Pacific bluefin tuna (PBF), Thunnus orientalis (Temminck and Schlegel, 1844), is highly migratory and capable of traversing large distances throughout the North Pacific Ocean. The majority of spawning activity has been reported only from the Sea of Japan and western parts of the Pacific Ocean. We have conducted larval surveys in these areas since 2009 (late period), and used generalized additive models (GAM) with presence/absence data to analyze larval distribution with respect to oceanographic conditions, such as sea surface temperature and chlorophyll-a concentration. Here, we compare our survey data with some historical data collected from 1979 to 1988 (early period). In total, 9192 individual PBF larvae were collected from 1979 to 2015, and body lengths ranged from 2 to 11 mm. In the early period, the relatively higher probability area for the presence of PBF larvae in the Pacific Ocean was wider than that in the Sea of Japan. However, in the late period, the relatively higher probability area for presence in the Sea of Japan was wider than in the early period. The spline function of sea surface temperature for PBF larvae was lower in the early period than in the late period, though large uncertainty in the Pacific Ocean in the late period was observed. These results suggest that the change in the distribution pattern of PBF larvae may be caused by ocean warming.
The age and growth of North Pacific albacore Thunnus alalunga were investigated using obliquely sectioned sagittal otoliths from samples of 126 females and 148 males. Otolith edge analysis indicated that the identified annulus in a sagittal otolith is primarily formed during the period from September to February. The assessments of the fish age at first annulus formation indicated that the first annulus represents an age of <1 year. This study presents an age estimate (0·75 years) for the formation of the first annulus. The oldest fish ages observed in this study were 10 years for females and 14 years for males. The von Bertalanffy growth parameters of females estimated were L ∞ = 103·5 cm in fork length ( L F ), K = 0·340 year −1 and t 0 = −0·53 years, and the parameters of males were L ∞ = 114·0 cm, K = 0·253 year −1 and t 0 = −1·01 years. Sexual size dimorphism between males and females seemed to occur after reaching sexual maturity. The coefficients of the power function for expressing the L F –mass relationship obtained from sex‐pooled data were a = 2·964 × 10 −5 and b = 2·928.
Establishing sampling techniques for juveniles of tuna species is essential for understanding their ecology and population dynamics because only individuals showing rapid growth during larval and juvenile stages are able to survive. This study is the first report on the successful sampling of juveniles of tuna species using the larva catcher (LC)-type trawl net, a pelagic trawl gear with a 10 × 10-m mouth opening, 40-m total length, and 5 × 5-mm mesh size. It was used in a research cruise in May-June 2009. The LC net was horizontally towed for 30 min at depths of 0 m-10 m at speeds of about 1.8 m/s. Overall, 66 and 163 juveniles of Pacific bluefin tuna (PBT) and yellowfin tuna (YFT) were collected; the standard length (SL) of the PBT and YFT ranged from 15.0 to 29.7 mm and 11.7 to 34.8 mm, respectively. The total operation time for the trawls of the LC-type net was much shorter than that of the usual trawls; therefore, a larger number of operations can be conducted in a day. We concluded that the LC-type trawl net can be used for collecting tuna juveniles because of its catchability and ease of operation.
The vertical distribution of juvenile skipjack tuna Katsuwonus pelamis was investigated in the tropical western Pacific (0-25°N and 130-160°E) based on midwater trawl sampling from October to December in 1992 to 1996. Most juveniles were sampled between depths of 40 and 120 m, that is, at depths ranging immediately above and below the thermocline, and at temperatures between 20 and 30°C. Relatively lower temperatures were observed in the pelagic zone of the research area from 1992 to 1994 (period of a shallow thermocline), in contrast to relatively higher temperatures from 1995 to 1996 (with a deep thermocline). The vertical distribution of skipjack juveniles became shallower from 1992 to 1993, whereas it became deeper in 1995 and 1996. These findings suggest that the vertical distribution of skipjack tuna during the juvenile period changed annually relative to the vertical temperature profile. Moreover, fluctuations in vertical temperature are believed to affect the expansion or contraction of the vertical habitat of skipjack juveniles in the pelagic zone. The mean standard length of juveniles collected in 1994 at a depth of 80-100 m in the North Equatorial Counter Current area was significantly larger than that of juveniles collected at 40-60 m. These findings suggest that the vertical distribution of juvenile skipjack tuna becomes deeper in line with their growth.
In order to clarify the ecological habit of the skipjack tuna, Katsuwonus pelamis, at the juvenile stage, we implemented a research program from 1992 to 1996. For the main activity of the program, we conducted survey cruises to determine the distribution of juvenile skipjack tuna in the offshore areas of Palau and Micronesia, in the tropical western Pacific, during October to December. We developed a new sampling gear, a high-speed midwater trawl net with a large mouth that enables to collect large numbers of juvenile tunas. We analyzed the relationship between the horizontal and vertical distribution of the juveniles and oceanographic conditions, feeding habit of the juveniles based on stomach content analysis, and growth process of juveniles by otolith increment analysis. The results of our analyses indicated that juvenile skipjack tuna was widely distributed in the offshore area from the North Equatorial Current to the North Equatorial Counter Current, around the Palau and Micronesian Islands. We concluded that these areas were important for juvenile skipjack tuna as a nursery ground.
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