SCRS/21/74 Collect. Vol. Sci. Pap. ICCAT, 66(3): 1216-1226 (211) A REVISION OF WESTERN ATLANTIC BLUEFIN TUNA AGE OF MATURITY DERIVED FROM SIZE SAMPLES COLLECTED BY THE JAPANESE LONGLINE FLEET IN THE GULF OF MEXICO (1975-198) Guillermo A. Diaz 1 SUMMARY The maturity function developed by Diaz and Turner (27) was updated in view of the newly adopted growth curve developed for the western stock by Restrepo et al. (211). The present study used different size samples than those used by Diaz et al (27). The estimated age of 5% maturity was 15.8 yr, which is 4 yr older than the ages estimated by Diaz and Turner (27) using the growth curve previously adopted by ICCAT. The estimated ages of 5% maturity using size samples collected in the bluefin tuna western stock spawning grounds of the Gulf of Mexico do not support the currently assumed age of maturity of 8 yr for this stock. RÉSUMÉ La fonction de maturité développée par Diaz et Turner (27) a été actualisée compte tenu de la courbe de croissance nouvellement adoptée pour le stock Ouest par Restrepo et al. (211). La présente étude a utilisé différents échantillons de taille de ceux utilisés par Diaz et al. (27). L'âge estimé d acquisition de la première maturité était 15,8 ans, ce qui représente 4 ans de plus que les âges estimés par Diaz et Turner (27) en utilisant la courbe de croissance antérieurement adoptée par l'iccat. Les âges estimés d'acquisition de la première maturité en utilisant des échantillons de taille prélevés sur les zones de frai du stock de thon rouge de l Ouest du Golfe du Mexique n appuient pas l âge de maturité actuellement postulé à 8 ans pour ce stock. RESUMEN Se actualiza la función de madurez desarrollada por Díaz y Turner (27) teniendo en cuenta la curva de crecimiento recientemente adoptada y desarrollada para el stock occidental por Restrepo et al. (211). Este estudio utilizó muestras de talla diferentes a las utilizadas por Díaz et al. (27). La edad estimada a la que el 5% de los peces son maduros era 15,8 años, es decir 4 años más que las edades estimadas por Díaz y Turner (27) utilizando la curva de crecimiento previamente adoptada por ICCAT. Las edades a las que el 5% de los peces son maduros, estimadas utilizando muestras de talla recogidas en las zonas de desove del stock occidental de atún rojo del Golfo de México no respaldan la edad de madurez actualmente asumida para este stock, de 8 años. KEYWORDS Sexual maturity, longlining 1 NOAA Fisheries Service. Office of Science and Technology. 1315 East West Highway. Silver Spring, MD 291, USA; Guillermo.Diaz@noaa.gov. 1216
Introduction Atlantic bluefin tuna (Thunnus thynnus) is managed by the International Commission for the Conservation of Atlantic Tuna (ICCAT) under a two stock scenario: the western Atlantic stock (western stock) and the eastern Atlantic and Mediterranean stock (eastern stock). Although there is indication of a certain degree of mixing between both stocks (Rooker et al. 28), they also exhibit strong natal homing behavior. The western stock spawns in the Gulf of Mexico while the eastern stock spawns in the Mediterranean Sea. Tagging studies have shown no indication of mature individuals from one stock entering the spawning grounds of the other stock. Currently, ICCAT assumes an age of 5% maturity for the eastern stock of 4 years old, while for the western stock the assumed age of 5% maturity is 8 years old. However, Diaz and Turner (27) used size frequency distribution of bluefin tuna landed in the Gulf of Mexico to estimate age of maturity and they suggested that age of maturity could be around 12 years. In 29, ICCAT adopted an updated growth curve for the western stock of Atlantic bluefin tuna developed by Restrepo et al. (211). This updated growth curve was very similar to the curve developed by Cort (1991) for the eastern stock. The present study updated the maturity function developed by Diaz and Turner (27) using a different source of size frequency data which was aged using the newly adopted growth curve developed by Restrepo et al. (211). Materials and Methods The size samples used in this study corresponded to samples collected from bluefin tuna caught by the Japanese longline fleet that operated in the Gulf of Mexico in the late 7s early 8s. The samples were provided by the ICCAT Secretariat, lengths corresponded to fork length (FL) measured in cm and class intervals were 2 cm. The data set provided number of individuals (frequency) by class interval in each month/area stratum. The area stratum corresponded to 5 o x5 o or 5 o x1 o cells. Only data from those cells for which the entire area was contained within the Gulf of Mexico were included in this study. Age was estimated from FL using the ICCAT bluefin tuna growth equation developed by Restrepo et al. (211) and slicing conventions adopted by the SCRS. A maturity function of a logistic form was estimated using a catch-curve approach and assuming a natural mortality vector for all ages M=.14. All calculations were carried out using the solver function in Excel. Results The size samples provided by the ICCAT Secretariat contained a total of 7256 records corresponding to 15,585 length measurements. This study used a subset of that dataset that contained 864 records (777 length measurements). These selected records corresponded only to samples collected in 5 o x5 o or 5 o x1 o squares that were fully contained inside the Gulf of Mexico. Sample sizes by month and year are presented in Table 1. Most samples were collected in 1978 and 1979 (81%) and 42% of all samples corresponded to the month of April. Figure 1 shows the size frequency distribution of the samples. Note that for simplicity length classes correspond to 2 cm bins and fish less than 18 cm (2 fish) were not included in the graph. The smallest bluefin tuna in the samples was 114 cm FL and the largest 318 cm FL. The cumulative frequency by 2 cm length bins is shown in Figure 2. Eighty percent of all observations were contained within the range 234-262 cm FL. The size frequency distribution by months for all years combined are shown in Figure 3. One noticeable difference is that the size frequency distributions for March and April show a higher proportion of larger animals than June and July. The proportion of bluefin tuna 27 cm FL was in the order of 13-14% for March and April and only about 5-7% for May and June. July showed an increase in the proportion of large bluefin tuna (29%), however sample size for this month was small (Table 1). For comparison purposes, Figure 4 shows the age distribution of the size samples used by Diaz and Turner (27) and the samples used in the present study. Both samples were aged following the same slicing protocol and applying the Turner and Restrepo (1994) growth parameters. Is it noticeable that the samples used by Diaz and Turner (27) contained a larger proportion of fish in the age range 9-11 yr while the present study used samples with a higher proportion of fish 12-14 yr. 1217
If we compare only the age distribution of the samples from Japanese longline fleet used by Diaz and Turner (27) with the age distribution of the Japanese samples used in this study (both aged with the Turner and Restrepo (1994) growth curve) similar differences can be observed. The age distribution of the Japanese size samples from Diaz and Turner (27) have a higher proportion of age 8-1 fish while the samples used in this study showed an age distribution with higher proportion in ages 12-14 (Figure 5). The difference in the age distribution of the samples used in the present study resulting from using the Turner and Restrepo (1994) or the Restrepo et al. (211) growth curves are presented in Figure 6. As expected, the age distribution of the samples aged using the Restrepo et al. (211) growth curve contained a larger proportion of older fish (mode=16 yr) than the age distribution obtained with the Turner and Restrepo (1994) curve (mode=12 yr). This is because the L parameter value estimated by Restrepo et al. (211) was lower than the one estimated by Turner and Restrepo (1994). Therefore, the Restrepo et al. (211) growth curve assigned older ages to bluefin tuna with a FL > 135 cm than the Turner and Restrepo (1994) curve (Figure 7). As previously mentioned, the growth curve developed by Restrepo et al. (211) for the western stock is very similar to the curve developed by Cort (1991) for the eastern stock (Figure 8). Diaz and Turner (27) estimated 11.8 yr to be the age of 5% maturity for western bluefin tuna. The present study estimated age of 5% maturity to be 12.8 yr (Figure 9) when using the same growth curve used by Diaz and Turner (27). These results reflect the difference in the age distribution between the samples used in these two studies (Figure 3). The estimated age of 5% maturity of the age distribution obtained when using the growth parameters estimated by Restrepo et al. (211) was 15.8 yr, which is 3 yr older than the maturity age estimated using Turner and Restrepo (1994) growth curve. Once again, these results reflect the difference in the age distribution resulting from ageing the same size samples with two different growth curves (Figure 6). Figure 1 compares the estimated maturity curves using the two mentioned growth curves. Discussion Because the Gulf of Mexico is a bluefin tuna spawning ground, it is assumed that all bluefin tuna found in the Gulf are sexually matured. Therefore, the estimation of age of maturity based on size/age frequency is based on this assumption and not on histological study of gonads. The size samples used by Diaz and Turner (27) and the ones used in the present study showed a different age distribution (Figure 4). The samples used by Diaz and Turner (27) corresponded to fish landed by the U.S. pelagic longline fleet (611 fish) during the period 198-25 and fish sampled by the Japanese longline fleet (1783 fish) during 1975-1981. The differences in the age distribution between the size samples used by these 2 studies could be due to different gear size selectivity between fleets (i.e., Japanese longliners catching larger fish than US vessels). In addition, the majority of the size samples used by Diaz and Turner (27) were collected during a later time period. Therefore, the lower proportion of older bluefin in these samples could also be the result of the heavy fishing pressure exerted on this stock during the late 7s and early 8s. Unlike the Turner and Restrepo growth curve (1994), the newly adopted growth curve developed by Restrepo et al. (211) is very similar to the growth curve developed by Cort (1991) for the eastern stock (Figure 8). However, even though these two stocks seem to grow in a very similar manner, there is a large discrepancy between the assumed ages of maturity (4 yr for the eastern stock, 8 yr for the western stock) and even more with the estimated ages by Diaz and Turner (26) and by the present study. All size samples used to determine age of maturity based on size distribution of fish caught in the spawning ground (i.e. Gulf of Mexico) were collected by longline gear. If longline gear were selecting (catching) in the Gulf of Mexico only fish larger than a certain size then the estimated age of maturity would be positively biased. However, a comparison with the size frequency distributions of bluefin tuna caught outside the Gulf of Mexico by US longline vessels clearly indicated that this gear is capable of catching smaller fish than those observed in the Gulf. Using satellite tags, several authors (e.g., Block et al. 25) showed that not all adult bluefin tuna visit the Gulf of Mexico spawning grounds every year. Several satellite tracks have shown bluefin tuna migrating south without entering the Gulf. Instead, they remain for a period of time in an area east of the Bahamas Islands before migrating back north. If these fish are spawning outside the Gulf of Mexico or if these tracks correspond to immature fish remains an open question. Although it is widely accepted that the Gulf of Mexico is a spawning 1218
ground, there is still uncertainty if there are other secondary areas where bluefin tuna might spawn outside the Gulf. This could be similar situation to what has been observed with the eastern stock, where different spawning grounds within the Mediterranean Sea have been recorded. If younger bluefin tuna spawn in other areas and do not enter the Gulf of Mexico until they reach older ages is, at this point, a speculation. However, if that is the case then age of maturity could be less than the estimated by Diaz and Turner (27) and in the present study. The assumed age of maturity currently used in ICCAT was based on the findings of Baglin (1982). Baglin indicated that relatively little development towards maturity was evident in the age 1 through 7 in the western stock and therefore unlikely to reach sexual maturity before age 8. Diaz and Turner (26) indicated that only 2% of the bluefin tuna in their samples were 8 yr or younger. The results of the present study using the Restrepo et al. (211) growth curve show this proportion to be even smaller (.3%). Block et al. (25) showed that the average size of bluefin tuna entering the Gulf of Mexico was 241 cm which corresponds to a 15 yr old fish according to the Restrepo et al. (211) growth curve. Nemerson et al. (2) also showed that the sizes of bluefin tuna found in the Gulf of Mexico are not consistent with the results obtained by Baglin (1982). A comparison between the results of Diaz and Turner (27) and this study also indicated that different size samples from the Gulf of Mexico can provide different results (see Fig. 4 and 5). Therefore, it is recommended that a detail analysis of all size samples from the Gulf of Mexico be conducted to identify the proper samples to estimate a western bluefin tuna maturity function. However, despite the different results that could be obtained using different size samples, the findings by Diaz and Turner (27) and by the present study and information of average size of bluefin tuna provided by other authors (Block et al. 25, Nemerson et al. 2) do not support the currently assumed age of 5% maturity (8 yr). Acknowledgements The author thanks Laurie Kell and Carlos Palma from the ICCAT Secretariat for providing and for aging the size samples used in the present study, and Clay Porch for useful comments that helped to improve this document. References Baglin, R. E., 1982, Reproductive biology of western Atlantic bluefin tuna. Fish. Bull. 8(1): 121-134. Block, B.A., Teo, S.L.H., Walli, A., Boustany, A., Stokesbury, M.J.W., Farwell, C.J., Weng, K.C., Dewar, H. and William, T.D. 25, Electronic tagging and population structure of Atlantic bluefin tuna. Nature, 434:1121-1127. Cort, J.L. 1991, Age and growth of the bluefin tuna (Thunnus thynnus) in the northeast Atlantic. Collect. Vol. Sci. Pap. ICCAT, 35(2): 213-23. Diaz, G.A., Turner, S.C. 27, Size frequency distribution analysis, age composition, and maturity of western bluefin tuna in the Gulf of Mexico from the U.S. (1981-25) and Japanese (1975-1981) longline fleets. Colloct. Vol. Sci. Pap. ICCAT, 6(4):116-117. Nemerson, D., Berkeley, S., Safina, C. 2, Spawning site fidelity in the Atlantic bluefin tuna, Thunnus thynnus: The use of size-frequency analysis to test for the presence of migrant east Atlantic bluefin tuna on Gulf of Mexico spawning grounds. Fish. Bull. 98:118-126. Restrepo, V.R., Diaz, G.A., Walter, J.F., Nielson, J., Campana, S.E., Secor, D. and Wingate, R.L. 211, Updated estimate of the growth curve of western Atlantic bluefin tuna. Aquatic Living Resources 23, 335-342. Rooker, J.R., Secor, D.H., De Metrio, G., Schloesser, R., Block, B.A., Neilson, J.D. 28, Natal homing and connectivity in Atlantic bluefin tuna populations. Science 322(592): 742-744. Turner, S.C. and Restrepo, V.R. 1994, A review of the growth rate of West Atlantic bluefin tuna, Thunnus thynnus, estimated from marked and recaptured fish. Collect. Vol. Sci. Pap. ICCAT, 42(1): 17-172. 1219
Table 1. Sample size by year and month. Month 1975 1976 1977 1978 1979 1981 Total 2 92 17 96 1,15 3 83 1 34 1,111 3 1,529 4 274 3 23 1,217 1,163 74 2,988 5 1 16 89 1,19 1,224 6 1 88 4 138 231 368 135 719 3,682 2,69 14 7,77 7 6 5 4 3 2 1 18 184 188 192 196 2 24 28 212 216 22 224 228 232 236 24 244 248 252 256 26 264 268 272 276 28 284 288 292 296 3 34 38 312 316 Figure 1. Size frequency distribution of bluefin tuna in the samples used in the present study. 1.9.8 Cumulative.7.6.5.4.3.2.1 18 184 188 192 196 2 24 28 212 216 22 224 228 232 236 24 244 248 252 256 26 264 268 272 276 28 284 288 292 296 3 34 38 312 316 Figure 2. Cumulative size frequency distribution of bluefin tuna in the samples used in the present study. 122
1221 1 2 3 4 5 6 7 8 9 1 18 185 19 195 2 25 21 215 22 225 23 235 24 245 25 255 26 265 27 275 28 285 29 295 3 35 31 315 February 2 4 6 8 1 12 14 16 18 185 19 195 2 25 21 215 22 225 23 235 24 245 25 255 26 265 27 275 28 285 29 295 3 35 31 315 March
3 April 25 2 15 1 5 18 185 19 195 2 25 21 215 22 225 23 235 24 245 25 255 26 265 27 275 28 285 29 295 3 35 31 315 16 May 14 12 1 8 6 4 2 18 185 19 195 2 25 21 215 22 225 23 235 24 245 25 255 26 265 27 275 28 285 29 295 3 35 31 315 25 June 2 15 1 5 18 185 19 195 2 25 21 215 22 225 23 235 24 245 25 255 26 265 27 275 28 285 29 295 3 35 31 315 Figure 3. Bluefin tuna size frequency distribution by month all years combined. 1222
.3 Relative.25.2.15.1.5 Diaz and Turner (27) This study (old growth curve). 3 4 5 6 7 8 9 1 11 12 13 14 15 16 17 18 19 2 21 22 Figure 4. Age frequency distribution of size samples used in the present study (black bars) and by Diaz and Turner (27) (gray bars). Note that the size samples from the Diaz and Turner (27) correspond to a combination of samples from the Japanese and US longline fleets. Both samples were aged following the same slicing protocol and applying the Turner and Restrepo (1994) growth parameters..3 Relative.25.2.15.1.5 Diaz and Turner (27) This study (old growth curve). 3 4 5 6 7 8 9 1 11 12 13 14 15 16 17 18 19 2 Figure 5. Age frequency distribution of size samples from the Japanese longline fleet used in the present study (red bars) and by Diaz and Turner (27) (blue bars). Both samples were aged following the same slicing protocol and applying the Turner and Restrepo (1994) growth parameters. 1223
.3 Relative.25.2.15.1.5 Restrepo et al. (29) Turner and Restrepo (1994). 3 4 5 6 7 8 9 1 11 12 13 14 15 16 17 18 19 2 21 22 23 24 25 26 27 28 29 3 Figure 6. Age frequency distribution of the size samples used in the present study resulting from using the Turner and Restrepo (1994) (red bars) and the Restrepo et al. (211) (blue bars) growth parameters. 35 3 25 SFL (cm) 2 15 1 Turner and Restrepo (1994) Restrepo et al. (29) 5 1 2 3 4 5 6 7 8 9 1 11 12 13 14 15 16 17 18 19 2 21 22 23 24 25 26 27 28 29 3 Figure 7. Western bluefin tuna growth curves. 1224
35 3 25 FL (cm) 2 15 1 Restrepo et al. (29) Cort (1991) 5 5 1 15 2 25 3 Figure 8. Growth curves developed for the western stock by Restrepo et al. (29) and for the eastern stock by Cort (1991). Percent matured 1.9.8.7.6.5.4.3.2.1 Diaz and Turner (27) This study 5 1 15 2 Figure 9. Maturity curves estimated by Diaz and Turner (27) and in the present study using the growth curve developed by Turner and Restrepo (1994). 1225
Percent matured 1.9.8.7.6.5.4.3.2.1 Restrepo et al. (29) Turner and Restrepo (1994) 5 1 15 2 25 3 Figure 1. Maturity curves estimated by Diaz and Turner (27) and in the present study using the growth curve developed by Turner and Restrepo (1994). 1226