BULLETIN OF MARINE SCIENCE, 71(2): 1105–1108, 2002

THE AGE AND GROWTH OF STHENOTEUTHIS OUALANIENSIS (CEPHALOPODA: OMMASTREPHIDAE) IN THE PACIFIC OCEAN

Kaori Takagi, Takeru Kitahara, Naoki Suzuki, Junta Mori and Akihiko Yatsu

Sthenoteuthis oualaniensis is distributed in the tropical and subtropical areas of the Pacific and the Indian Oceans. According to Nesis (1993), there is a complex population structure in S. oualaniensis, as is the case in many other ommastrephids and some loliginids. In the Pacific Ocean, there is the middle-sized squid which is a widespread and typical one (Nesis, 1993). Arkhipkin and Bizikov (1991) examined the statoliths of middle-sized female in the Indian Ocean and determined its growth. S. oualaniensis is, though, one of the most difficult species in the Ommastrephidae for the observation of statolith incre- ments due to the numerous occulting crystals and weak contrast in the increments (Uozumi, 1993). Using a newly developed heating technique in processing statoliths, we estimated the age and growth of S. oualaniensis, assuming the daily deposition of increments.

MATERIALS AND METHODS

Samples of S. oualaniensis were collected between September and December 1993 in the Pa- cific Ocean around the Hawaii and the Ogasawara (Bonin) Islands. We used the statoliths of 53 adults (112–284 mm in mantle length (ML), 21 males and 32 females) and 112 paralarvae (0.7– 13.5 mm in ML). The adults were captured by drift nets and jigs. The paralarvae were captured by bongo nets and a larval net. To examine the relationship between ML and age, we also used 6 other juveniles (39–50 mm in ML) captured using a dip net. They were collected between September and December 1997 off the coast of Peru and of Costa Rica. After the measurements of ML and body weight, we extracted statoliths and stored them in liquid paraffin. We heated the statoliths of squid larger than 10 mm ML at 180°C to enhance the contrast of increments. Then, we ground the statoliths according to Dawe’s method (Dawe and Natsukari, 1991). We counted the number of increments, i, from nucleus to dorsal dome edge. We measured the radii R in µm, the distance to the dorsal edge from the nucleus of statoliths. There were some thick and prominent increments ‘checks’ on the statolith. We also used this character for the measurement of radii ri in µm, and thus measured the distance from the nucleus to i-th incre- ment. Moreover, we calculated the mean growth coefficient Gt() from the ML measured above, as we describe below.

RESULTS

The estimated hatching months of adults, paralarvae and juveniles ranged from May to September, from September to November and from September to October, respectively. The hatching seasons suggest a long spawning period, and Harman et al. (1989) sug- gested the possibility of multiple spawning in S. oualaniensis.

We calculated the average of radii ri for the number of rings, i (i <40), to compare the radii between the adults and the paralarvae. The radii show linear relations to i in both adults and paralarvae as follows: ri = αi + β. The estimates of α and β are 7.33 and 13.5 µm (r2 = 0.920) for adults, and 4.69 and 33.4 µm (r2 = 0.930) for paralarvae. Here, r2 is the coefficient of determination. Because their slopes significantly differ from each other at

1105 1106 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 2, 2002 the significant level of 5%, we should not treat adults and paralarvae together in the growth analysis. According to Laptikihovsky et al. (1993), the daily growth coefficient is presented by

1 dL() t Gt()= , Lt() dt Eq. 1

where L(t) is ML(mm) at t (d) after hatching. Setting L(0) = L0, Eq. (1) gives

t Lt()= Lexp G()ττ d . Eq. 2 0 {}∫0

Accordingly, the mean growth coefficient Gt() (per d) is calculated as

1 t Gt()= Gd()ττ. Eq. 3 t ∫0

The combination of Eqs. (2) and (3) gives

1 Lt() Gt()= ln . Eq. 4 t L0

Since the smallest paralarva is 0.70 mm in ML in the present study, we set L0 = 0.70mm. Now, let us suppose that Gt() can be approximated by

Gt()= kta , where k and a are constants. The estimates of k and a are 0.08 and 0 for paralarva, 3.98 and −0.935 for adult male, and 3.32 and −0.887 for adult female. Thus, the growth equa- tion is given by

a+1 Lt()= L0 exp() kt .

Figure 1 presents the theoretical growth curves calculated by the above growth equa- tion. The theoretical growth curves are well fitted to the actual data, respectively, in male, female and paralarvae. In the range less than 50 d, however, the extrapolated curves from the growth equations of adults are not fitted to the actual data of paralarva and juvenile. Juvenile under 41 d is larger than paralarva in ML, and fit the growth curves of neither adults nor paralarva. NOTES 1107

Figure 1. Mantle length plotted against age t. Solid triangle, open triangle, open circle and open square show female, male, paralarva and juvenile, respectively. Thin and thick lines show the theoretical growth curve of male and female, respectively. Thin long dashed and dotted line curve show the extrapolation from the growth equations. The other thin line shows the theoretical growth curve of paralarva. Heavy broken line shows the growth curve of middle-sized female captured in the Indian Ocean (Arkhipkin and Bizikov, 1991).

DISCUSSION

The range of ML for over 20 d is wider than the one for under 20 d (Fig. 1). This is due probably to the separation of proboscis. The proboscis, an undifferentialed tentacle that is unique to the Ommastrephidae, characterizes paralarvae of less than 4 mm in ML. Such paralarvae are known as rhynchoteuthions. S. oualaniensis begins to separate its probos- cis into a pair of tentacles at 3.5–4.0 mm ML and the separation is completed at 9 mm ML (Wormuth et al., 1992). Since the tentacles are used to catch prey, it is reasonable to assume a change in the growth at the end of the rhyncoteuthion stage. Now, we discuss the differences of growth between adult and paralarva from the radii of statoliths. We obtained the relations of ri to i for i <40 in the adults and paralarvae and consequently the slope obtained was significantly smaller in paralarvae than in the adults. This presents the difference between adult and paralarva in the growth of statolith. From the growth analysis of the ML to age, the theoretical growth curves of adult and paralarva well fitted to the actual data, respectively. We extrapolated the theoretical growth curves for the paralarval stage from growth equations of male and female. The actual data of paralarva is lower apparently than the theoretical growth curves. Generally, the growth rate of squid varies for each ontogenetic phrase. Additionally, it is said that this growth 1108 BULLETIN OF MARINE SCIENCE, VOL. 71, NO. 2, 2002 discrepancy reflects the different hatching season and locality. The fact suggests that the paralarva grew up considerably slowly as compared with the others. Apparent paralarval slow-growth may also be due to net avoidance. And it follows that paralarva collected by the plankton nets has represented slow-growth individuals. Arkhipkin and Bizikov (1991) examined the statoliths of S. oualaniensis collected from the Indian Ocean and determined its growth of which the curve is quoted in Figure 1. However, we used only the statoliths extracted from the collected samples of immature and maturing squids for the age determination. We tried to observe statoliths of the ma- tured adults, and consequently the statoliths were difficult to observe. It is necessary to improve the preparation and observation methods.

ACKNOWLEDGMENTS

We are grateful to the staff of National Research Institute of Far Seas Fisheries for providing the materials. Additionally, we would like to express our sincere thanks to A. Arkhipkin, Falkland Islands Government Fisheries Department, S. Segawa and K. Sakuramoto, Tokyo University of Fisheries for their kindly help and advice. Anonymous reviewers helped to improve the manu- script.

LITERATURE CITED

Arkhipkin, A. A. and V. A. Bizikov. 1991. Comparatitive analysis of age and growth rates estima- tion using statoliths and gladius in squids. Pages 19–33 in P. Jereb, S. Ragonese, S. von Boletzky, eds. Squid age determination using statoliths. N.T.R.-I.T.P.P. Spec. Publ. 1. Dawe, E. and Y. Natsukari. 1991. Section 2 Practical procedures of squid aging using statolith. A laboratory manual. Light microscopy. Pages 83–95 in P. Jereb, S. Ragonese, S. von Boletzky, eds. Squid age determination using statoliths. N.T.R.-I.T.P.P. Spec. Publ. 1. Harman, R. F., R. E. Young, S. B. Reid, K. M. Mangold, T. Suzuki and R. F. Hixon. 1989. Evidence for multiple spawning in the tropical oceanic squid Sthenoteuthis oualaniensis (Teuthoidea: Ommastrephidae). Mar. Biol. 101: 513–519. Laptikihovsky, V. V., A. I. Arkhipkin and A. A. Golub. 1993. Larval age, growth and mortality in the oceanic squid Sthenoteuthis pteropus (Cephalopoda, Ommastrephidae) from the eastern tropical Atlantic. J. Plank. Res. 15(4): 375–384. Nesis, K. N. 1993. Population structure of oceanic Ommastrephids, with particular reference to Sthenoteuthis oualaniensis: a review. Pages 375–383 in T. Okutani, R. K. O’Dor and T. Kubodera, eds. Recent advances in fisheries biology. Tokai Univ. Press, Tokyo. Uozumi, Y. 1993. Efficacy of ageing by statolith microstructure in stock assessment of squid stock. ‘Ika-rui-sigen-gyokaikyou-kenkyu-kaigi-houkoku’ (report annual meeting on resource and fish- eries on squids for 1990 fiscal year), 22–40 (in Japanese) Wormuth, J. H., R. K. O’Dor, N. Balch, M. C.Dunning, E. C.Forch, R. F. Harman and T. W. Rowell. 1992. Family Ommastrephidae Steenstrup, 1857. Pages 105–119 in M. J. Sweeney, C. F. E. Roper, K. M. Mangold M. R. Clarke and S.v. Boletzky, eds. “Larvala” and juvenile Cephalo- pods: A manual for their identification. Smithson. Inst. Press, Washington, D. C.

ADDRESSES: (K.T., T.K., N.S.) Department of Fisheries Resource Management, Tokyo University of Fisheries, 4-5-7, Konan, Minato-ku, Tokyo 108-8477, Japan. (J.M.) Oceanic Squid Section, National Research Institute of Far Seas Fisheries, Fisheries Research Agency, 5-7-1, Orido, Shimizu, Shizuoka 424-8633, Japan. (A.Y.) Fisheries Biology and Ecology Division, National Research Institute of Fish- eries Science, Fisheries Research Agency, 2 12-4, Fukuoka, Kanazawa-ku, Yokohama, Kanagawa. 231-8648, Japan.