Hideaki Nakata

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.

Entrainment of coastal water into a frontal eddy of the Kuroshio and its biological significance

The Pacific coastal areas of Japanese Island are major spawning grounds of various fishes. It is considered that large amount of eggs and larvae are dragged into the Kuroshio front so that the survival of fish larvae at the front is important for their recruitment. From this viewpoint, a low-salinity water mass, which was withdrawn from the coastal area to the Kuroshio front, was investigated by drifters, in addition to fine-scale hydrographic observations and water sampling in and around the Kuroshio frontal area off Enshu-nada. The drifters were transported to the east within the low-salinity water along the Kuroshio front in the first stage, and were thereafter entrained into an eddy, which was caused by the frontal meander. They moved closely to each other along the front, but diverged in the eddy. This movement of the drifters coincided with the deformation of low-salinity water mass; the low-salinity water concentrated at the Kuroshio front surrounded by strong salinity gradients at first, while it spread out horizontally and became vague in the shallow surface layer in the frontal eddy. Comparing temperature sections across the front, the strong upwelling was detected in the eddy. Limiting factors for primary production and growth rates were calculated in six sections using the observed temperatures and concentrations of nutrients. In the frontal area of the Kuroshio, low concentration of nutrients limited the primary production shallower than 50 m. Due to the low productivity, concentration of chlorophyll a in the low-salinity water tended to decrease, although the initial concentration was high. Once the coastal water mass was entrained into the frontal eddy, on the contrary, the concentration recovered due to the enhanced primary production in the subsurface layer supported by the upwelling of nutrient-rich water. Fish larvae in the low-salinity water are assumed to use the new production in the eddy; otherwise, they would starve. The entrainment process, which was probably caused by offshoreward movement of the Kuroshio, holds the key to successive survival and recruitment of fish larvae in the Kuroshio system.

Effects of marine ciliates on survivability of the first-feeding larval surgeonfish, Paracanthurus hepatus: laboratory rearing experiments

The contribution of ciliates as a food source to survival of first-feeding surgeonfish larvae, Paracanthurus hepatus, was examined in rearing experiments. The larvae were exposed to eight treatments; i.e. a tintinnid, Amphorellopsis acuta (1.0 × 104, 5.1 × 103 and 2.2 × 103 cells l−1) and a naked ciliate, Euplotes sp. (1.3 × 104, 8.0 × 103 and 5.0 × 103 cells l−1), plus two controls without ciliates. Highest survival of the larvae over the first 4–8 days was observed in the highest density of A. acuta. Rearing experiments also showed that the survivals of larvae fed with A. acuta were higher than those fed with Euplotes sp. Gut content analyses revealed loricae of A. acuta in the larvae. Although Euplotes sp. (lacking loricae) was never recognized in those larval guts, feeding on Euplotes sp. by larvae was confirmed using the ciliate labeled with fluorescent microspheres, implying that the feeding on naked ciliates by fish larvae has been overlooked. The results strongly suggested that both tintinnid and naked ciliates play important roles as alternative food sources to copepod nauplii by enhancing the survivability of fish larvae, especially those with a smaller mouth.

Effects of frontal eddies on the distribution and food availability of anchovy larvae in the Kuroshio Extension

The frontal area of the Kuroshio Extension was observed in June 1997 and May 1999. In 1997, anchovy larvae were most abundant in a frontal region of the eddy generated in association with a northward intrusion of warm water from the Kuroshio Extension. High nutrient concentrations apparently overlapped with the larval aggregation, while chlorophyll-a concentration was rather low all over the region. In 1999, observations were made along a two-day trajectory of a drifter released from the eddy centre, which was recognized by a satellite image. Chlorophyll-a concentration was high in the eddy centre but decreased with time. By contrast, abundance of copepod nauplii in the eddy increased with time, and eventually doubled. These results suggest that the frontal eddy contributes to both copepod production and horizontal entrainment or aggregation of anchovy larvae in the frontal region. The frontal eddy possibly benefits the growth and survival of fish larvae in the oligotrophic oceanic water of the Kuroshio Extension region.

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 14.5 ± 2.9 (°C ± SD), which is significantly colder than the waters typically inhabited by bluefin tuna in their primary feeding grounds in the western and eastern Pacific (17.6 ± 2.1). 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.

Tidal-jet and vortex-pair driving of the residual circulation in a tidal estuary

Abstract Many tidal estuaries are composed of narrow constrictions and wide basins, forming a “strait-basin system”. A typical example of such a system is that of the Akashi Strait and Osaka Bay in the Seto Inland Sea, Japan. In this system, tidal currents in the straits are much stronger than those within the basins. Therefore the mass transport processes are advection rather than diffusion and governed by strongly non-linear effects. In order to reveal the flow and transport mechanism in the strongly non-linear effect, field observations have been conducted. The observations include flow-visualization experiments by the simultaneous use of several shipboard acoustic Doppler current profilers. In the strait-basin system, the tidal-jet from the strait generates a moving vortex-pair, which is smaller than the tidal excursion. This vortex-pair carries vorticity originally generated at the separation point in the strait far into the basin during each tidal cycle. The moving vortex replenishes the vorticity of the basin-wide residual circulation, which is larger than the tidal excursion. This residual circulation further acts on the trajectories of the moving vortices. Generation of moving vortices from a tidal-jet is not unique but common and essential phenomena in many strait-basin systems. These systems are filled with vortices.

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.

Feeding Strategy of Japanese Sand Lance Larvae in Relation to Ciliated Protozoa in the Vicinity of a Thermohaline Front

Larval feeding and survival strategies are described on a Japanese sand lance, Ammodytes personatus Girard, collected in the vicinity of a thermohaline front in the Ise Bay, in comparison with those of other predominant larvae, Hexagrammos spp. and Sebastiscus marmoratus (Cuvier). First-feeding A. personatus larvae (3.1–3.9 mm NL) fed primarily on tintinnid ciliates, subsequently switching to copepod nauplii (4.0–7.9 mm NL larvae) and post-naupliar copepods (8.0–11.3 mm NL larvae). First-feeding Hexagrammos spp. larvae (6.5–6.9 mm NL) fed primarily on post-naupliar copepods, and first-feeding S. marmoratus larvae (<4.0 mm NL), mostly on copepod nauplii. The different food preferences of these species at first-feeding were related to their different mouth widths (0.15–0.19, 0.52–0.56 and 0.32–0.40 mm, respectively) and/or body size (3.1–3.9, 6.5–6.9 and 3.2–3.9 mm NL, respectively). Ciliate-feeding by first-feeding A. personatus larvae was strongly related to the convergence of the larvae and their prey near the thermohaline front, densities of both being greater on the inshore side of the frontal zone. In conclusion, the aggregation of ciliates near the thermohaline front may have improved feeding conditions and survival of first-feeding A. personatus larvae.