BULLETIN OF MARINE SCIENCE, 71(1): 487–499, 2002

SEASONAL MIGRATIONS AND OCEANOGRAPHIC CONDITIONS FOR CONCENTRATION OF THE JAPANESE FLYING SQUID (TODARODES PACIFICUS STEENSTRUP, 1880) IN THE NORTHWESTERN JAPAN SEA

N. M. Mokrin, Yu. V. Novikov and Yu. I. Zuenko

ABSTRACT The distribution of the squid Todarodes pacificus in the north-western Japan Sea (38– 45°N, 130–139°E) is analyzed in relation to the water masses and vertical water struc- ture, using data from scientific and commercial catches of the squid and oceanographic data obtained by Russian and Japanese research vessels in 1986–1997. Seven zones of different types of water structure exist in the upper 200 m layer in the region: the Sub- tropical and Subtropical-transformed water types south of the Polar Front; the Interfron- tal and Interfrontal-transformed water types within the zone of the Polar Front; the Sub- arctic, Coastal, and Primorye Current types north of the Polar Front. The squid migrate into the Russian Exclusive Economic Zone (EZZ) of the Japan Sea in June. From July to October, the squid schools distribute widely over the whole Russian EEZ area, and con- centrate usually in the Subtropical and Subtropical-transformed zones from June to Sep- tember. In the Interfrontal zone, squid aggregations occur in June and September–Octo- ber, in the Interfrontal-transformed zone (in June–October), in the Subarctic zone (in July and September–October), and in the Coastal zone (in July–October). The squid con- centrations were located near the fronts (southern and northern divisions of the Polar Front offshore, or benthic front at the bottom of Primorye shelf). Concentrations of T. pacificus are formed under certain favorable environmental conditions: optimum water temperature, high gradient in the thermocline, and thin upper mixed layer. The highest concentrations of T. pacificus were formed in the zones with stable, steep, and shallow thermocline when the temperature was at its optimum for the squid.

Many cephalopod species are important for the world fisheries. According to FAO data, annual landings of cephalopods in the World Ocean increased from 1.1 million t in 1970s to 2.8 million t in early 1990s (Yatsu, 1999). Species of the family Ommastrephidae are a significant component of the world cephalopod catch (Fukuda and Okazaki, 1998; Yatsu, 1999). The Japanese flying squid, Todarodes pacificus, is one of the most impor- tant target species for fisheries. Japan, Korea, and China catch T. pacificus commercially. The total catch of this squid in the Japan Sea by Japanese and Korean fleets in the last decade was 260,000–420,000 t (Kidokoro et al., 2000). At the beginning 1990s, T. pacificus dominated in nekton communities in the upper epipelagic zone offshore in the north- western Japan (Shuntov et al., 1998). T. pacificus is an epipelagic, neritic-oceanic species. It is common in the southern Okhotsk (Japan), East China, Yellow, northwestern Philippine, and northern South China seas, and in the Pacific Ocean off the Kuril and Japanese Islands (Shevtsov, 1978; Okutani, 1983; Nesis, 1985; Dunning, 1988; Murata, 1990). The results of biological studies car- ried out by Japanese and Russian researchers in 1960s-1970s have been summarized in several papers on the life cycle and population structure of the squid. Principal studies are by Hamabe and Shimizu (1966), Zuev and Nesis (1971), Shevtsov (1978), Okutani (1983),

487 488 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002 and Murata (1990). Ecological studies on T. pacificus are less numerous, especially in the north-western Japan Sea, particularly, in the Russian Exclusive Economic Zone (EEZ). The purpose of this paper is to evaluate the distribution, seasonal migrations, and con- centration formation of this squid. The information presented in our study could be of value for a better understanding of the life cycle, and could be also useful for commercial fishing for T. pacificus.

MATERIALS AND METHODS

The area of investigation was located in the north-western part of the Japan Sea between 38– 45°N and 130–139°E (Fig. 1). Data on water temperature and squid distribution were obtained from scientific cruises on the Pacific Research Fisheries Centre’s research vessels between 1986– 1997, and from the Japanese research vessels TANSHU-MARU, CHOKAI-MARU and KOSHIJI-MARU in 1988–1997, conducted within framework of joint Japanese-Russian studies (Table 1). All data were collected during the period of fishing for squid in the northern Japan Sea (June-October). The squid individuals were caught by jigging from top 50 m (motorized and hand operated jigs) or pelagic trawls with the mouth vertical and horizontal openings of 40 m and 50 m, respectively and a towing

Figure 1. Map of the surveyed area in the Japan Sea. MOKRIN ET AL.: THE JAPANESE FLYING SQUID (TODARODES PACIFICUS) 489

Table 1. Todarodes pacificus collections in the Japan Sea.

Research vessel Number of Daate (month, year) Are stations ARTYK (6Russia) 0S9.8 J31 06−0S9.88 J65 08−0S9.89 J02 IRKUTSK (6Russia) 0S9.8 J41 1S0.87 J61 TANSHU-MARU (8Japan) 0S8.8 J01 0S8.89 J01 CHOKAI-MARU (2Japan) 0S7.9 J51 0S7.94 J81 0S6.96 J71 PROFESSOR LEVANIDOV (3Russia) 1S0.9 J43 0S9.97 J83 KOSHIJI-MARU (7Japan) 0S9.9 J8 TINRO (5Russia) 1S0.9 J26 OSMOTRITELNY (8Russia) 0 −0B9.97 P9G KALMAR (7Russia) 0 −0B8.93 P6G KALMAR,EDULIS (7Russia) 0 −0B9.95 P8G 1 KALMAR,EDULIS (7Russia) 0 −0B9.96 P7G 1 Abbreviations: JS − offshore Japan Sea; PGB − Peter the Great Bay speed of about 4.5 kt. Data from fisheries statistics for T. pacificus from the Japanese and Russian commercial jigging vessels operating in the Russian EEZ during 1982–1992 were also used (350 daily catches) in the analysis of conditions for squid concentrations. Water temperature was mea- sured by CTD and BT casts or by Nansen thermometers. Recent results describing the thermohaline water structure of the Japan Sea (Zuenko and Yurasov, 1995; Zuenko, 1998) were used. Four main water masses exist which differ mainly in temperature: Surface Subtropic (SST, the warmest), Surface Subarctic (SSA), Subsurface Subtropic (SSST), and the Japan Sea Proper Water (JSP). Several modifications of these water masses appear as a result of mixing or another processes (Fig. 2). The most important feature in respect of the squid distribution

Figure 2. Schematic presentation of water masses and water structure types for sections across the northwestern Japan Sea in mid-summer. Water masses: SC – Surface Coastal, PC – Primorye Current, SSA – Surface Subarctic, SSTT – Surface Subtropic transformed, SST – Surface Subtropic, DS – Deep Shelf, JSP – Japan Sea Proper Water, SSSTT – Subsurface Subtropic transformed, SSST – Subsurface Subtropic. Water structure: PC – Primorye Current, SA – Subarctic, IFT – Intertrontal transformed, IF – Interfrontal, SST – Subtropic transformed, ST – Subtropic. 490 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002

Figure 3. Climatic positions of the North Polar Front (A) and South Polar Front (B) at the sea surface monthly. 4 – April, 5 – May, 6 – June, 7 – July, 8 – August, 9 – September, 10 – October, 11 – November. is horizontal mixing between SST and SSA. This intermixing produces Surface Subtropic-trans- formed water (SSTT), and divides the Polar Front into two frontal divisions: North Polar Front (NPF) and South Polar Front (SPF). Similar mixing takes place in the subsurface water layer. As a result, several types of vertical water structure appear. Seven zones of different types of water structure can be observed in the upper 200 m layer in the north-western Japan Sea in the summer: Subtropic (SST/SSST) and Subtropic-transformed (SST/SSSTT) south of SPF; Interfrontal (SSTT/ SSSTT) and Interfrontal-transformed (SSTT/JSP) within the Polar Front zone between SPF and NPF; Subarctic (SSA/JSP), Primorye Current (PC/JSP), and Coastal (SC/DS) north of NPF (Fig. 2). There are seasonal changes in temperature and salinity of the water masses (Table 2). The arrangement of water masses also changes seasonally according to seasonal shifts of the Polar Front divisions (Fig. 3). Distribution of the water masses and fronts at the sea surface and in the subsurface layer (50 m) was determined using maps which were drawn on the basis of water temperature data in both layers, over a 10-d period (10-d maps). The zones of different types of water structure were deter- mined for each 10-d period. Temperature data were also used to determine the thickness of the mixed layer (a layer with vertical gradient of temperature <0.1 C m−1), upper and lower limits of the thermocline (a layer with vertical gradient of temperature >0.1 C m−1), and mean temperature gra- dient in the thermocline (a difference between temperature at the upper and lower limits of the thermocline relative to its thickness, C m−1). The squid distribution was characterized using CPUE (catch per unit effort) as the number of specimens per machine/man per hour jigging or number of specimens per hour trawling. These data were plotted in 10-d maps for each survey. The catch was considered to be commercial when CPUE was over 20 ind hr−1 for jigging (Mokrin and Filatov, 1999) or CPUE over 50 ind hr−1 for trawling, (corresponding to an estimated density of 12,500 ind km−2). Both jig and trawl data were used in the analysis of squid migration and distribution in September and October. In June–August the squid distribution was analyzed from jig catches only. MOKRIN ET AL.: THE JAPANESE FLYING SQUID (TODARODES PACIFICUS) 491

Table 2. Physical parameters of water masses in the upper 200 m layer in the north-western part of the Japan Sea (values in the upper (first) row are for February, lower row for August), (Zuenko and Yurasov, 1995).

Tsype of Wmater mas L,ayer, T,emperature Salinity wCater ºups structure Srubtropic S0urface Subtropic Wate −200 > 8933. −34.0 (0SST) −20 >261 33. −33.8 S*ubsurface Subtropic Water a––bsent (0SSST) 3 −2000 1 −115 34. −34.5 Srubtropic S0urface Subtropic Wate −200 >8933. −34.0 t)ransformed (0SST −20 >261 33. −33.8 S*ubsurface Subtropic a––bsent T0ransformed Water (SSSTT) 3 −2400 −9833. −34.1 Icnterfrontal S0urface Subtropi −530 −6933. −34.0 T0ransformed Water (SSTT) −380 1 −250 33. −33.9 S*ubsurface Subtropic a––bsent T0ransformed Water (SSSTT) 3 −2400 −9833. −34.1 Icnterfrontal S0urface Subtropi −530 −6933. −34.0 t)ransformed T0ransformed Water (SSTT −380 1 −250 33. −33.9 J*apan Sea Proper Water *0−3633. −34.1 (JSP) >20 0−5933. −34.1 Srubarctic S*urface Subarctic Wate *0−3633. −34.1 (0SSA) −260 1 −118 33. −33.7 J*apan Sea Proper Water *0−3633. −34.1 (JSP) >200 −5933. −34.1 Croastal Sturface Coastal Wate a––bsen (0SC) −460 1 −199 <32. D*eep Shelf Water * −1−1733. −34.0 (DS) >440 −9833.4– 33. Prrimorye P0rimorye Current Wate −100 −1−1733. −34.0 C)urrent (0PC −40 >104 33. −33.5 Japan Sea Proper Water >1000 −3633. −34.1 (JSP) >200 −5933. −34.1 * in the winter, SSST and SSST water masses do not penetrate into the north-western Japan Sea; ** in the winter it is impossible to distinguish between the surface and subsurface water masses in the Subarctic and Coastal zones because of convection.

RESULTS

Migrations and distribution of T. pacificus were analyzed in relation to water structure by comparing 10-d maps of CPUE and water types in June–October over the investigated years. Figure 4 shows a typical example. Both squid distribution and water mass arrange- ment have considerable seasonal changes. However, general features of squid distribu- tion relative to water structure in certain seasons were basically the same in different years. Probability of squid catches and commercial catches in each zone, averaged over the whole period of studies, is presented in Figure 5. We observed the following changes in squid distribution and concentration in relation to water structure: (1) in June squid 492 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002

Figure 4. Distribution of Todarodes pacificus catches in zones of different water structure types in the last ten days of June 1996: PC – Primorye Current, SA – Subarctic, IFT – Interfrontal transformed, IF – Interfrontal, SST – Subtropic transformed, ST – Subtropic; 1 – Primorye Current Front, 2 – North Polar Front, 3 – subsurface North Polar Front, 4 – South Polar Front, 5 – subsurface South Polar Front were caught from the Subtropic to Subarctic zones, but commercial concentrations were found mainly in the Subtropic, Interfrontal, and Interfrontal-transformed zones; the high- est probability of commercial catch for squid was in the Interfrontal-transformed and Subtropic-transformed zones; (2) in July squid were distributed in all zones, but the prob- ability of commercial catches was higher in the Subarctic and Subtropic zones; (3) in August the probability of commercial catches decreased in the Subarctic zone, and in- creased in the Coastal and Interfrontal-transformed zones; the highest probability was in the Subtropic and Subtropic-transformed zones; (4) in September squid were also distrib- uted through all zones with the highest probability of commercial catch in the Subtropic- transformed zones; in the Coastal and Subtropic zones the probability of commercial catch decreased, and increased in the Primorye Current, Subarctic, and Interfrontal zones; (5) in October squid were also distributed throughout all zones, but the probability of MOKRIN ET AL.: THE JAPANESE FLYING SQUID (TODARODES PACIFICUS) 493

Figure 5. Probability of Todarodes pacificus catches in the zones of different types of water structure. Details are in the text. 494 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002 commercial catch was the highest in the Interfrontal and Interfrontal-transformed zones (no data from the Subtropic transformed zone were available). The squid distribution pattern in the Coastal zone, in Peter the Great Bay, was the most complex. This zone has three parts determined by subsurface conditions. In the shallow area (C1) there exists the Coastal Water (SC) only. In the deeper area (C2), relatively warm brackish Deep Shelf Water (DC), formed by tidal mixing, underlays the SC, and they are separated by a weak thermocline (the upper benthic front is formed by this ther- mocline). Further offshore (C3), cold and more saline Japan Sea Proper Water (JSP) is found beneath the SC separated by the thermocline that is very strong in the summer. As usual, T. pacificus did not concentrate in the zones C1 and C3. In the C2 zone, the squid were observed under and within the thermocline in the daytime. In the C1 zone, the squid concentrations were occasionally formed just above the bottom, while in the C3 zone the squid appeared in the thermocline layer. At nighttime, squid were observed in the upper 20 m layer in all zones. The thermocline condition is also very important for squid distribution in the offshore areas of the Japan Sea (Fig. 6). This is not a new observation. Some authors have previ- ously concluded that a sharp thermocline is an important factor for the presence of con- centrations of T. pacificus (Kim and Lee, 1981; Kim et al., 1984; Tameishi, 1991, 1993). In this study, we have found that the concentrations were formed mainly when the ther- mocline gradient was strong (0.8–1.0ºC m−1 offshore and 0.6–0.8ºC m−1 in the coastal zone), and the thermocline depth (mixed layer thickness) was less than 10 m offshore and 15–20 m in the coastal zone. The higher the gradient and the thinner the mixed layer, the higher the CPUE value for the squid. However, in 5% of cases, concentrations were observed when the mixed layer depth was less than 6 m (Mokrin and Filatov, 1999).

DISCUSSION

Before migrating into the Russian EEZ, the squid are widely distributed in the warm subtropical waters. In June the squid schools cross the SPF, in July–October are distrib- uted through all zones, but concentrate mainly in the warm Subtropic and Subtropic- transformed zones and in relatively cold Coastal zone (in August). The squid concentra- tions in both Subtropic zones are stable during the whole warm season, but in the Coastal zone they are observed during the warmest period only. In the Subarctic and Primoye Current zones squid are rare, but may form concentrations in both Interfrontal zones during northward (June) and southward (September–October) migrations. The distribu- tion area of T. pacificus in the summer is divided into two major parts (Fig. 5): some animals concentrate at the Polar Front, while the others migrate northward to the Russian Primorye coast. In the summer-autumn, squid are widely distributed, and their distribution is not re- stricted by the temperature regime, though commercial concentrations are formed in a rather narrow temperature range (Fig. 7). Usually the sea surface temperature is consid- ered as a factor influencing the squid distribution pattern. T. pacificus occurs within a wide range of temperature, from 4–28°C. In Korean waters, the range was 5.5–22.5°C with temperatures favorable for commercial concentration of 14–20°C( Lee et al., 1985). In Japanese waters, the range was determined at 13–28°C with the optimum temperature for fishery of 20–24°C (Kasahara, 1978). In Russian waters, the temperature range was 6–27°C with the best catches at 14–25°C (Mokrin and Filatov, 1999). Thus, the tempera- MOKRIN ET AL.: THE JAPANESE FLYING SQUID (TODARODES PACIFICUS) 495

Figure 6. Probability of commercial catch for Todarodes pacificus and catch per unit effort (CPUE, ind. per hour per jigging machine) in relation to thermocline parameters in the offshore Japan Sea: A – depth of the upper mixed layer, B – seasonal thermocline gradient. Commercial catch corresponds to the CPUE > 20. ture range in the Russian EEZ appeared somewhat wider than in Korean and Japanese waters. Optimum temperature range for the squid fishery changed seasonally, being at its maximum of 17–25°C in August, with a subsequent decrease (Mokrin and Filatov, 1999; Fig. 7). No strong relation between catch and water temperature was found (Fig. 8). When water mass structure and water temperature are considered, it appeared that the squid concentrations depend indirectly on temperature, through water structure. The tempera- ture values are indicative of certain water masses, and they in turn could be favorable for squid concentrations. 496 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002

Figure 7. Range of the sea surface temperature for several months on the feeding grounds of Todarodes pacificus in the north-western Japan Sea (Mokrin and Filatov, 1999),

Even in favorable water masses, the distribution of the squid is not uniform. Within the SST, the squid concentrate in the Subtropic-transformed zone, and within the SSTT in the Interfrontal-transformed zone. In the Coastal zone the squid distribute mainly in the sub- surface layer within DS. These zones are adjacent to corresponding fronts: SPF, NPF, and the benthic front. In the frontal zones there are certain factors favorable for squid concen- tration, e.g., a strong and narrow thermocline, and relatively high abundance of zoop- lankton. T. pacificus usually concentrates near the frontal zones (Zuev and Nesis, 1971; Kasahara, 1978; Tameishi, 1991). Some authors suggest that plankton concentrates at fronts because of the convergence effects (Levastu and Hela, 1974; Bakun and Csirke, 1998), while the others suggest that enhanced biological productivity of frontal zones occurs because of high water stability (Tameishi, 1993), or due to local upwellings on the warmer side of the front (Zuenko et al., 1992). Notwithstanding the favorable conditions near the offshore fronts, the squid migrate through these fronts. According to Sugimito and Tameishi (1992) and Tameishi (1991, 1993), the squid may cross fronts and move to another front using small warm eddies and streamers. Our results suggest that long-distance northward migrations of the squid to- wards the Russian Primorye coast are related to seasonal shifts of fronts as well as to the cross-frontal movement of the squid, although some of the animals remain near the SPF. The exact mechanism of the squid migration is not clear. We suppose that better feed- ing conditions is the main reason for migration of the squid. Small-sized squids feed on zooplankton (Hamabe and Shimizu, 1966; Okutani, 1983; Murata, 1990), and large-sized squid mainly consumes nekton (small-sized fish and squid, including conspecific juve- niles), hence the squid concentrations tend to form within high productivity areas (e.g., on fronts and shelves) or in the areas with high concentration of plankton with strong seasonal thermocline. MOKRIN ET AL.: THE JAPANESE FLYING SQUID (TODARODES PACIFICUS) 497

Figure 8. Probability of commercial catch for Todarodes pacificus in relation to water temperature at 0, 20 and 50 m depths in the north-western Japan Sea in summer-autumn (Mokrin and Filatov, 1999). Commercial catch corresponds to the CPUE > 20 ind per hour per jigging machine. 498 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 1, 2002

CONCLUSION

Our results indicate that the distribution of T. pacificus changes considerably with sea- son due to long-distance feeding migrations. The general direction of migrations is north- ward in the summer and southward in the autumn. Some squid concentrates in the Sur- face Subtropic water mass near the South Polar Front during the entire warm season. Others move through the front and continue migration to the north: in June they concen- trate within the Polar Front zone (Interfrontal and Interfrontal-transformed zones), in July in the Subarctic zone, in August in the Coastal zone, and in September–October again within the Polar Front zone. Favorable conditions for squid feeding occur near both divisions of the Polar Front, as well as in the Coastal zone. Those conditions include an optimum temperature gradient in the thermocline and thickness of the mixed layer, and are supposed to form a background for high productivity and enhanced food availability for T. pacificus.

ACKNOWLEDGMENTS

We are grateful to S. Hasegawa and H. Kidokoro for useful information during preparation of this paper. We thank two anonymous reviewers for their helpful comments on the manuscript. Special thanks to O. N. Katugin for editing the final version of the text.

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ADDRESS: Pacific Research Fisheries Centre (TINRO-Centre), 4 Shevchenko Alley, Vladivostok, 690950, Russia.