Preliminary results of age and growth study of bigeye tuna in the western Pacific

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1 Preliminary results of age and growth study of bigeye tuna in the western Pacific Chi-Lu Sun, Chien-Lung Huang and Su-Zan Yeh Institute of Oceanography National Taiwan University Taipei, Taiwan, R.O.C. Introduction The Taiwanese offshore longline fleets in the western Pacific, whether based in Tungkang or foreign fishing ports, are landing more bigeye tuna than in the past. Several studies were conducted on the growth of Pacific bigeye tuna in the 1950's and 's. These were based upon the increments between modal points in the size composition data (i.e. Iversen 1955, Yukinawa and Yabuta 1963, Shomura and Keala 1963, Kume and Joseph 1966, Suda and Kume 1967) and the number of annual markings (annuli) on the scale (Nose et. al. 1957). Recently, SPC began a growth study using otolith increment counts. The purpose of this study is to provide up to date estimates of the age and growth rate of bigeye tuna by counting the growth rings on sections of the first dorsal spine. This information is critical because it will allow the age-composition of the catch to be determined, which in turn will allow the status of the western Pacific bigeye stock to be assessed by use of yield-per-recruit and sequential population analysis. Materials and Methods Length, weight, and first dorsal spine samples from bigeye tuna were collected from the Tungkang fish market on a monthly basis between February 1997 and January In total, 1096 first dorsal spines were collected. Three cross sections were taken along the length of each spine above the condyle base (Fig. 1A) using a low-speed "ISOMET' saw. The thickness of the sections ranged from 0.8 to 1.0 mm. The cross-sections were read with a binocular using passing light. The age of each fish was determined from the number of growth rings visible. The basic assumption behind this method is that two rings are formed each year a translucent (light colored) ring formed during slow growth in the winter and an opaque (dark colored) ring formed during fast growth in the summer. The number of pairs of alternating dark and light rings in the cross-section is equated with the age of the fish. l

2 The distance between the center of the dorsal spine and the outer edge of each annual ring was measured using an optical micrometer. To estimate the center of the spine, measurements were made from the outside edge of each ring to the opposite edge of the cross section (d t ) following Cayre and Diouf (1981) (Fig. 1B). These distances were then converted into radii (R,) by the following formula used by Gonzalez-Garces and Farina-Perez (1983): R, = d-d/2 where Rj is radius of the ring i (distance between the center of the cross section and the outside edge of the ring /) d, is distance from the outside edge of ring i to the opposite edge of the cross section, d is diameter of the spine (distance from the edge of the cross section to the opposite edge of the cross section). The relationship between fork length (FL) and dorsal spine radius (R ) was modeled by the linear equation FL = a + br The fork length of the fish was then back-calculated for each one of different rings, using the formula where FL t is predicted fork length of the fish corresponding to age or ring / in cm; FL is observed fork length of the fish in cm; Ri is radius of the ring calculated as the average value observed in ring i (Fig. IB); R is dorsal spine radius; a is ordinate in the origin of the equation FL = a + b R. These fork lengths, back-calculated for each estimated age were used by the Ford-Walford method to fit von Bertalanffy growth model and obtain vital parameters. Weight was related to fork length using the power function W = a"fl b ' 2

3 An analysis of covariance was conducted to examine whether the weight-length relationship varies by sex. Results and Discussion Spines from 1,096 specimens ranging in size from 45.6 to cm FL were examined (Fig. 2). A significant linear relationship was found between spine radius (R) and fork length (FL = R, r 2 = 0.89, Fig. 3). The adjusted Ford- Walford relationship between /, and l,+, was very close, giving an r 2 of The parameter estimates for the von Bertalanffy growth model were: AM). 125"' ;!«, = 2.4 cm, and t 0 = yr (Fig. 4). The relationship between FL and weight (W) by sex and with the sexes combined are shown in Fig.5. There was no significant difference between male and female. The estimates of the von Bertalanffy growth parameters from this study are compared to those from previous studies in Table 1. Our estimates are most similar to those of Yukinawa and Yabuta (1963), who also worked with western Pacific samples. The value of K in our study was somewhat lower than that of Yukinawa and Yabuta (1963), because different methods were used. The early growth bands were increasingly obscured by the vascular core as the size of the fish increased. Therefore, the number of bands was estimated for the larger bigeye based on the average position (radius) of the rings in younger specimens. The use of spinal sections to estimate age has the advantage of easy sampling and the growth bands stand out clearly. An additional advantage of the method is easy storage of samples for future reexamination. (Compean-Jimenez and Bard, 1983) However, estimates of age from spine cross sections has not been validated, therefore we are going to study the vertebrae to help validate our spine methodology in the near future. References Cayre, P.M. and T. Diouf, Estimating age and growth of little tunny, Euthynnus alletteratus, off the coast of Senegal, using dorsal fin spine sections. In Prince E.D. and L.M. Pulos (eds.), Proceedings of the international workshop on age determination of oceanic pelagic fishes: tunas, billfishes, and sharks, p Cayre, P.M. and T. Diouf, Croissance du thon obese (Thunnus obesus) 3

4 de L'Atlantique d'apres les resultats de marquage. Pap. 20(1): ICCAT Col. Vol. Sci. Compean-Jimenez G. and F.X. Bard, Growth increments on dorsal spines of eastern Atlantic bluefin tuna, Thunnus thynnus, and their possible relation to migration patterns. In Prince E.D. and L.M. Pulos (eds.), Proceedings of the international workshop on age determination of oceanic pelagic fishes: tunas, billfishes, and sharks, p Gonzalez-Garces, A. and A.C. Farina-Perez, Determining age of young albacore, Thunnus alalunga, using dorsal spines. In Prince E.D. and L.M. Pulos (eds.), Proceedings of the international workshop on age determination of oceanic pelagic fishes: tunas, billfishes, and sharks, p Iversen, E.S., Size frequencies and growth of central and western Pacific bigeye tuna. Spec. Sci. Rep. U.S. Fish. Wildl. Serv. (Fish.), 162: 1-. Kume, S. and J. Joseph, Size composition, growth and sexual maturity of bigeye tuna, Thunnus obesus (Lowe), from the Japanese long-line fishery in the eastern Pacific Ocean. Bull. Inter-Amer. Trop. Tuna Comm. 11(2): Nose, Y., H. Kawatsu and Y. Hiyama, Age and growth of Pacific tunas by scale readingl. In Suisan Gaku Shusei, Tokyo Univ. Press. P Shomura, R.S. and B.A. Keala, Growth and sexual dimorphism in growth of bigeye tuna {Thunnus obesus) A preliminary report. Proceedings of the world scientific meeting on the biology of tunas and related species, La Jolla, California, 2-14 July FAO Fisheries Report, 6(3): Suda, A. and S. Kume, Survival and recruit of bigeye tuna in the Pacific Ocean, estimated by the data of tuna longline catch. Rep. Nankai. Reg. Fish. Res. Lab. (25): Yukinawa, M. and Y. Yabuta, Age and growth of bigeye tuna, Parathunnus mebachi (Kishinouye). Rep. Nankai Reg. fish. Res. Lab. 19:

5 SPINE SHAFT CROSS SECTION AREA mm ' CONDYLE BASE Fig 1. First dorsal spine and the location of cross section (A) and cross section of the first dorsal spine showing group rings and measurements taken (B). R = radius of spine, i?, = radius of ring 50 n = 1096 >. u I 30 cr u T jyutt " '!' Fork length (cm) Tflili-^ Fig 2. Length-frequency of bigeye tuna caught in western Pacific from February 1997 to January 1998.

6 \ 00 c o 200-j - FL = ^ r 2 = 0.89 n = ' Spine radius (mm) Fig 3. Relationship between spine radius (rhrn) and fork length (cm) of western Pacific bigeye tuna. S o g> 100 jo o u. 80 H T r Age Fig 4. von Bertalanfly growth curve estimated for bigeye tuna of the western Pacific based on measurements of growth bands on dorsal spine cross section.

7 180 1 'a 1 * 120.2? Male W = 4E-05 FL r n=257 I I 1 1 I Fork Length (cm) 1-4 gll 4> Female W * 2E-05 FL r 2 = 0.94 n= Fork Length (cm) J? 120 w "S Total PT = 3E-05FI r 2 = 0.97 n = 856 t 1 1 r r w^«2jb* Fork Length (cm) «_»«4H* 44^4% f Fig 5. Relationship between weight (kg) and fork length (cm) for male, female and sex combined bigeye tuna from western Pacific Ocean.

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9 AGENDA FOR THE ALBACORE RESEARCH GROUP 8. Albacore Research Group 8.1 Regional Fishery Developments ~/ y ' / Albacore Fishery Overview (longline & troll) - Bigelow Data Situation - Lawson Catch Trends Relevant Paper - WP 36 (Uosaki) - Recent status of the Japanese albacore fisheries in the SPAR area / Relevant Paper - WP 22 (Uosaki) - Standardization of CPUE for albacore caught by Japanese longline fishery in the SPAR area ^ 8.2 Biological/Ecological Research Stock Structure Age & and Growth Reproduction Movement 8.3 Stock Assessment Relevant Paper - WP 23 (Wang) - Fluctuation of the south Pacific albacore stocks (Thynnus alalunga) relative to sea surface temperature Relevant Paper - WP 24 (Wang) - A new method applying in assessing south Pacific albacore stocks (Thynnus alalunga) Relevant Paper - WP 25 (Hampton) - MULTJFAN CL: a length-based age-structured model for fisheries stock assessment, with application to south Pacific albacore (Thunnus alalunga) El Nino Effects on Productivity/Recruitment - Hampton 8.4 Research Coordination and Planning