SCRS//00 Col. Vol. Sci. Pap. ICCAT, 0(): 0-0 () SIZE FREQUENCY DISTRIBUTION ANALYSIS, AGE COMPOSITION, AND MATURITY OF WESTERN BLUEFIN TUNA IN THE GULF OF MEXICO FROM THE U.S. (-) AND JAPANESE (-) LONGLINE FLEETS Guillermo A. Diaz and Stephen C. Turner SUMMARY Bluefin tuna size frequency data from the U.S. and Japanese pelagic longline fleets in the Gulf of Mexico were analyzed. Analyses were performed on data from fish caught within the period January-June to limit it to sexually mature individuals. Results from the ageing process (both data sets combined) indicated that approximately % of all fish present in the Gulf of Mexico were year old or older. Age of 0% maturity was estimated to be at around years. RÉSUMÉ Le présent document inclut une analyse des données de fréquence de tailles du thon rouge des flottilles palangrières pélagiques des Etats-Unis et du Japon dans le Golfe du Mexique. Les analyses ont été réalisées sur les données de poissons capturés entre janvier et juin pour les limiter aux spécimens arrivés à la maturité sexuelle. Les résultats du processus de détermination de l âge (les deux jeux de données combinés) indiquaient que près de % des poissons présents dans le Golfe du Mexique avaient ans ou plus. L âge à maturité (0% des individus) a été estimé s élever à ans environ. RESUMEN Se analizaron los datos de frecuencias de talla del atún rojo de las flotas palangreras pelágicas de Japón y Estados Unidos en el Golfo de México. Se realizaron análisis de los datos de los ejemplares capturados en el periodo de enero a junio para limitarse a ejemplares sexualmente maduros. Los resultados del proceso de determinación de la edad (combinando ambos conjuntos de datos) indicaban que aproximadamente el % de todos los ejemplares presentes en el Golfo de México tenían nueve o más años. Se estimó que la edad de madurez del 0% se situaba en torno años. KEYWORDS Bluefin tuna, maturity, size distribution, age composition U.S. Department of Commerce, National Oceanographic and Atmospheric Administration, National Marine Fisheries Service, Southeast Fisheries Science Center, Sustainable Fisheries Division, Virginia Beach Dr., Miami, FL. This is a Sustainable Fisheries Division contribution no. SFD--0. 0
. Materials and methods Bluefin tuna are thought to spawn in the Gulf of Mexico and the Florida Straits (Richards ), and they are know to migrate to and from the Gulf of Mexico before and after the spawning season. Size frequency distributions of bluefin tuna caught by the U.S. pelagic longline fleet in the Gulf of Mexico from to were estimated. Round weights from dealer reports were used to estimate fork length (FL) using the monthly equations developed by Parrack and Phares () and adopted by the SCRS. Following ICCAT convention, we refer to FL as the straight length measurement from the snout to the fork. Size frequency distributions were also estimated for bluefin tuna sampled by the Japanese fleet in the Gulf of Mexico for the period - (data provided by ICCAT). Size information for the Japanese data set corresponded to FL. Only bluefin tuna reported caught from January through June were included this analysis. For both data sets, age was estimated from FL using the ICCAT bluefin tuna growth equation developed by Turner and Restrepo () and slicing conventions adopted by the SCRS. A maturity function of a logistic form was estimated for each fleet separately and for both fleets combined as well. The estimation procedure assumed a constant natural mortality for all ages throughout the period analyzed (M=). Mature biomass at age was estimated using weight as a proxy by multiplying the estimated maturity at age by the average observed weight at age in the period -.. Results and discussion The U.S. bluefin tuna data set consisted of 0 weight records. Table shows that the highest landings from the Gulf of Mexico by the U.S. fleet corresponded to the period -. The total number of bluefin tuna measured by the Japanese longline fleet was (Table ). Relative FL frequency distributions of bluefin tuna caught by the U.S. pelagic longline fleet in the Gulf of Mexico are presented in Figure. In the entire time series, only fish (%) measured < cm FL. The largest estimated FL was cm. The relative proportion of the largest bluefin tuna in this time series decreased from the mid-0s onward. Years - also showed a decrease in the proportion of BFT < cm FL. Table shows the estimated FL at age by month. Fish larger than the growth model predicted length at age were assigned to the age group +. Results of the ageing process showed that only 0.% ( fish) of all BFT landed by the U.S. fleet (all years combined) were younger than years old; while % ( fish) were older than yr old. Figure shows the relative age frequency distribution of BFT landed by the U.S. pelagic longline fleet in the Gulf of Mexico by year. Applying the same ageing technique to the Japanese data yielded similar results (Figure ). Compared to the U.S. data, the Japanese samples had higher proportions of fish younger than year old (.%) and older than year old (%). But, both data sets showed similar modal ages (Figure ). The lower proportion of fish older than years old in the U.S. data could be interpreted as a result of heavy fishing pressure in the early years of the fishery. The data sets used in this document were limited to fish caught between the months of January and June in an attempt to age sexually mature bluefin tuna present in the area. Both data sets showed the modal age to be around -. Data from the U.S. fleet seems to indicate that a negligible proportion of fish (0.%) attained sexual maturity prior to age. The Japanese data showed a higher proportion of younger fish in it samples. Unlike the U.S. data, in the Japanese data the proportion of fish age (.%) was comparable to the proportion of fish year old (.%). These differences in the proportion of younger fish between both data sets are not interpreted as changes in the maturity schedule of this species since both distributions showed the same modal ages. The estimated maturity curves from the U.S. and Japanese data sets indicated that age of 0% maturity correspond to. and. years, respectively (Figure ). Estimated mature biomass at age is presented in Figure. In the context of stock assessments, the estimated age of 00% maturity will require VPA models to be set to use a 0+ or + age group.
References PARRACK, M. and P. Phares.. Aspects of the growth of Atlantic bluefin tuna determined from markrecapture data. Col. Vol. Sci. Pap. ICCAT, (): -. RICHARDS, W.J.. Spawning of bluefin tuna (Thunnus thynnus) in the Atlantic Ocean and the adjacent seas. Col. Vol. Sci. Pap. ICCAT, (): -. Table. Number of bluefin tuna landed by the U.S. pelagic longline fleet in the Gulf of Mexico from 0 to. Year No. records Year No. records 0 0 0 0 Table. Number of bluefin tuna measured from the Japanese longline fleet operating in the Gulf of Mexico from to. Year Fish sampled 0 0 0
Table. Estimated upper limit of fork length (cm) at age by month based on the growth model adopted by the SCRS for western Atlantic bluefin assessments. Fish larger than the predicted length at age were assigned to the age group +. Month Age 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0. 0. 0 0. 0. 0 0 0 0 0 0 Figure. Relative fork length (FL) frequency distribution of bluefin tuna caught by the U.S. pelagic in the Gulf of Mexico (-).
Figure (continued) 0 0 0 0
0 0 y Figure. Relative age frequency distribution bluefin tuna caught by the U.S. pelagic in the Gulf of Mexico (-).
Figure (continued)
0 0 0 0 0 0 0 0 Figure. Relative age frequency distribution bluefin tuna sampled by the Japanese pelagic in the Gulf of Mexico (-).
US -0 JAP - 0 Figure. Relative age frequency distribution of bluefin tuna caught in the Gulf of Mexico by the U.S. pelagic longline fleet (blue bars) in - and sampled by the Japanese fleet (red bars) in -. Relative frequencies for each data set were estimated by combining data from all years. 0. Proportion mature 0. 0. 0. 0. US -0 JAP - Combined 0 0 0 Age Figure. Estimated maturity curves for western bluefin from U.S. landings (blue line), Japanese data (red line) and both fleets combined (green line).
00 Biomass mature (kg) 00 00 00 00 US -0 JP - combined last assessment 00 0 0 0 Age Figure. Estimated mature biomass at age using weight as a proxy from U.S. landings (blue line), Japanese data (red line) and both fleets combined (green line). The dashed line corresponds to the maturity function used in the previous ICCAT assessment. 0