Harumi Yamada

Distribution, growth and hatch date of juvenile Pacific bluefin tuna Thunnus orientalis in the coastal area of the Sea of Japan

Mid-water trawl surveys were conducted from late August to late September in 1999 and 2004 in order to investigate the distribution pattern, hatch date, and growth of juvenile Pacific bluefin tuna Thunnus orientalis in the Sea of Japan. Juveniles were collected at the stations where ambient water temperature (mean water temperature from surface to 30 m deep, WT0−30) was 23.4–25.9°C, and most of them were found in waters where WT0−30 was higher than 24°C. Sampled juveniles ranged 108–280 mm fork length. Based on otolith analysis, they were estimated to grow to approximately 180 and 250 mm at 60 and 90 days old, respectively, and showed similar growth to that of Atlantic bluefin tuna in the Mediterrannean Sea. The back-calculated hatch date of the samples was mostly in July and most juveniles spawned in the Sea of Japan.

Thermal adaptation of pacific bluefin tuna Thunnus orientalis to temperate waters

Immature Pacific bluefin tuna Thunnus orientalis, tagged with archival tags, were released near Tsushima Island in the East China Sea during the winters of 1995 through 1998. Time-series data for ambient and peritoneal cavity temperatures, recorded every 128 or 256 s for 23 fish recovered, were analyzed. The objective of this study was to clarify the process of development of thermoconservation ability with growth in relation to adaptive mechanisms to cooler temperate waters. According to the results, mean ambient temperatures ranged from 14.9 to 20.7°C, which is almost within the optimum temperature range according to previous reports. Mean peritoneal temperatures were higher than ambient temperatures (19.7–27.3°C), but never reached 35°C, which would induce overheating. Although the mean thermal differences between peritoneal and ambient temperatures increased with body size, the rate of increase decreased with body size. A heat budget model suggests that as the insulation of the body develops, the estimated mean values of internal heat production decrease with body size. This is probably due to the allometric scale effect and explains why the thermal difference does not increase quickly with body size. It is likely that Pacific bluefin tuna inhabit cooler temperate waters in mid-latitude regions to avoid overheating.

Differences in vertical distribution and movement of Pacific bluefin tuna (Thunnus thynnus orientalis) among areas: the East China Sea, the Sea of Japan and the western North Pacific

Immature Pacific bluefin tuna, Thunnus thynnus orientalis, marked with archival tags, were released off Tsushima Island in the East China Sea. Timed data on swimming depth, ambient temperature and peritoneal-cavity temperature were recorded every 128 s to evaluate seasonal and spatial changes and the effects of ambient temperature on vertical distribution. The tuna swam within the surface mixed layer in the East China Sea during both night and day in winter, whereas they spent most of their time at the surface in summer when a thermocline developed. Bluefin tuna migrating into the Sea of Japan and the western North Pacific usually stayed at the surface and did not show large vertical migrations. This was presumably because the vertical distribution of their prey in these regions is markedly different from that in the East China Sea. This suggests that seasonal and spatial differences in the vertical distribution of water temperature and prey determine the pattern of vertical distribution and movements of bluefin tuna.

Immature Pacific bluefin tuna, Thunnus orientalis, utilizes cold waters in the Subarctic Frontal Zone for trans-Pacific migration

The habitat and movements of a Pacific bluefin tuna were investigated by reanalyzing archival tag data with sea surface temperature data. During its trans-Pacific migration to the eastern Pacific, the fish took a direct path and primarily utilized waters, in the Subarctic Frontal Zone (SFZ). Mean ambient temperature during the trans-Pacific migration was 14.5u2009±u20092.9 (°Cu2009±u2009SD), which is significantly colder than the waters typically inhabited by bluefin tuna in their primary feeding grounds in the western and eastern Pacific (17.6u2009±u20092.1). The fish moved rapidly through the colder water, and the heat produced during swimming and the thermoconservation ability of bluefin tuna likely enabled it to migrate through the cold waters of the SFZ.

Why do young Pacific bluefin tuna repeatedly dive to depths through the thermocline

Young Pacific bluefin tuna Thunnus orientalis with archival tags were released in the East China Sea. Time-series data for depth, and ambient and peritoneal temperatures for nine fish, recorded every 128 s, were analyzed. Our objectives were to describe monthly changes in diving patterns in relation to the ambient thermal structure and the occurrence of feeding events during March–June, and to discuss possible reasons why bluefin tuna repeatedly dived to depths below the thermocline in terms of their thermoconservation mechanisms. It was found that the fish repeatedly dived through the thermocline at intervals of 1.2 h on average, and the dive frequency was high during March–May. However, the dive frequency and periodicity decreased in June, when the gradient became steeper. In contrast, feeding events increased in June. These results indicate that from March to May, bluefin tuna repeatedly dive because food biomass is inadequate at the surface, and they stop undertaking repeated dives in June when food becomes more readily available at the surface, in addition to low visibility caused by low solar radiation. Further, the range of heat transfer times for these fish was so long that their peritoneal temperature was probably maintained by engaging in brief dives. The periodicity of dives may lead to a lower fluctuation in the peritoneal temperature, suggesting that the dives are a kind of behavioral thermoregulation.

The effect of water temperature on habitat use of young Pacific bluefin tuna Thunnus orientalis in the East China Sea

Immature Pacific bluefin tuna Thunnus orientalis, tagged with archival tags, were released near Tsushima Island in the East China Sea (ECS) during the winters of 1995, 1996 and 1997. Geolocations were estimated using the archival tags from recovered fish. These data, together with sea surface temperature (SST) data from satellite remote sensing, are used to describe the habitat used by these bluefin in the ECS from January to June for 3 years (1996, 1997, 1998), and to asses the effect of water temperature on fish distribution and movement. The results indicate that their geolocations ranged from the area north-east of Tsushima Island to the offshore area in the southwest. However, the area of highest density differed among years, being furthest south in 1996 and furthest north in 1998. The differences were probably caused by changes in SST associated with La Niña (1996) and EI Niño (1998) events. Another densely populated area was identified in offshore waters of latitude 28–30 oN in 1996 (only), on the cold side of the Kuroshio front. These fish may have been prevented from moving northwards by an intrusion of Kuroshio water of approximately 25°C into the region immediately to the north-east.

Recruitment abundance index of Pacific bluefin tuna using fisheries data on juveniles

The recruitment abundance index of Pacific bluefin tuna Thunnus orientalis was estimated from 1980 to 2003 fishing year by using the troll fishery data in Nagasaki Prefecture, western Japan. It has been shown that the troll fishery in Nagasaki Prefecture operates with good time-area coverage of the species habitat, and that the fishing power slightly changed during the period analyzed, based on fisheries statistics, published information, and interviews with the fishers. Average catch per unit effort (CPUEs) were standardized by a generalized linear model (GLM) considering the effects of fishing year, season and landing area. Standardized CPUE of age-O bluefin tuna showed larger fluctuations year by year than the nominal CPUE combined for all ages. High CPUEs in fishing years of 1981, 1994, 1996 and 1999 were observed. Data from these years agreed with the higher recruitments estimated by virtual population analysis (VPA) or higher catch of age-O fish reported for the Pacific side. The age-specific standardized CPUE of age-O bluefin tuna in this study was judged to be a useful indicator of recruitment.