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To examine the habitat usage of adult Pacific bluefin tuna (PBF), electronic tagging was conducted in the Sea of Japan during May and June of 2012-2017. Archival tags were internally implanted and pop-up satellite archival transmitting tags were deployed; data on the horizontal movements and diving behaviours of 36 individual PBF were successfully retrieved. In the summer spawning season, the tagged PBF were concentrated near Sado Island and Oki Island in the Sea of Japan, and they were distributed widely to the southwest (near Tsushima Island) or northeast (near the Tsugaru Strait) in the autumn and winter. We obtained the first long-term tracking record (246 d) for adult PBF, and this individual exhibited residency in a known spawning region during the spawning season in the proximity of warm-core eddy features. This fish spent most of the daytime below the thermocline between 30 and 150 m depths where the surface ambient temperature was 26.0 ± 1.5°C, but at night it ventured into the warm surface layer. Its whole-body heat transfer coefficient increased when it experienced warm waters (≥24°C), which we suggest is a physiological response to avoid overheating. The mean peritoneal cavity temperature was only 1.8°C higher than the ambient temperature, compared with 6.9°C higher during the cooler autumn-winter period. Our hypothesis is that the warm surface temperatures found in the spawning grounds induce a physiology-reproduction trade-off in adult PBF, which must behaviourally and physiologically thermoregulate their body temperature to gain spatial and temporal access to oceanographic conditions that may promote larval survivorship and growth.

Differences in the reproductive traits of Pacific bluefin tunaThunnus orientalis(PBF) in the Sea of Japan (SOJ) were investigated to evaluate the reproductive potential in 3 fishing grounds (i.e. northern, central and southern areas). The size distribution of PBF differed among the fishing grounds, with smaller fish (100-130 cm in fork length, FL) being caught in the central and northern areas of the SOJ. Spawning-capable females were found at sea surface temperatures (SSTs) between 17 and 25°C, with most SSTs >21°C. Seasonal changes in SST differed among the fishing grounds, and the period when SST exceeded 21°C differed in each fishing ground. Spawning-capable females were found at different times among the fishing grounds, and were observed throughout the fishing season in the southern area of the SOJ. The mean ± SD relative batch fecundity was estimated to be 69.8 ± 45.3 and 92.7 ± 43.1 oocytes g-1in the central and southern areas of the SOJ, respectively. The total spawning fraction was 0.2, 0.38 and 0.73 in the northern, central and southern areas of the SOJ, respectively. SST and FL were positively correlated with the batch fecundity. In addition, FL, SST and location affected the probability of occurrence of spawning-capable fish among mature fish. Therefore, the difference in the observed reproductive potential of PBF among fishing grounds in the SOJ can be linked to the differences in seasonal increases in SST and the size distribution of adult fish.

Yellowtails Seriola quinqueradiata are an important fishery resource around Japan. Here we investigated the movement ecology and habitat utilization of this migratory fish. Archival tags were implanted in 26 adult yellowtails (61-90 cm in fork length) to examine their seasonal movement patterns and vertical distribution. Yellowtails were captured and released around Noto Peninsula in the Japan Sea on 27 May 2004. Eight individuals were recaptured more than 2 mo later, and we analyzed their daily position and vertical movement with ambient water temperature recorded in 60 or 120 s intervals. Most yellowtail individuals moved from the central coast of Japan to the north (from the west of Tsugaru Strait to the west of Hokkaido) in June. Individuals resided in the northern part of the Japan Sea from summer to mid-fall (late July to late October). Seasonal thermoclines developed during this northward movement and subsequent period of residency, with individuals primarily occupying the surface mixed layer during the daytime and at night; however, individuals made frequent short dives through the thermocline, especially during the daytime. These phenomena may be related to foraging based on the distribution of their prey. In late October, yellowtails started a rapid southward movement, primarily remaining in vertical thermal mixing coastal areas, and not entering the coldwater masses that formed offshore. This southward movement pattern might indicate that yellowtails avoid cold offshore water temperatures.

Archival electronic tags were internally implanted in 713 age-0 Pacific bluefin tuna (PBF) caught in their nursery waters off the southern coast of Japan and in the East China Sea over an extended study period (1995–2015) to clarify the spatial and temporal variability of their trans-Pacific migration. Two hundred twenty-five of these tagged tuna were recaptured by fisheries (31.6%), and we successfully retrieved tag data from 14 of 21 individuals recovered in the Eastern pacific. Furthermore, one archival tag recovered in the Western Pacific revealed that the individual had performed a trans-Pacific migration, so in total 21 tagged PBF were shown to have migrated to the Eastern Pacific (2.9% of the total tags released). We successfully downloaded data from 15 of these 21 archival tags, which revealed that some age-1 PBF migrate rapidly (123.9 ± 82.8 km day−1) and directly from waters offshore of Japan to the eastern Pacific (160.0°E to 130.0°W), a journey that takes an average of 2.5 months (ranging from 1.2 to 5.5 months) through relatively cool waters (14.7 ± 2.0 °C). All juvenile PBF began their trans-Pacific migration shortly after exposure to cooler water temperatures (

A large number of fish species are associated with drifting seaweeds; however, the ecological significance of such seaweeds for fishes remains unclear. Here, we developed a raft equipped with a seaweed clump, interval still/video cameras, and a GPS satellite buoy. This novel monitoring system was used to monitor the schooling and associative behavior of Seriola spp. juveniles with seaweed for up to 1 wk in the East China Sea. We observed diel behavioral patterns of the fish, which swam around the seaweeds during the day and remained ‘attached’ to the seaweed or to conspecifics at night. This nighttime behavioral pattern suggests that the fish may use drifting seaweed to maintain schools at night when vision is less effective. Solitary individuals and those in smaller schools tended to remain close to the seaweed, whereas fish in larger schools were observed swimming actively around the seaweed. Additionally, some of the solitary fish and small schools escaped into the seaweed when potential predators appeared. As the school size of the fish increased over time, solitary individuals and small schools may have used drifting seaweeds as a shelter from predators until the fish could gather to form larger schools. We suggest that drifting seaweeds have multiple ecological functions for Seriola spp. and other seaweed-associated fishes, and the knowledge of these functions will be useful in designing conservation and management measures for the associated fishes., Marine Ecology Progress Series, 573, pp.101-115; 2017

The reproductive performance of barfin flounder (Verasper moseri), a multi-batch spawner spawning during February-April (boreal winter) in the wild, is particularly sensitive to temperature perturbations. Final oocyte maturation (FOM) of captive flounder occurred only when post-vitellogenesis fish experienced a slowly-elevated water temperature regime (SETR) (from 2 °C to 6 °C over a month). We used electronic tags to show how post-vitellogenesis flounder experience SETR during its pre-spawning migration, during their southward migration, to a location about 750 km from the summer feeding grounds in the coastal waters of Hokkaido (CWH; 40.2–43.4°N). From 2010 through 2012, we tagged a total of 208 V. moseri in early December at the feeding grounds of the CWH, during in which vitellogenin incorporation into oocyte was almost completed. One hundred and two tags were recovered and the data from 98 were downloaded. All of the tagged females moved offshore by the end of January. Fish spent more time in cold and deeper waters (3–4 °C, 351–380 m) in January than they did in December (6–8 °C, 33–59 m), but experienced warmer temperatures (5.1–5.7 °C, 367–396 m) in February and March around the spawning period. All tagged fish experienced SETR and then experienced the onset of SETR in January, although SETR of tagged fish have large variations among individuals. A comparison between depth-temperature data recorded by data-logger and oceanic horizontal temperature conditions suggests that fish did not experience warm surface water of the Tsugaru Warm Current, a branch of the Kuroshio that flows into the Pacific Ocean via the Tsugaru Strait, and instead were in the colder Oyashio water by migrating deep and finally experiencing warmer water in February and March. In conclusion, SETR appears to be one of the exogenous factors that induces FOM in the wild.

Although archival tags have been deployed in Thunnus since the early 1990s, few studies have placed such loggers on the small-size growth stage. In this study, Pacific bluefin tuna (PBT, Thunnus orientalis) 20–26.5 cm in fork length (FL) were fitted with archival tags and released off the southern coastal area of Japan from July to August in 2012, 2013, and 2014. Time-series data on swimming depth, ambient water temperature, and peritoneal cavity temperatures collected at 30-s intervals during 78–144 days were analyzed in 11 age-0 PBT. In August, the age-0 PBT spent 69.5 to 83.4% of their time within the mixed layer at mean temperatures between 27.6 and 28.3 °C, while previous studies reported that PBT with FL > 50 cm in the East China Sea are found most often in zones with temperatures between 14 and 19 °C. The thermal difference between the peritoneal cavity and ambient temperature was less than 1 °C for age-0 PBT in August, indicating that they were not in danger of overheating. The mean daytime swimming depths of age-0 PBT were significantly and positively correlated with the depth of the thermocline, especially from August to October. By focusing their swimming depths around the thermocline, the age-0 PBT potentially maximize the probability of encountering prey. There was a significant correlation between the mean nighttime depth and lunar illumination. The change in vertical distribution with the lunar cycle during nighttime is likely a form of predator avoidance.

To better understand habitats of larval Pacific bluefin tuna (PBT; Thunnus orientalis) in the nursery grounds, the vertical distribution of larval PBT was collected using a 1-m2 Tucker trawl and a 3.14-m2 ring net at depths of 0, 10, 20, and 30 m during the spawning seasons of 2016 and 2017. Most of the larvae in the Japan Sea (756 of 938 individuals) were collected from the surface layer using a ring net, while similar numbers were collected from 0 to 20 m depth in the Nansei area. The temperature at ≥20 m depths in the Japan Sea was cooler than the optima temperature, and the thermocline was steep at 10 m depth, and the cold subsurface water was suggested to make the PBT larvae aggregate near the surface. A weak ontogenetic and diel migration was observed; the body size at 10 m depth during the day and 0 m at night were significantly larger than those at 0 m during the day in the Japan Sea. These results suggested that some small larvae sink to 10 m and/or large larvae move to the surface at night. Our results showed that the larval PBT habitat is strongly controlled by water temperature, and that global warming would have a positive effect on PBT larvae in the Japan Sea, which was evidenced by the higher density of PBT larvae after 2010 compared to before 1990.

Broadcast-spawning scleractinian corals annually release their gametes with high levels of synchrony, both within and among species. However, the timing of spawning can vary inter-annually. In particular, the night of spawning relative to the full moon phase can vary considerably among years at some locations. Although multiple environmental factors can affect the night of spawning, their effects have not been quantitatively assessed at the multi-regional level. In this study, we analysed environmental factors that are potentially correlated with spawning day deviation, in relation to the full moon phase, in Acropora corals inhabiting seven reefs in Australia and Japan. We accordingly found that sea surface temperature and wind speed within one to two months prior to the full moon of the spawning month were strongly correlated with spawning day deviations. In addition, solar flux had a weak effect on the night of spawning. These findings indicate that Acropora have the capacity to adjust their development and physiology in response to environmental factors for fine-tuning the timing of synchronous spawning, thereby maximizing reproductive success and post-fertilization survival.

Pacific bluefin tuna, Thunnus orientalis, and dolphinfish, Coryphaena hippurus, are temporally sympatric top predators in the north East China Sea. To clarify their vertical habits in relation to the thermocline, we analyzed swimming depth and ambient temperature data for Pacific bluefin tuna that were obtained from animal-borne data-loggers in April, May, and November 2007. We also reanalyzed published electronic tagging data for free-ranging dolphinfish in the Tsushima Straight. When the vertical temperature structure was homogeneous, Pacific bluefin tuna made vertical excursions to the bottom layer in early May and November. When the thermocline developed in late spring, bluefin tuna made frequent dives below the thermocline, and their main distribution depth shifted to above the thermocline from the surface. During the same period, dolphinfish in this area remained near the surface and did not dive across the thermocline. However, swimming data for dolphinfish that were obtained in September and October showed that dolphinfish extended their vertical depth ranges as the thermocline depth increased.

The growth and reproductive characteristics of dolphinfish Coryphaena hippurus collected in the waters off western Kyushu from May 2008 to April 2011 were determined based on scale and otolith readings and gonad histological examinations, respectively. Based on annual increments in scales and daily increments in sagittal otoliths, the von Bertalanffy growth curves in male and females were determined as $$ FL_{t} = 1049[1 - \exp \{ - 0.835(t + 6.975 \times 10^{ - 14} )\} ] $$ and $$ FL_{t} = 938[1 - \exp \{ - 1.029(t + 6.975 \times 10^{ - 14} )\} ] $$ , respectively, where FL t is the mean fork length (mm) at age t. The spawning period was found to last from June to August for dolphinfish, based on an examination of the monthly changes in the gonadosomatic index and histological observations. Therefore, based on the relationship between the fork length and the developmental stage of the testes or ovaries, male and female dolphinfish were found to reach sexual maturity by the following spawning season after hatching in the northern East China Sea.

Environmental changes influence foraging behavior for most animals. Dolphinfish, Coryphaena hippurus, are epipelagic predators and have a cosmopolitan tropical to warm-temperate (>20°C) distribution. We simultaneously obtained the ambient temperature and the foraging behavior (i.e., swimming speed, depth and tailbeat acceleration) of dolphinfish, using an acceleration data-logger in May, September, October, November 2007, June 2008, May and July 2010 for 8 individuals. Although the dolphinfish spent a mean ± standard deviation of 43.4 ± 27.7% of their time at the surface (0–5 m), dive excursions from the surface (DES) were observed in all individuals and maximum DES depths ranged from 50.1 to 95.4 m. DES events resulted dives below the thermocline for these dolphinfish, and there was a significantly positive relationship between the isothermal layer depth (ILD) and DES depth. Our results demonstrate that dolphinfish avoided the rapid thermal change beyond the thermocline, and their prey is most likely found in the upper layers of the thermocline. Gliding behavior during the DES phase was also observed and dolphinfish gradually descended to deeper waters with gliding. The gliding time was longer when the ILD was deeper, and fish tended to dive deeper. We suggest that dolphinfish adopt gliding behavior to search a broader range of depths for prey, while minimizing energy use.

Finless porpoises in Omura Bay is the smallest among five populations in Japanese waters. This is a newly established population after the global warming of 9000 years ago, consisting of approximately 300 individuals and believed to be confined in Omura Bay. At the major corridor to the ocean from this bay, presence of finless porpoises was monitored acoustically from November 2007 to May 2009. A stereo acoustic event recorder (A‐tag) was deployed at this strait. A‐tag stored the intensity and the sound source direction of biosonar signals that provides independent traces of sound source corresponding to each animal. During 1.5 year observation, 226 individuals of porpoises were detected. Of them, 76% were presented at night and 73% observed during March to April. In the same season, anchovy landings in the Omura bay was recorded. 70% percent of the porpoises were observed during the tidal current that went out of Omura Bay that suggests the porpoises swam back and forth around the monitoring station. Although finless porpoises have been known to be confined in a local area, occasional migration of this species induced by prey availability might extend their habitat at different waters in Asian coasts.