by two fishermen with jiggers, hand nets (diameter about 1 m). The large-sized S. oualaniensis (>30 cm ML) were caught on each such station from the windward side of the ship by two fishermen with manual reinforced jiggers with sinkers (and without bait) from depths down to 120 m. The biomass of young and middle-sized S. oualaniensis on the surface was estimated on the drift light stations using the method of visual observations of Zuyev et al. (1985). It is based on the behaviour of the squids to approach the surface and to be attracted by the lights of a stopped ship so that they are easily counted from shipboard. The biomass of squids was estimated by multiplying their number during 1 h of observation with the average weight of the different size-groups, and dividing by the speed of vessel drift and the width of the lighted zone. For estimation of the biom- ass (B, kg km−2) of large-sized S. oualaniensis at depth we used a new method, based on jig catches of one fisherman for 1 h,

B = Y / (N . T . V . D), where Y is the total catch (kg); N is the number of fishermen; T is the duration of fishing effort (h); V is the drift speed of the ship (km h−1); and D is the width of fishing zone for one jig (0.005 km).

RESULTS

TAXONOMIC COMPOSITION.—Nineteen species were identified from our collection (Table 1). Three species were added in the list from Piatkowski and Welsch (1991) and eleven species were not recorded by these authors. Sixteen species belong to ten families of order Teuthida and three species are from three families of order Octopoda. Planktonic paralarvae of the benthic sublittoral Octopus defilippi evidently had been carried away by currents beyond the natural coastal biotope. The other cephalopod spe- cies are oceanic. Only Abralia marisarabica can be considered as endemic to the Arabian Sea. This species was described by Okutani (1983) from the northern Arabian Sea, but there is a questionable record in the region of Seychelles Islands (Nesis, 1985). According to zoo- geographical classification of Nesis (1985), 63% of the found species are cosmopolitan, 16% are Indo-West Pacific and 16% are Atlantic-Indo-West Pacific. Most species inhabit tropical and subtropical waters (74%) other species have tropical ranges. The bulk of the teuthofauna is made up by seven species: Sthenoteuthis oualaniensis, A. marisarabica, Abraliopsis lineata, Ancistrocheirus lesueurii, Onychoteuthis banksii, and (Table 2). The most abundant species were representatives from six families: (51%), (28%), Onychoteuthidae (8%), Chtenopterygidae (8%), Ancistrocheiridae (3%) and (2%). Species Enoploteuthis anaspis, Pterygioteuthis gemmata, Octopoteuthis sicula, Taningia danae and Taonius pavo each occurred in single specimens. In the trawl samples, cephalopods were represented mainly by paralarvae. Juvenile and adult specimens occurred rarely, except for small-sized adult enoploteuthids of the gen- era Abralia and Abraliopsis. length of cephalopods in the samples ranges from 2 to 45 mm. Paralarvae S. oualaniensis of 3–5 mm ML predominated in the majority trawl catches. The species composition in the observations and in the catches from the water surface was not diverse. Three species of nektonic oceanic squids, S. oualaniensis, Thysanoteuthis 212 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002

Table 1. Pelagic cephalopod fauna of the Arabian Sea with some zoogeographical and ecological remarks

Oyrder, family and species Ztoogeograph Habita Teuthida Enoploteuthidae Enoploteuthis (Paraenoploteuthis) anaspis Roper+ AS-IWP TME/ Abralia (Enigmoteuthis) marisarabica Orkutani AME/ Abraliopsis (Micrabralia) lineata (PGoodrich) ITBW E/M/ Ancistrocheiridae Ancistrocheirus lesueurii (CSd'Orbigny) TBE/M/ Pyroteuthidae Pterygioteuthis gemmata Chun+ AS-IWP TME/ Octopoteuthidae Octopoteuthis sicula (üR ppell )+ AS-IWP TME/ Taningia danae Joubin+ CSTBM/ Onychoteuthidae Onychoteuthis banksii (CSLeach) TBE/M/ Chtenopterygidae Chtenopteryx sicula (éV rCSany ) TBE/M/ Bathyteuthidae Bathyteuthis bacidifera RPoper* ITBW M/ Ommastrephidae Sthenoteuthis oualaniensis (PLesson) ITMW E/ Hyaloteuthis pelagica (PBosc)?* ATM-IW E/ Thysanoteuthidae Thysanoteuthis rhombus Troschel+ CSTME/ Chiroteuthidae Grimalditeuthis bonplandi (éV rany )+ CTME/ Chiroteuthis picteti JPoubin ITBW M/ Cranchiidae Liocranchia reinhardti (CSSteenstrup) TBE/M/ Heliocranchia pfefferi Massy+ CSTBE/M/ Taonius pavo (Lesueur)+ CSTBn E/M/ Octopoda Bolitaenidae Japetella diaphana HCSoyle* TBE/M/ Octopodidae Octopus defilippi Véran y+ CSTME/ Tremoctopodidae Tremoctopus violaceus delle Chiaje+ CSTE Argonauthidae Argonauta argo Léinn + CSTE * from Piatkowski and Welsch (1991) + newly recorded, absent in the list of Piatkowski and Welsch (1991) A - Atlantic; IWP - Indo-West Pacific; C - Cosmopolitan; Ar - Arabian; T - tropical; TS - tropical-subtropical; TnS - tropical-north-subtropical. E - epipelagic; M - mesopelagic; B - bathypelagic. CHESALIN AND ZUYEV: PELAGIC CEPHALOPODS OF THE ARABIAN SEA 213

Table 2. Number and sizes of cephalopods, identified from trawl samples in the Arabian Sea, February to April 1990.

Sspecies Trawl nLight oblique nLight SS DmS ML, m E. anaspis --10/ 145. A. marisarabica 321+5651/10 27 +2100/8 1 +70/5 2. −30.0 A. lineata 151+3006/9 31 +3680/8 1 +206/6 3. −31.0 A. lesueurii 54+04/8 11 +04/7 +00/1 2. −12.5 P. gemmata -11-/ 3. 3 O. sicula --10/ 121. T. danae 150+175/9 25 +41/8 15/ 9. O. banksii 33+00/7 39 +02/6 2 +30/5 3. −40.0 C. sicula 540+128/10 82 +241/8 1 +00/5 3. −12.0 S. oualaniensis 12-Jan +08/1 2 +20/6 2. −27.0 T. rhombus ---55. −6.6 G. bonplandi --0+50/2 11. −30.0 C. picteti 62+47/7 2 +60/6 +40/1 11. −16.0 L. reinhardti -111/ +10/2 4. −33.0 H. pfefferi ---09. T. pavo --10/ 135. O. defilippi -2+0-/2 6. 8−9.0 A. argo 3+03/3 +0-/2 3. 0−8.6 Numerator, first term, paralarvae; second term, juveniles and adults; denominator, number of hauls. rhombus and O. banksii were recorded. In addition, ten pelagic octopuses Argonauta argo and one specimen of Tremoctopus violaceus were caught by hand nets, which pas- sively drifted with the current. The squid T. rhombus occurred in most cases in pairs. A total of 15 specimens of 15–33 cm ML were caught. The squid O. banksii was represented in the surface layer by indi- viduals with sizes of up to 12 cm ML; only one specimen of 4.5 cm ML was caught at the surface by a hand net. The ommastrephid squid S. oualaniensis absolutely dominated numbers and biomass among all cephalopods at the water surface. Schools of S. oualaniensis, which totaled up to a hundred and more individuals, concentrated each night around the drifting vessel where they actively hunted for lantern fishes and flying fishes. Aside from the trawl samples, a total of 3476 specimens of S. oualaniensis were collected and analyzed. VERTICAL DISTRIBUTION.—Samples were collected by a non-closing trawl, but at differ- ent depths and times, providing some information about the vertical distribution of cepha- lopods. Paralarvae of most cephalopod species perform short diel vertical migrations. Gener- ally, the abundance of cephalopod paralarvae reached maximum values in the SSL that occupied water column from 20 to 120 m, but usually was at 60–90 m. The mean num- ber of the enoploteuthid paralarvae in all oblique night hauls (0–100 m) was 47.2 n tr−1, while in the hauls from the maximum SSL it was 77.3 n tr−1. The mean number of the paralarvae of S. oualaniensis and O. banksii in oblique night hauls was 54.0 and 16.6 n tr−1, respectively, and increased in the SSL hauls approximately two times. The mean number of A. lesueurii was 6.7 in the oblique hauls and 16.3 n tr−1 in the SSL hauls. 214 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002

Numbers of C. sicula increased from 3.3 in the oblique night hauls to 48.8 n tr−1 in SSL hauls. In contrast, the cranchiid paralarvae were more abundant in the oblique night hauls. Juveniles were distributed in the wide water column. Only abundance of enoploteuthid juveniles in the hauls was close or more than that of the paralarvae, while the number of juveniles of other species was markedly lower than that of their paralarvae. The greatest abundance of enoploteuthid (85.7 n tr−1) juveniles was found in the oblique night hauls. The number of ommastrephid, onychoteuthid and cranchiid juveniles were less than 3.0 n tr−1 and close in the SSL and the oblique hauls. In the daytime samples from the mesopelagic DSL (225–475 m) the numbers of cepha- lopod paralarvae and juveniles were low. Six species (E. anaspis, O. sicula, G. bonplandi, C. picteti, T. danae and T. pavo) were found here that were absent in the night epipelagic hauls (down to 120 m). These are mainly mesopelagic and bathypelagic species, which have no extensive vertical migrations. The most abundant families in the daytime hauls were: Enoploteuthidae (41%), Ommastrephidae (20%), Onychoteuthidae (17%), Chtenopterygidae (9%), Chiroteuthidae (6%), Ancistrocheiridae (3%) and Cranchiidae (2%), that is close to the night hauls. The mean number of enoploteuthid paralarvae, paralarvae of S. oualaniensis and O. banksii were approximately 5 n tr−1 for each. We observed and caught paralarvae of S. oualaniensis (from 6 mm ML), juveniles (3– 10 cm ML, modal ML 3–6 cm) and middle-sized adults up to 33 cm (10–30 cm ML, modal ML 15–18 cm) at the water surface, where they were hunting on lantern fishes and flying fishes in light of the drifting vessel. Large-sized females from 30 to 62 cm (modal ML 48–54 cm) were caught at the depth down to 120 m using jiggers. On more than 75% of stations, the catch depth for the giant females of S. oualaniensis was 50–90 m; maxi- mal catches occurred at 70–80 m, usually near the maxima of the SSL and thermocline. GEOGRAPHICAL DISTRIBUTION.—The geographical distribution of the cephalopod paralarvae of A. marisarabica, S. oualaniensis, O. banksii, and L. reinhardti over the studied area are shown in Figure 2. Abundance of S. oualaniensis paralarvae increased in the northern part of the grid to a maximum density of 4.11 n m−3. The paralarvae of A. marisarabica had a maximum density of 3.36 n m−3 in the southwestern region. The paralarvae of O. banksii were more frequent in the southwestern and central parts of the grid, where their densities reached 0.87 and 0.47 n m−3. L. reinhardti paralarvae were rare everywhere and their maximal density was 0.47 n m−3 in the central part of the grid. The distribution of biomass of adult S. oualaniensis at the water surface, based on data of visual observations, is shown in Figure 3A. The mean biomass in the southern part of the studied area during February was estimated as 54 kg km−2; in the northern part in March and April it was 38 kg km−2. The highest value reached 240 kg km-2. The most productive stations (biomass >200 kg km−2) were those at 15oN, 60o45'E and 18oN, 63o22'E. The quantitative distribution of S. oualaniensis at depths of up to 120 m, according to jig catches, differed significantly (Fig. 3B). The mean biomass of large-sized females (>30 cm ML) for the entire investigated area was evaluated as 4.5 t km−2, the highest value being 26.4 t km−2 at 21oN, 64oE. The most productive regions (biomass >10 t km−2) were noted between 20o30' and 21o30'N, 63o30' and 64o30'E, as well as at 18o30' to 19o30'N and 62o30' to 63oE. CHESALIN AND ZUYEV: PELAGIC CEPHALOPODS OF THE ARABIAN SEA 215

paralarvae in the top 100 m from night trawl hauls, February to

L. reinhardti

and

O. banksii, A. marisarabica O. banksii,

,

S. oualaniensis

Figure 2. Spatial distribution of April 1990. 216 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002

Figure 3. Biomass distribution of S. oualaniensis based on observations at the surface (A) and in jig catches from depths down to 120 m (B), February to April 1990. Stations were positioned every 30 mi from 15°N–18°N and 59°E–62°20'E (4–22 February), then from 18°30'N–21°30'N (4–24 March); the final grid (from 3–11 April) was nearly the same as the latter. CHESALIN AND ZUYEV: PELAGIC CEPHALOPODS OF THE ARABIAN SEA 217

The total stock of S. oualaniensis over the entire investigated grid (equal to 159,560 km2) was estimated as 750,000 t. Moreover, approximately 200,000 t of squid was con- centrated on one tenth of the grid in aggregations with biomass more than 10 t km−2.

DISCUSSION

According to the zoogeographic division of Nesis (1985), the Arabian Sea is part of the West-Indian area of the North-Central subzone of the Tropical Zone of the World Ocean. The sea is characterized by great faunistic impoverishment, especially among the meso- pelagic fauna. More than 30 species of oceanic cephalopods that occur in the equatorial subzone of the Indian Ocean are absent from the Arabian Sea (Nesis, 1985). Obviously, this is related to the oceanographic features, especially (1), the nearness of Asia creating the domination by the monsoons, which seasonally reform the current system and the vertical advection and (2), the oxygen minimum layer from 100–200 m down to 1000– 1700 m, depending on the region (Banse, 1984; Currie, 1973; Neyman and Burkov, 1989; Olson et al., 1993; Rogers, 2000; Wyrtki, 1973). While the tropical fauna is impoverished in the Arabian Sea, there is a high development of the most adapted species, noted, for example, among the zooplankton (Moryakova, 1971), myctophids (Gjøsaeter, 1984) and, obviously, S. oualaniensis. We compared our list of cephalopod species with that by Piatkowski and Welsch (1991), which they collected on RV METEOR, using also modified Isaacs Kidd midwater trawl with similar mesh size, in the Arabian Sea between 24° and 18°N latitude and 62° and 67°E longitude from April to June 1987. We added eleven species to the list of these authors (see Table 1). The most abundant species in their collection was the cranchiid L. reinhardti, also enoploteuthids A. marisarabica and A. lineata were numerous, other spe- cies occurred in isolated specimens. Our mass species were same enoploteuthids, S. oualaniensis, O. banksii and C. sicula, while Liocranchia reinhardti were not so numer- ous. Differences may be connected with better catching ability of our trawl. Also, the sampling season in Piatkowski and Welsch (1991) was from April to June (the transi- tional period from northeastern winter monsoon to southwestern summer monsoon), while we worked from February to April, during the northeastern monsoon. The vertical stratification of paralarvae of Enoploteuthidae, Ommastrephidae, and Cranchiidae in the surface layer of the Arabian Sea was described in detail by Piatkowski et al. (1993). These authors further showed that in most cases the paralarvae are concen- trated below the mixed layer (>30 m) during the day and night, but at night they were more abundant in the mixed layer than during the day. According to our investigations and confirming the data of the German scientists, the abundance of paralarvae in most cases increased in the hauls from the maximum epipelagic SSL, which, as a rule, associ- ated with the thermocline and pycnocline. The vertical distribution of juvenile and adult squids of the majority of species is clearly complicated. In most cases, they conduct extensive vertical migrations between the epipelagial and mesopelagial. The vertical distribution of the most abundant species, S. oualaniensis, in our data is as follows. Paralarvae inhabit the epipelagic SSL (20–120 m) during the night; juvenile, young and middle-sized specimens are dispersed in the upper mixed layer (0–30 m) at night, while the giant females do not reach the surface but concentrate at night in the depth range of 30–120 m (mostly in the SSL). According to the literature on direct under- water observations (Gutsal, 1991; Kolodnitsky and Moiseev, 1987; Zuyev and Gutsal, 218 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002

1994), S. oualaniensis was found at the maximum diving depth of the submersible (400 m) at night, juvenile and middle-sized specimens dominate in the top layer (0–200 m), whereas giant forms concentrate at a depth of 200–350 m, where the oxygen saturation does not exceed 1.8–10%. Furthermore, the squids actively hunt on lantern fishes in this oxygen minimum layer. The occurrence of squids in the oxygen minimum layer suggests special physiological-biochemical adaptations. Preliminary analyses have shown that under an oxygen deficit the squids can use proteins and the break-down products (free amino acids) for anaerobic metabolism (Shulman et al., 1992). The daytime depths of S. oualaniensis have not yet been accurately determined. According to underwater observa- tions from manned submersible SEVER-2 in the northwestern Indian Ocean the juveniles and adult specimens in daytime were found between 200 and 1150 m (Gutsal, 1991). On the other hand, their giant forms were caught by jiggers at depths of 150–250 m during the day (pers. comm. of Yu Korzun). The geographical distribution of studied paralarvae from Ommastrephidae, Enoploteuthidae, Onychoteuthidae and Cranchiidae family according to our and litera- ture data (Piatkowski et al., 1993) shows that the spawning of these oceanic squids occurs in the open Arabian Sea, but separate spawning sites may exist for each species. In par- ticular, paralarvae of S. oualaniensis were mostly distributed in the northern region of the sampling grid, the nursery areas of A. marisarabica were located mainly in the southern part, whereas more dense concentrations of O. banksii and L. reinhardti paralarvae were recorded in the central part. According to Piatkowski et al. (1993) the various species of cephalopod paralarvae do not co-occur in high numbers. The authors noted the absence of paralarval enoploteuthids in the central part of the Arabian Sea and proposed that adult enoploteuthids may belong to a so-called ‘mesopelagic-boundary community’ as it has described in the waters of Hawaii (Reid at al., 1991). We found paralarvae of Enoploteuthidae in all of our study areas, so that we considered that representatives of the Enoploteuthidae, with the largest number of species, are spawning both in the shelf-slope region and in the open sea. Obviously, further studies are needed to define the location of the spawning areas of different species. Different size classes of S. oualaniensis show distinct spatial distribution pattern. It is known, both the middle-sized Red Sea-Arabian form and the giant Arabian form occur in the Arabian Sea, with similar geographical distribution (Bizikov, 1995; Nesis, 1993). However, the vertical and horizontal distribution of the forms differs. Adult specimens of the middle-sized form and young specimens and males of the giant form occur near the surface, therefore they were lumped together in the visual observations. Nevertheless, the mean of squid surface biomass in the Arabian Sea is very low and estimated as 40–50 kg km−2, which corresponds to the average means for the entire tropical zone of the Indian Ocean (Pinchukov, 1989; Zuyev et al., 1985). At the same time the bulk of S. oualaniensis, which are the females of the giant form, are concentrated at depth, where the biomass of squids reaches enormous densities. According to our calculation, based on data of jig catches from depths up to 120 m, the mean biomass was estimated as 4.5 t km−2, that is one hundred times more than near the surface. On our station grid, the highest catches were obtained in the northern part (21oN and 64oE), where the biomass reached up to 26.4 t km−2. Previously, a report of RV SHOYO-MARU expedition to the Gulf of Arabia de- scribed the existence of large school of S. oualaniensis from the waters of Karachi (Okutani and Tung, 1978), as well as from a submersible (Gutsal, 1991). CHESALIN AND ZUYEV: PELAGIC CEPHALOPODS OF THE ARABIAN SEA 219

The high number of paralarvae of S. oualaniensis in the catches of RV PROFESSOR VODYANITSKY from February to April 1990 gives evidence for intensive spring spawning. Considering that the life span of S. oualaniensis is close to 1 yr (Bizikov, 1995), one may assume that most squids die after the spring spawning. During the summer, the total biomass is very low, the new generation will grow until the end of autumn and winter, and the spawning begins in the middle of winter. Large top predators (tunas, lancet fish, sharks, dorados, swordfish, dolphins, etc.) oc- cur in the Arabian Sea in small numbers. Therefore, the squid S. oualaniensis practically occupies the uppermost trophic level in the pelagic Arabian Sea (Chesalin et al., 1995).

ACKNOWLEDGMENTS

We sincerely thanks our colleagues O. P. Ovcharov , V. N. Tyupa (IBSS), V. A. Bizikov (VNIRO, Moscow), and S. I. Moiseyev (GIDRONAVT, Sevastopol) for help in sampling of material. In particular, we thank A. M. Amelekhina for her assistance in processing cephalopods from the trawl samples. We are grateful to anonymous reviewers for critical reading of the manuscript, many valuable comments and a great editorial work.

LITERATURE CITED

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