Kenzo Yoseda

Reproductive season and female maturity size of coconut crab Birgus latro on Hatoma Island, southern Japan

Information about reproductive season, size at sexual maturity and size-dependent reproductive potentials is important for efficient resource management. Such information is necessary to determine seasonal closures and minimum legal size limits. To conserve resources of coconut crab Birgus latro, which are in danger of extinction due to excessive exploitation, their reproductive season, female size at functional maturity and relationship between female size and reproductive potentials were investigated. The reproductive season started in early June and ended late August, and females with higher reproductive potentials showed a tendency to spawn early in the reproductive season. The prohibition of exploiting the resources during the reproductive season, especially early in the season, would be appropriate for sustainability of high reproductive outputs. Fifty percent of investigated females matured functionally at 24.5-mm thoracic length (TL). However, the size at which all females reached functional maturity was 32.3 mm TL, and there was a wide variation in the size of functional maturity among individuals. We must determine minimum legal size by taking the variation into account to allow all individuals to mate at least once before they are caught, which may prevent serious depression in reproductive rate and genetic diversity.

Male maturity, number of sperm, and spermatophore size relationships in the coconut crab Birgus latro on Hatoma Island, southern Japan

Abstract To consider effective resource management strategies for the coconut crab, Birgus latro, we determined the size of males at sexual maturity by histological methods and investigated an optimal method to estimate the number of sperm in spermatophores. The size of 50% sexual maturity in male coconut crabs was estimated at 22.2 mm thoracic length (TL) on the Hatoma Island, Japan. We established a procedure to soak spermatophores for more than 120 min in a 20% NaOH solution before mixing, and to count the sperm within 24 h after mixing, thereby enabling us to assess the number of sperm per individual and estimate male reproductive potential. Using this method, we investigated the relationship between male size and number of sperm retained by males. Larger male coconut crabs have a higher number of sperm in both the vasa deferentia and testis. The relationships between male size and spermatophore volume also increased with increasing male size, and the sperm quantity packed in a spermatophore increased with increasing spermatophore volume. Thus, male coconut crabs display a size-dependent reproductive output, as is reported for other species of anomurans.

Post-release movement and diel activity patterns of hatchery-reared and wild black-spot tuskfish Choerodon schoenleinii determined by ultrasonic telemetry

Post-release movement and diel activity patterns of hatchery-reared and wild black-spot tuskfish were examined using ultrasonic telemetry. Five hatchery-reared and four wild fish were released in the sandy bottom of Urasoko Bay, Ishigaki Island, Okinawa, Japan, and monitored using automated monitoring receivers from November 2005 to February 2006. Both hatchery-reared and wild fish tended to stay near the release site for over two weeks, before leaving the release site. Both hatchery-reared and wild tuskfish showed diurnal rhythm intermittently; signals were recorded more frequently in the daytime and less frequently in the nighttime, suggesting that the fish of both origins were active during the day and inactive during the night. These findings indicate that the one-year-old hatchery-reared tuskfish have some consistent behavioral characteristics with those of the wild.

The regularity of dive performance in sea turtles: a new perspective from precise activity data

Air-breathing divers are assumed to dive and surface in a way that maximizes their realized benefit. Dive duration in diving animals seems to depend on the purpose of the dive. Although the purpose of dives in most marine mammals and birds is undoubtedly foraging, this is not always true for so-called ‘surfacers’ that spend most of their time underwater for various purposes (e.g. foraging, resting, mating and migration). In this study, we investigated the relationships among dive duration, the amount of activity per unit time, the air volume in the lungs and postdive surface duration of four hawksbill turtles, Eretmochelys imbricata, which are surfacers, using multisensor data loggers. Our results demonstrated that hawksbills generally perform dives with consistent relationships between dive duration, the amount of activity per unit time and the air volume in the lungs, and that dives are followed by short periods at the surface, indicating that hawksbills rarely perform anaerobic dives. This longer dive duration compared to the length of surface resting indicates that the diving strategy of hawksbill turtles maximizes their time spent submerged. This strategy seems unique to sea turtles and surfacers in general.

Prediction of timing of mating and egg extrusion in the coconut crab Birgus latro judged from female pleonal expansion

A temporal pattern of reproductive activities of biological resources is essential information for the implementation of specific seasonal closures. First we investigated the relationship between the female pleonal expansion and female reproductive condition (i.e., non-ovigerous, ovigerous, or with spermatophores) to clarify the morphological characteristics of female coconut crabs that were able to mate. Second, we mated females with males in captivity to examine the relationship between the degree of female pleonal expansion and mating success. The index of pleonal expansion (ratio of inter-tergite distances to thoracic length) was significantly different among female reproductive conditions; females with spermatophores just before egg extrusion had the most expanded pleon. In the mating trials, all females showing index values higher than 0.35 mated with males. These results indicate that the female pleonal expansion has a strong relation to reproductive activities of females and can be applied as a morphological criterion for selecting females being able to mate. Next, we investigated whether reproductive activities of female coconut crabs are related to the lunar cycle. The reproductive activities were synchronized with the lunar cycle, and most females mated and extruded eggs around the new moon phase. These results have considerable implications for resource management and aquaculture for this species.

Effect of shelter acclimation on the post-release movement and putative predation mortality of hatchery-reared black-spot tuskfish Choerodon schoenleinii , determined by acoustic telemetry

In this study, the effect of shelter acclimation on the post-release movement and putative predation mortality of hatchery-reared black-spot tuskfish Choerodon schoenleinii was examined using acoustic telemetry. We acclimated four 1-year-old fish to shelters in cages before release and compared their movements with six nonacclimated fish. Since it was not possible to compare the behavioral pattern between the former and the latter fish due to the short periods the latter fish were available to be monitored, we also compared their movements with those of large nonacclimated fish that were less likely to be preyed upon. Sixty-seven percent of the nonacclimated fish showed atypical movements before the signals ceased to be detected, a pattern that suggested a predation event had occurred, whereas none of the acclimated and large nonacclimated fish showed the atypical movements. In addition, the probability of detection cessation was about 13 times lower in the acclimated than nonacclimated fish. The signal detection patterns suggest that the acclimated fish utilized night-time shelters from the first night after release, while the large nonacclimated fish started to utilize shelters several days after release. Therefore, it is likely that the shelter acclimation enhanced the shelter utilization by tuskfish, possibly decreasing post-release predation mortality.

Effects of a tropical cyclone on the distribution of hatchery-reared black-spot tuskfish Choerodon schoenleinii determined by acoustic telemetry.

The effects of a tropical cyclone on the distribution of hatchery-reared black-spot tuskfish Choerodon schoenleinii were examined using acoustic telemetry. Nine fish were released in Urasoko Bay, Ishigaki Island, Japan, in September 2006, and another nine were released in June to July 2007, before a cyclones passing through the area in September 2007. Data for the fish released in 2006 were used as the cyclone-inexperienced group to compare their distribution pattern to that of the 2007 cyclone-experienced group. Both groups of fish were monitored for up to 150 days. Of the nine fish in each group, four (44%) and two (22%) were monitored for over 150 days in the cyclone-inexperienced and the cyclone-experienced groups, respectively. Three of the five fish that had settled in the monitoring area left the area within a few days of the cyclone event. To estimate the time of disappearance of the fish, maximum wind speed during a period of 7 days (indicating the occurrence and intensity of the tropical cyclone), fish size and release year were evaluated as explanatory variables using a Cox proportional hazards model with Akaikes information criterion. The best predictive model included the effect of maximum wind speed. One fish that left the monitoring area displayed movement patterns related to strong winds, suggesting that wind-associated strong currents swept the fish away. No relationships were found between the movement patterns of the other two fish and any physical environmental data. The daily detection periods of one of the two fish gradually decreased after the cyclone hit, and this fish eventually left the monitoring area within 3 days, suggesting that it shifted to a habitat outside the monitoring area. These results indicate that tropical cyclones have both direct and indirect effects on the distribution of hatchery-reared C. schoenleinii.

Effect of shelter acclimation on the post-release survival of hatchery-reared black-spot tuskfish Choerodon schoenleinii: laboratory experiments using the reef-resident predator white-streaked grouper Epinephelus ongus

A critical component of many releasing projects is the identification and subsequent implementation of optimal release strategies that can decrease post-release predation mortalities. We performed laboratory experiments to investigate whether acclimation to shelters affects the post-release survival of hatchery-reared black-spot tuskfish Choerodon schoenleinii in the presence of a reef-resident predator, the white-streaked grouper Epinephelus ongus. Tuskfish were exposed to groupers under three different experimental conditions/treatments: (1) acclimation of fish to shelters prior to their exposure to groupers; (2) no acclimation of fish to shelters, but with shelters available during their exposure to groupers; (3) fish not acclimated to shelters and no shelters available during their exposure to groupers. Tuskfish that were acclimated to shelters utilized shelters more frequently than did non-acclimated fish, and the survival rate of acclimated fish was higher than those of fish in the other treatments. These results suggest that pre-release acclimation to shelters improves the post-release survival of hatchery-reared black-spot tuskfish.