<I>Todarodes Pacificus</I>

BULLETIN OF MARINE SCIENCE, 71(2): 987–992, 2002

INVESTIGATION ON THE EARLY STAGES OF THE OMMASTREPHID SQUID TODARODES PACIFICUS NEAR THE OKI ISLANDS (SEA OF JAPAN)

Jun Yamamoto, Shinya Masuda, Kazushi Miyashita, Ryosuke Uji and Yasunori Sakurai

ABSTRACT The MOCNESS (Multiple Opening Closing Net and Environmental Sampling Sys- tem) was used to investigate the distribution of the early life stages of the Japanese common squid Todarodes pacificus in the southwest Sea of Japan near the Oki Islands, a suspected spawning ground of this species. The largest catches of hatchling-sized paralarvae occurred at the surface layer (0–25 m), and paralarval mantle lengths increased with increasing sampling depth, suggesting that T. pacificus paralarvae gradually descend in the water column life cycle as they grow. There was no clear difference in the horizontal distribution among the different size groups.

The ommastrehid squids Illex illecebrosus and Todarodes pacificus lay spherical, pe- lagic egg masses, which are slightly negative or neutrally buoyant (O’Dor and Balch, 1985; Bower and Sakurai, 1996). The optimum temperature range for T. pacificus embryo development and hatchling survival is 15–23ºC (Sakurai et al., 1996). After hatching, paralarvae swim vertically to the surface from the egg masses (Bower and Sakurai, 1996). Based on these observations, a hypothetical scheme of the reproductive process of T. pacificus was proposed (Sakurai et al., 2000) in which the egg masses occur near the pycnocline, and after hatching, paralarvae swim to the surface. Information derived from field studies show most paralarvae occur at 25–50 m depth (Watanabe, 1965). The paralarval distribution patterns of I. illecebrosus in the Northwest Atlantic and T. pacificus near the Kuroshio off southern Japan are thought to be affected by convergent flows that occur near fronts in these regions (Dawe and Beck, 1985; Bower et al., 1999). However, ommastrephid squid egg masses have never been observed in nature, and field studies have not clearly defined paralarval movement, or changes in distribution patterns related to paralarval growth. The objectives of the present study are to examine the abundance, size and vertical distribution of the early stage of T. pacificus in a region where spawning is thought to occur.

MATERIALS AND METHODS

The survey was conducted near the Oki Islands in the southwest Sea of Japan (Fig. 1) during 8– 17 November 1999. The autumn spawning group of T. pacificus spawns in this area and season (Murata, 1990). T. pacificus paralarvae were sampled at 14 stations using the RV TOTTORI-MARU NO. 1 (199 t) from the Tottori Prefecture Fisheries Experimental Station. Oblique tows were conducted from 100 m to 0 m depth using a 1 m2 MOCNESS equipped with five 0.333 mm mesh nets. The nominal sampling depth intervals for each net were 0 m, 0–25 m, 25–50 m, 50–75 m and 75– 100 m. The collected samples were immediately fixed with 4% formalin, and then rinsed and preserved in a 50% isopropyl alcohol and distilled water solution. T. pacificus paralarvae were identified using descriptions and figures from Okiyama (1965) and Okutani (1965). The dorsal mantle length (ML) of each paralarvae was measured under a stereomicroscope. The mantle lengths

987 988 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 2, 2002

Figure 1. A map of the study area in the southwest Sea of Japan. ×: hydrographic data only, Ț: no data for surface layer. of damaged paralarvae (e.g., those with inverted mantles) were estimated by comparing the damaged paralarvae to undamaged paralarvae. Temperature and salinity data were collected at each station from the sea surface to the seafloor or to 300 m depth with a CTD. Sea surface temperature was also measured on 8 November from the AVHRR sensor (Channel-4) on the NOAA-14 satellite. DATA ANALYSIS.—The abundance of T. pacificus paralarvae in the MOCNESS samples was cal- culated as number per 1000 m3 of seawater sampled. Catch abundances were compared among depth intervals by dividing the total of all catches at each interval by the total of all volumes sampled excluding the two sites where no paralarvae were found. Artificially incubated T. pacificus paralarvae hatch at approximately 0.95 mm ML (Watanabe et al., 1996), so all paralarvae ML<1.0 mm were considered to be hatchling-sized.

RESULTS

A total of 441 T. pacificus paralarvae were collected. Mantle lengths ranged between 0.7 and 6.7 mm. Changes in the vertical distribution of different size groups are shown in Figure 2. The largest catches of hatchling-sized (ML < 1 mm) paralarvae occurred in the 0–25 m layer. Most (90%) hatchling-sized paralarvae were collected at 0–50 m depth, and most (71%) paralarvae >2 mm ML were collected at 25–75 m. The temperature range of sampled water was 15–22.3°C, and the pycnocline occurred below 50 m depth (Fig. 3). Thus, the waters sampled occurred within the ideal temperature range for embryo development and hatchling survival (Sakurai et al., 1996), but the pycnocline occurred below the layer where the largest catches of hatchling-sized paralarvae occurred. The total distribution and abundance of T. pacificus paralarvae was overlaid on a satellite image showing sea surface temperature (Fig. 4). Although paralarvae were distrib- YAMAMOTO ET AL.: INVESTIGATION ON THE EARLY STAGES OF TODARODES PACIFICUS 989

Figure 2. Vertical distribution and abundance of Todarodes pacificus paralarvae at each towing depth (ind 1000 m−3).

Figure 3. Vertical distributions of temperature (°C) and density (Sigma-t) over the study area. uted widely in the study area, the largest catches occurred near a mixing zone that occurred between cool water northwest of the survey area and warm water in Oki Strait. Maps of the temperature distributions at 0 m, 50 m and 100 m depth (Fig. 5) show a general crowding of isotherms from northwest to southeast, which coincide with the lo- cation of the mixing zone. The distributions and abundances of each size groups overlaid on the isotherms at the depth where the highest numbers of paralarvae occurred for each group (Fig. 6). The abundance and horizontal distribution patterns of different size groups paralarvae were similar. For all groups, largest catches were made in the 21–22°C water zone occurring from northwest to southeast. 990 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 2, 2002

Figure 4. The total distribution and abundance of Todarodes pacificus paralarvae (ind 1000 m−3) overlaid on a satellite image (NOAA /AVHRR Channel-4) captured on 8 November 1999. light tone: warm water; dark tone: cool water.

Figure 5. Temperature (ºC) distributions at 0 m, 50 m and 100 m depth. YAMAMOTO ET AL.: INVESTIGATION ON THE EARLY STAGES OF TODARODES PACIFICUS 991

Figure 6. Horizontal distribution patterns of different size groups of Todarodes pacificus paralarvae (ind 1000 m−3) and isotherms at the depth where the largest catch occurred for each group. Arrows: sites where no paralarvae were collected.

DISCUSSION

The largest catches of hatchling-sized paralarvae occurred at 0–25 m (Fig. 2). Before hatching, ommastrephid embryos develop in gelatinous egg masses (O’Dor et al., 1982; Bower and Sakurai, 1996), which presumably occur near the pycnocline (O’Dor and Balch, 1985). The pycnocline in the present study area occurred below 50 m depth (Fig. 3), but most hatchlings were captured between 0 and 50 m depth, which suggests hatchlings ascend to the surface soon after hatching, as reported by Bower and Sakurai (1996). The depth where most paralarvae were collected increased with increasing size, suggesting that paralarvae gradually descend in the water column as they grow. Although the area sampled was limited, most T. pacificus paralarvae were collected near the front that occurred near mixing waters (Fig. 4), especially where water tempera- tures ranged 21–22°C (Fig. 6). Few were sampled in the warm waters near Oki Strait or the cool waters in the northeast part of the study area (Figs. 4,6). Peaks in abundance near frontal zones have been reported for T. pacificus paralarvae captured near the Kuroshio 992 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 2, 2002 off southern Japan (Bower et al., 1999) and for I. illecebrosus paralarvae in the Northwest Atlantic (Dawe and Beck, 1985). The accumulation of paralarvae near these fronts is presumably caused by convergent flow (Bakun and Csirke, 1998).

ACKNOWLEDGMENTS

We thank the crew of RV TOTTORI MARU NO. 1, the technicians at the Tottori Prefecture Fisheries Experimental Station and two anonymous reviewers of the manuscript. We also thank J. R. Bower, R. Rigby and N. Eda for their assistance.

LITERATURE CITED

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ADDRESSES: (J.Y., K.M.) Field Scince for Northern Biosphere, Hokkiado University, Hakodate, Hokkaido, 041-8611, Japan (S.M., R.U.) Tottori Prefecture Fisheries Experimental Station, Sakaiminato, Tottori, 648-0046, Japan. (Y.S.) Graduate School of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido, 041-8611, Japan. CORRESPONDING AUTHOR: (J.Y.) E-mail: .