Reproductive biology of female Pacific bluefin tuna, Thunnus orientalis, in the Sea of Japan
Introduction
Pacific bluefin tuna, Thunnus orientalis (Temminck & Schlegel), is one of the largest tuna species and is found throughout the Pacific Ocean. This species is an international fishery resource with high commercial value and is fished in many countries with various fishing equipment (ISC, 2014). The stock assessment for Pacific bluefin tuna is conducted by the International Scientific Committee for Tuna and Tuna-like Species in the North Pacific Ocean (ISC). The latest assessment, in 2014, concluded that the spawning stock biomass is near its historically lowest level (ISC, 2014). Biological parameters such as age, maturity schedule, fecundity, spawning frequency, and duration of the spawning period are essential for assessing the stock status as fishery resources. However, information on the reproductive parameters of Pacific bluefin tuna is scarce (ISC, 2013). Pacific bluefin tuna have two spawning grounds, one in the northwestern Pacific Ocean during May and June and one in the Sea of Japan during July and August. Larvae have been collected in May and June from waters off Taiwan to areas near the Ryukyu Islands in the northwestern Pacific Ocean (Yabe et al., 1966, Ueyanagi, 1969, Tan and Chen, 1975, Tanaka et al., 2006, Abe et al., 2014). Histological observations of ovaries indicate that the spawning activities in the northwestern Pacific Ocean start in May and peak in late May and early June (Chen et al., 2006). In the Sea of Japan, information on spawning grounds and spawning seasons has mainly been derived from observations of the occurrence of larvae and juveniles. Larvae have been collected in July and August (Okiyama, 1974, Okiyama and Yamamoto, 1979, Kitagawa et al., 1995, Abe et al., 2014), and spawning date estimates based on counting daily increments in otoliths have revealed that juveniles collected in the Sea of Japan hatch during July and August (Tanaka et al., 2007, Itoh, 2009). Nonetheless, few histological investigations of gonads, which is the best method to investigate fish reproductive biology, have been conducted in the Sea of Japan. The objective of this study is to better understand the reproductive biology of female Pacific bluefin tuna caught in the Sea of Japan using histological analyses.
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Sample collection and sex ratio
A total of 1040 fresh ovaries from landed female Pacific bluefin tuna caught with commercial purse seines in the Sea of Japan were sampled in 2011 and 2012 at Sakai Port in Tottori Prefecture (Fig. 1). The landing period at Sakai Port was late May to early August. The proportion of port sampling times to landing times were 84.4% in 2011 and 85.7% in 2012. Port samplings were conducted around 0800 h and the mean time between catch (ending time of operation) and sampling was 29 h 24 min (SD 14 h 56
Sex ratio
The numbers of males and females landed in 2012 were aggregated by month and classified as early, middle and late. Chi-square tests indicated no differences in the numbers of males and females in each period (Table 2; P > 0.1). The difference in the numbers of males and females through the total sampling period was not significant (P = 0.5983), indicating nearly equal sex ratios.
Occurrence of ovarian developmental classes
Of the 1040 ovaries, 139 ovaries were immature class, 20 ovaries were developing class, 88 ovaries were spawning capable
Sex ratio
The nearly equal sex ratios observed in the present study are consistent with results for Pacific bluefin tuna in the waters off Taiwan, in the spawning ground of the northwestern Pacific Ocean (Chen et al., 2006). For Atlantic bluefin tuna (T. thynnus), sex ratios differ by sizes for the eastern stock around the Balearic Islands spawning ground (Aranda et al., 2013b) and for larger fish in spawning schools of the western stock (Baglin, 1982). The possibility of such differences in size classes
Conclusion
This investigation describes some reproductive parameters for smaller and younger female Pacific bluefin tuna, especially from spawning schools in the Sea of Japan. Investigations on maturity and reproductive activity of fish caught with other fishing gear throughout the Sea of Japan are also needed to clarify their reproductive biology. Spawning frequency and batch fecundity evaluated in this study were higher than those of larger and older Pacific bluefin tuna in the northwestern Pacific
Acknowledgements
This research was implemented as part of the Research Project on Japanese Bluefin Tuna conducted by the Fisheries Agency of Japan. We sincerely appreciate the support of the Tottori Prefectural Fisheries Experiment Station, the Fisheries Cooperative Association of Sakaiminato, and the fishermen who cooperated in sample collection.
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