Program

Leptocephali are the marine larvae of anguillid and marine eels and other elopomorphs that are present worldwide from tropical to southern temperate latitudes. Their unique laterally-compressed transparent bodies have only a thin layer of muscle tissue overlaying a mucinous-pouch containing energy storage compounds that forms the supporting structure of the body. All organs except the eyes are small or greatly reduced. Their tubular guts appear to absorb easily assimilatable materials from particulate organic materials called marine snow, which is consumed in the euphotic zone. This combination of biological features allows them to feed on readily available materials and have low respiration rates due to having minimal amounts of respiring tissue as they grow to large sizes. Their transparency and shape-change behaviors that mimic gelatinous zooplankton generally avoided by fish predators may provide greater protection from predation compared to other fish larvae. The body structure and physiology of leptocephali appears highly adapted for efficient swimming, and various evidence suggests that some large larvae may swim directionally to reach their recruitment areas. Their large energy reserve could be used for swimming until metamorphosis occurs near recruitment areas, and captive anguillid leptocephali have been observed to swim continuously for more than a month without food. This swimming ability could be used by species of Anguilla and Conger to exit subtropical gyres and cross boundary currents such as the Kuroshio Current and Gulf Stream. These characteristics of potentially low predation rates, feeding on a ubiquitous food source, and strong swimming ability supported by a large energy reserve to facilitate reaching recruitment areas at large body sizes may be key aspects of the long-term success of marine eels that inhabit most types of ocean habitats and anguillid eels living in freshwater and estuarine habitats in many regions of the world.

Environmental DNA (eDNA) refers to genetic materials from excretions and sloughed-off skin or tissue of organisms in the water column. eDNA analysis has enabled non-invasive biological and ecological surveys of aquatic organisms to be made that only use water samples, for example, to study the distribution, biomass and biodiversity of various types of species. We applied this method to search for spawning events of the Japanese eel during an R/V Natsushima cruise in May 2015 and detected eDNA from a seawater sample at one location, suggesting the presence of Japanese eels. This validated an effectiveness of eDNA analysis for the detection of their spawning events. However, interpretation of the results of eDNA analysis is difficult, so basic information associated with eDNA release rate, degradation and dispersal by water currents is important. In order to correctly interpret eDNA experimental results obtained in the ocean, we conducted four fundamental experiments in the laboratory. The amount of eDNA exponentially increased in tanks with more specimens (0, 1, 3, 9 and 27 eels). According to the advancement of developmental stages (egg, larva, juvenile and adult), the eDNA released in tanks also rapidly increased, and male silver eels released 5-40 times more eDNA than females despite their smaller body size. We also found that the amount of eDNA after spawning was 10-200 times higher compared to that before spawning. The eDNA degradation was slower at 5˚C than 25˚C showing the estimated times of 2.9 and 1.7 days for 90% degradation of the initial amount of eDNA, respectively. These fundamental results will help correct interpretations of the results of eDNA analysis in the open ocean to be made and will contribute to revealing the spawning events of the Japanese eel for the first time in the world.

The spawning area of the Japanese eel is located along the West Mariana Ridge in the western North Pacific, where preleptocephali, eggs and spawning-condition adults have been collected. However, the estimated spawning area is large and the exact timing of spawning events has remained unknown. New approaches are needed to investigate the spatiotemporal characteristics of specific reproductive events. We tested the correspondence between egg collection-points and the distribution of internal-tide energy during 3 R/V Hakuho Maru cruises (KH-09-2, KH-11-6, KH-12-2) along the southern West Mariana Ridge. The egg collection locations, observed ADCP current patterns and distribution of simulated internal-tide energy indicated that there were high-energy patches of internal tides upstream of the egg collection points in all cruises. To estimate the spawning times, we used the developmental stages of eggs collected during the 3 cruises, their collection times and presumed elapsed times from fertilization, and the possible experienced temperatures for each egg from equations in Ahn et al. (2012), which showed the relationship between water temperature and development stages. This was combined with CTD temperature data at the depth where the eggs were caught along with an experimental egg rising speed (3 m/h), and the swimming depth of silver eels observed by pop-up tags to estimate the exact spawning times. We could determine the development stages of 53 eggs among the 490 collected eggs (7 eggs: ear-eye vesicle formation; 9 eggs: heart formation; 37 eggs: just before hatching). The experienced temperature estimated from the development rate of the collected eggs was not significantly different from the environmental temperature from CTD data only if spawning occurred at 1:00 in all cases. Therefore, we hypothesize that Japanese eels spawn at about 1:00 a.m. and internal tide patterns may contribute to the mechanisms that determine the places they spawn in the open ocean.

To gather information on hiding-place use of yellow eels we monitored the behaviour of yellow-phase Japanese eels using ultrasonic telemetry in a semi-natural experimental pond (20 m×50 m×1 m) in Miyazaki Prefecture, Japan. Four wild eels (A-D, body lengths: 601±39.3 mm) caught in the Mimikawa River near the experimental pond were tagged with ultrasonic pingers (V9 series, Vemco, Canada) and then were released into the pond on 6 July 2015. Five acoustic monitoring receivers (VR2W, Vemco, Canada) were deployed to make arrays for acoustic localization. We analysed detection records from 7 July to 24 September. Usage areas (95% utilization distribution) of the tagged eels were calculated every hour by kernel density estimation. Movement distance was defined as a distance between two consecutive determinations of locations detected every 15 minutes. We regarded that eels used a pipe when they stayed in a 3-m circle around the pipe. Data of eel A were obtained only for 86 hours after the release because there was almost no detection afterwards. Eel B made a burrow near the center of the pond and both eel C and D used one of 4 pipes together as a hiding-place about 20 days after release. Eels showed habitat fidelity to their hiding-places. Eel B showed nocturnal behavior during the whole observation period. Movement distance of eel B became longer than that before making the burrow (p<0.001, Mann-Whitney U-test). However, the movement distances and usage areas of eel C and D were smaller than those before settlement in the pipe, probably because they stayed in a hiding place (pipe) that was located in a food-rich area in the pond and they can easily take food near the pipe. The results of our study suggest that yellow-phase Japanese eels have hiding-place fidelity for at least about two months.

We investigated the horizontal and vertical movement patterns of eleven black sea bream in littoral zone and an oyster farming area in Hiroshima Bay by active tracking. Wild captive six fish (fish IDs #1–6) in the littoral area were released at capture site, whereas five fish (#7–11) were captured and then released in the oyster farming area. These fish equipped with depth transmitters had been tracked for 99 days. Fish #1–4 and #6 released in the littoral had been found around the low tide line throughout the tracking period (41 days), further only one fish (#5) was found in littoral zone and oyster farming area. Five fish (#7–11) had been migrated within the oyster farming area for a cumulative duration of 50 days. The daily movement distance of all fish were recorded as 0.11–0.78 km/day. Although the home range sizes of oyster farm residents were larger than the other fish. The minimum convex polygon (MCP) home range sizes of these fish (#7–11) were noted at 0.11 - 1.14 km². The swimming depth in the oyster farming area was highly variable among the tracked fish (1–15 m), which coincided with the depths of the submerged oyster wires. These results indicate that black sea bream in Hiroshima Bay inhabits the oyster farming area and highly dependent on the spatial arrangement of the oyster rafts. Also, our findings here imply that oyster rafts have a function as artificial reefs and contribute for an abundant stock of black sea bream in Hiroshima Bay.

The spawning area of the Japanese eel, Anguilla japonica, is located along the West Mariana Ridge in the Pacific Ocean. Despite some recent progress, there are still large gaps in knowledge about the swimming routes and behavior of Japanese eels during their long-distance oceanic migration and about their reproductive ecology in the spawning area. To achieve a better understanding of the behavior at their spawning area, three large female silver eels (Eel A: total length<TL> 898 mm, body weight<BW> 1110 g; eel B: TL 992 mm, BW 1545 g; eel C: TL 792 mm, BW 913 g) were equipped with pop-up satellite archival tags (PSATs: MiniPAT, Wildlife Computers) and released at 3 different locations along the West Mariana Ridge on 19, 20 and 30 May 2014. Two of three PSATs (Eel A and B) ascended successfully near their release points and one (Eel B) provided clear records of consistent diel vertical migration (DVM) with daytime dives to depths of about 800 m, where temperatures were near 5°C and nighttime ascents to depths of 200 to 350 m. A clear relationship between the upper limit of migration depths and the lunar cycle was found. This daily rhythm of DVM observed in their spawning area was the same behavior found in other areas during the spawning migration, which may be explained as a being the result of trade-offs between predator avoidance (deep-depths with low-light) and the necessity to maintain a sufficiently high metabolism for migration and maturation of the gonads (shallow-depths with warmer water). We propose that their swimming depths are controlled by a fixed light intensity and water temperature. The results of the study allow the spawning depth the Japanese eel to be estimated as being at about 200 m along the West Mariana Ridge.

Hatchery-reared seedlings for stock enhancement often have inferior behavioral characteristics unsuited for natural environments. Such behavioral characteristics are formed by rearing in safe and comfortable tank environments, and thus the rearing environment of seedlings should be altered to improve them. Repeated net-chasing in a rearing tank would allow them to learn avoidance responses for quasi-predator attacks and when faced with threatening situations. We investigated the effect of a net-chasing treatment to improve behavioral characteristics of fish (Japanese flounder Paralichthys olivaceus and red sea bream Pagrus major) and a crustacean (Indian bait prawn Palaemon pacificus). For Japanese flounder, we tried to improve maladaptive off-bottom swimming during foraging of juveniles by net-chasing treatment (two weeks). When fed at the surface, the off-bottom swimming of net-chased fish was repressed, whereas control fish showed an increase of off-bottom swimming. For red sea bream, we investigated the change in their behavioral characteristics by a net-chasing treatment (three weeks), and then a predator (marbled rockfish Sebastiscus marmoratus) exposure test was conducted. Net-chased fish exhibited higher tolerance to stress induced by environmental change and better alertness to an unknown object than did control fish. Furthermore, net-chased fish also exhibited a better survival rate than control fish in the predator exposure test. For Indian bait prawn, we investigated the effect of net-chasing treatment for crustaceans. Net-chased (eight day treatment) prawns entered a shelter more quickly, and showed higher avoidance rate to a contact stimulus than control. These studies showed that net-chasing treatment will be useful for behavioral improvement of released fish and crustaceans during the rearing period. Compared to the previous training, e.g., learning specific predator information, the net-chasing treatment may have wide applicability for the training of seedlings before release to improve survival traits of the fish.

The information about pregnancy of harbour porpoise is quite limited because parturition events of porpoises are difficult to observe in nature and because few aquarium keep harbour porpoise in Japan. Otaru aquarium, Hokkaido, Japan, is keeping four harbour porpoises which were rescued from by-caught in set nets. One of them, Atsuko had been pregnant twice, first childbirth in 14th January 2011 and second childbirth was in 15th May 2015. First childbirth was resulted in stillbirth. About second childbirth, she was delivered of a boy but the calf dead nine minutes after the birth. The aim of this study is to report the detail about parturition including physiology parameter, behaviour like surfacing time, clicks and lactation rate which were observed during second birth. These behaviour observations were recorded for 10 minutes at every 30 minutes from 13th to 15th May 2015. Maximum lactation rate was 1.0/min when 67 minutes before the childbirth. Breath rate and surfacing time increased towards to the end of the birth. The number of clicks detected by A-tag set in the pool was largest (1.97/min) just after the A-tag was put in the pool, but after that, no clicks were observed even during childbirth. The physiology parameter, food intake, weight and progesterone level in monthly average were compared between first and second childbirth. About first childbirth, the progesterone level changed less than 20 ng/ml and food intake and weight had seasonal change although during pregnancy. On the other hand, food intake and weight during second childbirth had followed the seasonal patterns before July 2014 when progesterone level was 10.2ng/ml, but after then progesterone level continued increasing until birth to 40ng/ml, and weight and food intake also increased. This information would be useful for captive breeding treatment of this species.