First attempt to age yellowfin tuna, Thunnus albacares, in the Indian Ocean, based on sectioned otoliths

Fisheries Research 149 (2014) 19–23

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Short Communication

First attempt to age yellowfin tuna, Thunnus albacares, in the Indian Ocean, based on sectioned otoliths Chia-Lung Shih a,∗ , Chien-Chung Hsu a , Chiee-Young Chen b a b

Institute of Oceanography, College of Science, National Taiwan University, Taipei 106, Taiwan Department of Marine Environmental Engineering, National Kaohsiung Marine University, 142, Haichuan Road, Nantzu District, Kaohsiung 811, Taiwan

a r t i c l e

i n f o

Article history: Received 28 June 2013 Received in revised form 4 August 2013 Accepted 1 September 2013 Keywords: Age validation Length–weight relationship Sectioned otoliths Sex ratio Sex-specific growth

a b s t r a c t This study first attempts to investigate the age and growth of yellowfin tuna in the Indian Ocean and clarifies its sex-specific growth using sectioned otoliths. The results indicate that the assumption of annual opaque zones may be reliable and fish age at the first opaque zone formation is about 0.75 years. The age estimates cover a large range of lifespans (age estimates range from 0.75 to 10.50 years) in which the maximum age estimate shows that males (9.50 years) are older than females (6.50 years). The estimated von Bertalanffy growth curves are shown to be significantly different between sexes, while the von Bertalanffy growth parameters (L∞ , k and t0 ) are 123.6 cm, 0.846 year−1 and −0.449 years for females and 162.9 cm, 0.244 year−1 and −2.139 years for males. The growth curves of both sexes are similar before 4 years, after which males start to grow larger than females. Sexual size dimorphism in growth curves occurs after mature age. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Yellowfin tuna, Thunnus albacores, is a large oceanic fish found in the waters between 35 ◦ N and 35 ◦ S of the Pacific, Indian and Atlantic Oceans, with more abundance in the tropics (Collette and Nauen, 1983). This species in the Indian Ocean is a single stock based on genetic studies (Ward et al., 1997). Yellowfin tuna in the Indian Ocean has been exploited since the 1950s and it is mainly caught by longliners, purse seines, gillnets and pole-andline (Anon., 2011). The recent stock assessment of yellowfin tuna in the Indian Ocean shows that it is suffering from high fishing pressure with an annual catch between 300,000 and 410,000 tons for the period 2007–2010 (Anon., 2011). Growth parameters are essential input data for age-based stock assessments of fish (King, 2007). Although several age and growth studies of yellowfin tuna in the Indian Ocean have been conducted (Table 1), only one has adopted hard tissue and the growth equation estimated by this research may be applicable only for younger fish because of the lack of samples older than four years old (Stéquert et al., 1996). By contrast, annual sectioned otolith increment counts have been adopted to determine the age of several tuna species (bigeye tuna, Thunnus obesus, Pacific bluefin tuna, Thunnus orientalis, South bluefin tuna, T. orientalis, and North Pacific albacore

∗ Corresponding author. Tel.: +886 2 33661393; fax: +886 2 23661198. E-mail address: [email protected] (C.-L. Shih). 0165-7836/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.fishres.2013.09.009

tuna, Thunnus alalunga), and these are suited for the whole life history of these tuna species (Chen et al., 2012; Gunn et al., 2008; Farley et al., 2006; Shimose et al., 2009). However, no age determination studies of yellowfin tuna in the Indian Ocean have used the annual increments of otolith counting method. Alternatively, sexual divergent growth can remarkably affect the outputs of stock assessments (Wang et al., 2005). Stéquert et al. (1996) has investigated the sex-specific growth of yellowfin tuna in the Indian Ocean, but growth information was limited to young fish (4 years old). The objectives of this study first attempt to age yellowfin tuna in the Indian Ocean using otoliths, to estimate its growth equation (which covers a larger size range) and to clarify its sex-specific growth. 2. Materials and methods 2.1. Sample collection For age determination, the sagittal otoliths of yellowfin tuna were collected by scientific assistants onboard four Taiwanese longline vessels operating in the western tropical Indian Ocean (1–8 ◦ S/45–59 ◦ E; 6 ◦ N–8 ◦ S/68–75 ◦ E) from April to December in 2006 and from July to December in 2007. These samples included information on fishing date and location, fork length (FL) (to the nearest 1 cm), processed (gilled and gutted) weight (to the nearest kg) and sex. To examine the sex ratio, the sex of the 2007 catch was recorded when possible.

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Table 1 Growth parameters [L∞ (cm), K (year−1 ) and t0 (years)] of the von Bertalanffy growth equation and the corresponding growth performance index ( (Thunnus albacares) sampled in the Indian Ocean estimated from previous studies.



) for yellowfin tuna

Author

Sampling area

Method

L∞

K

t0

This study This study (females) This study (males) Rohit et al. (2012) Shono et al. (2007) Somvanshi et al. (2003) Stéquert et al. (1996) John (1995) Pillai et al. (1993) Marsac (1991) John and Reddy (1989) Mohan and Kunhikoya (1985)

Western Indian Ocean Western Indian Ocean Western Indian Ocean East coast of India Indian Ocean Indian EEZ Western Indian Ocean Andaman and Nicobar, India Minicoy and south-west coast of India Indian Ocean West coast India Minicoy, India

OTO (annual) OTO (annual) OTO (annual) LFA LFA LFA OTO (daily) LFA LFA LFA LFA LFA

166.9 123.6 162.9 197.4 166.1 193.0 272.7 171.5 144.1 173.1 175.0 145.0

0.209 0.846 0.244 0.300 0.380 0.200 0.176 0.316 0.440 0.650 0.290 0.320

−2.663 −0.449 −2.139 −0.116 NA NA −0.266 −0.305 −0.448 NA NA −0.340



8.67 9.47 8.78 9.37 9.26 8.92 9.48 9.14 9.12 9.88 9.09 8.81

OTO = otoliths, LFA = length frequency analysis, NA = not available.

2.2. Length–weight relationship The customary mathematical formula for the length–weight relationship (Sparre and Venema, 1992) is W = aLb , where W represents the weight (kg) of fish at L (cm) FL, and a and b are parameters to be estimated. The differences in this relationship between sexes were examined using the likelihood ratio test (Kimura, 1980). 2.3. Sex ratio Length data were arbitrarily grouped into 5-cm class intervals by sex to investigate the sex ratio (females/total) by length in this stock. The equality (p = 0.5) of the sex ratio was then hypothesized and examined using the exact binomial test. 2.4. Otolith observation and age validation To prepare otolith sections, we followed the process of the technical manual of age determination for southern bluefin tuna (Anon., 2002). At the beginning, transverse sections (Anon., 2002) were cut but the opaque zones in otolith sections were unclear for reading. Then, we tried to cut oblique sections (Chen et al., 2012) and these opaque zones seemed to be clearer. Thus, two kinds of sections were obtained for aging; no differences between them in age reading were assumed in this study. To obtain clear increment images, pictures of the transverse and oblique sections were captured using a compound microscope under transmitted light (Fig. 1a) and a dissecting microscope using reflected light (Fig. 1b), respectively. The criterion to determine the counts of otolith sections was adopted from the technical manual (Anon., 2002). Two readings were taken by one reader without reference to the information on fish length, catch date and previous reading; the interval between these two readings was at least one week. The precision of these two readings was then estimated by using the average percentage error (IAPE) (Beamish and Fournier, 1981):

 R  NA    −1   IAPE = 100 Xij − Xj X N −1 , j

j=1

A

i

where NA is the number of aged fish, R the number of readings, Xij the ith reading of the jth fish and Xj the mean counts of the jth fish. If the two counts of the same individual were different, final ages were determined by a third reading (i.e., by referring to the previous two readings).

Fig. 1. Otolith sections for yellowfin tuna (Thunnus albacares) in the western Indian Ocean. (a) Transversely sectioned otoliths and (b) obliquely sectioned otoliths. Red circles indicate opaque zones identified.

To verify if the opaque zone formed once per year, an edge type analysis (ETA) was carried out (Shimose et al., 2009). In the ETA, the otolith edge condition was classified as opaque, translucent or unidentifiable and the monthly proportion of fish that had an opaque edge was calculated. Additionally, the age of the first opaque zone formation was estimated and 20 obliquely sectioned otoliths were randomly selected for the microincrement analysis (assumed to be daily growth increments). For this analysis, sectioned otoliths were ground into very thin sections (about 40 ␮m). To enhance microincrements, the surfaces of sections were decalcified with 5% ethylenediaminetetraacetic acid for about 30 s. Microincrements were then examined under high magnification (×400) with a compound microscope

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using transmitted light. Two counts of the microincrements from the core to the end of the first opaque zone were taken; as before, the interval between these two counts was at least one week.

Table 2 Result of binomial test for the hypothesis that the sex ratio (females in the total samples) is equal to 0.5 for the yellowfin tuna (Thunnus albacares) sampled from the longline fleets in the Indian Ocean during 2007 fishing season. Length class

2.5. Estimation of growth curve To assign the count to an exact age, the date of the opaque zone formation was assumed to be 1 October based on the ETA of this study. If samples were collected in the second quarter (April–June), third quarter (July–September) or fourth quarter (October–December), +0.25, +0.50, and +0.75 years were added to the final increment count of the otolith for age determination, respectively. The von Bertalanffy growth equation (von Bertalanffy, 1938) is usually used to model the growth of yellowfin tuna. The growth parameters were estimated using the length at age data by the nonlinear least squares method and the equation was then expressed as Lt = L∞ [1 − exp(−k(t − t0 ))],

Female

<70 70–75 75–80 80–85 85–90 90–95 95–100 100–105 105–110 110–115 115–120 120–125 125–130 130–135 135–140 140–145 145–150 150–155 >155 Total

3. Results 3.1. Length–weight relationship Sex-specific length–weight relationships were found to be not significantly different (likelihood ratio test, 2 = 4.29, df = 2, P = 0.117). A sex-combined relationship was estimated for fish that ranged from 66 cm to 165 cm FL and the equation was W = 3.8 × 10−6 L3.276 (R2 = 0.94, n = 1047, P < 0.001). Because of individual differences and seasonal changes in body condition, variation in processed weight was high even in the same length class.

*

Male

Sex ratio

P

7 9 8 8 6 4 13 45 85 76 69 35 18 15 5 6 4 2 0

7 16 9 10 4 6 17 58 117 122 82 49 39 25 21 21 14 9 6

0.50 0.36 0.47 0.44 0.60 0.40 0.43 0.44 0.42 0.38 0.46 0.42 0.32 0.38 0.19 0.22 0.22 0.18 0.00

0.791 0.230 1.000 0.815 0.344 0.754 0.585 0.237 <0.05* <0.001* 0.329 0.156 <0.01* 0.154 <0.01* <0.01* <0.05* 0.065 <0.05*

415

632

0.40

<0.001*

P < 0.05

100

opaque edge d (%)

where Lt is the FL (cm) at age t (year), L∞ the theoretical asymptotical FL, k the growth coefficient (per year) and t0 the theoretical age (years) at zero length. Sex-specific growth functions were then examined using the likelihood ratio test (Kimura, 1980). To compare the growth curves obtained by different studies, the growth performance index (  = ln K + 2ln L∞ ) (Pauly and Munro, 1984) was used.

21

69 82

3 34

80

15

60

4

2 3

40 20 0 May

un. Ju

Jul.

Aug. Sep.

Otc.

Nov.

Month Fig. 2. Monthly frequency of occurrence for opaque zone on the otolith edge of yellowfin tuan (Thunnus albacares) in the western Indian Ocean. The number above each plot indicates the sample size.

3.2. Sex ratio

3.4. Age validation

For all sized samples combined, there were significantly fewer females than males according to the exact binomial test (sex ratio = 0.40, n = 1047, P = <0.001). The sex ratios were low but not significantly different for most length classes from the smallest length class to 135 cm FL, except for the length classes of 105–115 cm and 125–130 cm FL. Then, the sex ratios decreased as length class increased and males outnumbered females significantly (Table 2).

In the ETA analysis, 209 individual (51%) otolith edges were identified. The opaque zones of otoliths sections were primarily formed from August to November (Fig. 2). The highest proportion (88%) of fish that had opaque zone edges was in October and the lowest proportion (33%) was in June. The proportions seem to peak once a year but lacked samples in some months. For following analyses, the opaque zones were assumed to form annually. In the investigation into the age of the first opaque zone formation, the precision of aged fish between the two counts of microincrements was high (IAPE = 3.8%). The mean (±SD) value of the two counts was 255 (±45), while the range was from 167 to 361. These results indicate that the age of the first opaque zone was close to 0.75 years. Thus, all age estimates were subtracted by 0.25 years, which was the estimated time interval between the times of birth and opaque zone formation. The birthday was assumed to be 1 January, in the medium of the spawning season between November and February (Shung, 1973; Hassani and Stéquert, 1990; Stéquert et al., 2001), and the opaque zone was assumed to form on 1 October based on the preliminary result of ETA of this study.

3.3. Otolith structure The structure of opaque zones between transversely and obliquely sectioned otoliths showed few differences. For transverse sections, opaque zones usually included multiple opaque bands, which increased the difficulty of age reading. By contrast, oblique sections showed clearer and more distinct opaque zones, although multiple opaque bands also existed in these sections. Thus, oblique sections were mainly adopted for reading; however, if oblique sections were unavailable, transverse sections were used for this purpose in which there were 317 oblique sections and 91 transverse sections.

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Frequency (ind.)

80

4. Discussion

70

Unknown sex s

60

Male

50

Female

40 30 20 10 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165

0

Forrk length (cm) ( Fig. 3. Length distributions by 5 cm intervals for yellowfin tuna (Thunnus albacares) with otoliths used in the final age determination.

3.5. Precision of aging Of the 408 otolith sections, 22 (5%) fish had no clear opaque zones. Among the two counts of 386 otoliths, 221 (57.3%) showed complete agreement and 142 (36.8%) had only one-year differences. The precision of aged fish was high (IAPE = 6.8%) in which oblique sections had higher accuracy (IAPE = 6.7%, n = 308) than transverse sections (IAPE = 7.4%, n = 78). 3.6. Age and growth The 386 otoliths of age estimates comprised 128 females, 222 males and 35 unknown sexes. Ages were estimated to be 0.75–10.50 years (female: 0.75–6.50 years; male: 0.75–9.50 years) and fish ranged from 66 to 165 cm FL (female: 67–145 cm FL; male: 69–165 cm FL) (Fig. 3). Sex-specific growth parameters had statistically significant differences tested by the likelihood test (2 = 23.65, df = 3, P < 0.001). The growth equations estimated were Lt = 123.6(1 − exp(−0.846(t0 + 0.450))) (R2 = 0.50, n = 128) for females, Lt = 162.9(1 − exp(−0.244(t0 + 2.139))) (R2 = 0.64, n = 222) for males and Lt = 166.9(1 − exp(−0.209(t0 + 2.663))) (R2 = 0.99, n = 386) for sex-combined. However, the variation in age estimates was high even among similar sized individuals. The growth trajectories younger than 4 years old were similar between sexes, after which males grew larger than females (Fig. 4). The estimated growth parameters showed that the L∞ of males (162.9 cm FL) was substantially larger than that of females (123.6 cm FL) but K was reversed.

Fig. 4. Observed length-at-age data and the fitted von Bertalanffy growth curves of yellowfin tuna (Thunnus albacares) in the western Indian Ocean.

Annual opaque zones in the otolith sections of yellowfin tuna in the Indian Ocean were herein investigated for the first time. Sex-specific growth curves, which covered a large size range, were estimated in this study and samples were collected from the major fishing grounds of this stock (Anon., 2011). The precision of age estimates using otoliths in this study (IAPE = 6.8%) was found to be at an acceptable level (IAPE < 10%) (Powers, 1983). However, age validation has not been completed by ETA in this study, but the daily microincrement analyses suggested the first opaque zone formation was close to 0.75 years. A large variation of age estimates even among similar sized individuals was observed in this study. Such variation may result from individual variability at the time of birth because they can spawn throughout the year, peaking between November and February (Hassani and Stéquert, 1990). Additionally, habitats have been found to influence the growth of albacore tuna in the South Pacific Ocean (Williams et al., 2012), but the limited sample size and area of this study did not allow for this assessment. A large size range in age estimates of approximately 1.5 years (length range about 70–120 cm FL) was also observed in yellowfin tuna based on the microincrements of sectioned otoliths (Lehodey and Leroy, 1999). The growth performance indices for particular species are similar even though they are sampled in different waters (Pauly and Munro, 1984). In comparison with the growth index of sexcombined yellowfin tuna in the Indian Ocean, the value of this study was the lowest (8.69) among previous results (8.81–9.88) (Table 1). Although Stéquert et al. (1996) used microincrement otoliths to investigate the age and growth of yellowfin tuna in the Indian Ocean, the growth parameter L∞ (272.7 cm) estimated was unreasonable and larger than those of previous results (Table 1) because that study used young individuals to estimate ages. There have been a few age and growth studies of this stock (Table 1), but they have all used length frequency analysis to estimate growth parameters. However, this method has been suggested to be suited only to young and fast-growth fish (Campana, 2001). In comparison to age and growth studies of yellowfin tuna in the Indian Ocean using hard tissues, even for the results of different stocks (as calculated by Rohit et al., 2012), the predicted mean length at 1 year (89 cm FL) in this study seemed to be higher than those presented by other studies (48.4–78.5 cm FL). However, those of ages 2 and 3 years herein (age 2 years: 104 cm FL; age 3 years: 116 cm FL) were similar to those of previous studies (age 2 years: 89.7–114.2 cm FL; age 3 years: 118–134 cm FL). After 3 years of age, however, the predicted mean length of fish in this study (age 4 years: 126 cm FL; age 5 years: 133 cm FL) became smaller compared with previous studies (age 4 years: 136.5–160.5 cm FL; age 5 years: 172 cm FL). These previous studies used dorsal spines, scales and microincrement otoliths to estimate ages. Hard tissues were reliable for young fish because scales and daily increments in otoliths have been shown to underestimate older fish (Gunn et al., 2008; Laurs et al., 1985), while dorsal fin spine has been suggested to be suited to young fish (<3 years old) due to the vascularization of the core (Stéquert and Conand, 2004). In addition, the growth rate of fish usually decreases when fish attain sexual maturity, and the age of 3 years for yellowfin tuna in the Indian Ocean is above the suggested age of maturity (about 100 cm FL corresponding to approximately 2 years of age) (Zhu et al., 2008). Thus, the smaller predicted mean lengths of older fish (>3 years old) estimated in this study may be reasonable. These comparisons indicate that the assumption of annual opaque zone for yellowfin tuna in the Indian Ocean may be reliable. The large disparity of this study in the growth of yellowfin tuna between sexes occurred at 4 years of age (120 cm FL). A similar

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result was also reported by Stéquert et al. (1996) that male yellowfin tuna in the Indian Ocean are slightly larger than females between ages 3 (male: 120.4 cm FL; female: 118 cm FL) and 4 years (male: 146.5 cm FL; female: 142.2 cm FL). The 120 cm FL is larger than the size at sexual maturity (100 cm FL) (Zhu et al., 2008), suggesting that reproduction seems to be a major factor influencing the sex-specific growth of mature Indian yellowfin tuna. Similar inferences were also made in studies of North Pacific albacore (Chen et al., 2012) and South Pacific albacore (Williams et al., 2012). However, we showed that large male Indian yellowfin tuna (>135 cm FL) outnumbered large females. Such differences in the sex ratio could also support the sex-specific growth results obtained in this study. In conclusion, the annual increments of otolith sections seem to be reliable for use in aging yellowfin tuna in the Indian Ocean. Even if the aging method was not fully validated, there should not have been a bias in their age estimates by sex. However, further investigations are necessary to collect additional samples from December to July to complete age validation by ETA, and also smaller sized fish (<65 cm) to derive more informative growth curves. Acknowledgments We thank Mr. Jimmy Liu, Jimmy Chen and Tzy-Lin Tzeng for their hard work on-board sampling. We are also grateful to anonymous referees for their valuable comments. This study was in part financially supported by the Fisheries Administration, Council of Agriculture, Taiwan, through the grants 96AS-FA-12 to Drs. Hsu and Chen for collecting the biological samples, and by National Science Council, Taiwan (NSC96-2616-B-002-MY3) to Dr. Hsu for the serial analysis. References Anon., 2002. A manual for age determination of southern bluefin tuna Thunnus maccoyii: otolith sampling, preparation and interpretation. In: Report of the Direct Age Estimation Workshop of the CCSBT, 11–14 June 2002, Victoria, Australia. Anon., 2011. Executive summary: status of the Indian Ocean yellowfin tuna (Thunnus albacres) resource. IOTC-2011-SC14-11. Beamish, R.J., Fournier, D.A., 1981. A method for comaring the precision of a set of age determinations. Can. J. Fish. Aquat. Sci. 38, 982–983. Campana, S.E., 2001. Accuracy, precision and quality control in age determination, including a review of the use and abuse of age validation methods. J. Fish Biol. 59, 197–242. Chen, K.S., Shimose, T., Tanabe, T., Chen, C.Y., Hsu, C.C., 2012. Age and growth of albacore Thunnus alalunga in the North Pacific Ocean. J. Fish Biol. 80, 2328–2344. Collette, B.B., Nauen, C.E., 1983. FAO Species Catalogue. Vol. 2: Scombrids of the World. An Annotated and Illustrated Catalogue of Tunas, Mackerels, Bonitos, and Related Species Known to Date. FAO Fish. Synop. 125. FAO, Rome, 137 pp. Farley, J.H., Clear, N.P., Leroy, B., Davis, T.L.O., McPherson, G., 2006. Age, growth and preliminary estimates of maturity of bigeye tuna, Thunnus obesus, in the Australian region. Mar. Freshwater Res. 57, 713–724. Gunn, J.S., Clear, N.P., Carter, T.I., Rees, A.J., Stanley, C.A., Farley, J.H., Kalish, J.M., 2008. Age and growth in southern bluefin tuna, Thunnus maccoyii (Castelnau): direct estimation from otoliths, scales and vertebrae. Fish. Res. 92, 207–220. Hassani, S., Stéquert, B., 1990. Sexual maturity, spawning and fecundity of the yellowfin tuna (Thunnus albacores) of the western Indian Ocean. In: Expert Consultation of Indian Ocean tunas, Bangkok, Thailand, 2–6 July 1990, IPTP Coll. Vol. Work. Doc. 4, pp. 91–107. John, M.E., 1995. Studies on yellowfin tuna, Thunnus albacares (Bonnaterre, 1788) in the Indian Seas. University of Mumbai, 258 pp. (Ph.D. Dissertation).

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