Tohya Yasuda

Fine-Scale Tracking of Marine Turtles Using GPS-Argos PTTs

Abstract High-accuracy location data of wildlife telemetry using conventional satellite location systems are difficult to obtain. However, such data are necessary to clarify the nature of movements and home range sizes of animals. In order to measure the high-accuracy location data, we developed new GPS-Argos Platform Terminal Transmitters (PTTs) which transmit both the conventional location and GPS location simultaneously. Two experiments, one in an artificial rearing pond and the other in the open sea, were performed. First, two hawksbill turtles were tracked with the PTTs in a 5 ha breeding pond in Thailand. Their home ranges using both data were calculated and found to be 2.96 ha and 0.93 ha by the GPS data, and 156,740 ha and 184,478 ha by a conventional data. Secondly, a female green turtle attached with the GPS-Argos was released from the coast of Pangnga Province, Thailand. There was a relationship between depth and speed of travel based on the GPS data. The data from the PTT showed that the turtle moved south along the coastline at the depth of less than 20 m for 5 days, and then stayed at a depth of less than 10 m for 4 days. However, we could not find any clear relationship using conventional data. Only a meandering movement at a variety of depths was observed. The results of the two experiments indicated the PTTs have an enormous potential for enhancing our understanding of fine-scale movement patterns and home ranges of marine turtles.

High-frequency depth recording reveals the vertical movement of flounder in the Tsugaru Strait of northern Japan

Analysis of high-frequency depth-recording data of adult Japanese flounder Paralichthys olivaceus (Temminck & Schlegel) with depth/temperature logging tags, released in the Tsugaru Strait of northern Japan, has yielded new insights into behavioural differences on vertical movement. Here, we document diel differences in fine-temporal-scale swimming behaviour observed from six flounder released during the no-spawning season. While the flounder remained on the seabed for the majority of the recording period, fish occasionally left the seabed, swam into the water column and then swam back to the seabed. The mean swimming duration per tagged fish ranged from 44 ± 49 to 94 ± 164 s, with a maximum observed swimming duration of 44.5 min. Vertical movements included one or more clear ascent and descent phases highlighted by a distinct peak. Our results reveal that in the no-spawning season nocturnal swimming is more active and frequent than during the day. Our results provide important information about diel differences in swimming behaviour with respect to vertical movement and also show the vulnerability of Japanese flounder to capture by bottom-fishing gear (e.g. trawling) during the day.

Biomass fluctuation of two dominant lanternfish Diaphus garmani and D. chrysorhynchus with environmental changes in the East China Sea

Acoustic surveys have been conducted for estimating the biomass of commercially important fish (e.g., anchovy, jack mackerel), lanternfish (Diaphusgarmani and D. chrysorhynchus), and pearlside (Maurolicus japonicus) in summer in the East China Sea (ECS) since 1997. The biomass of lanternfish and pearlside was 2.26–19.16 times that of commercially important fish, and these species represented substantial biomass in the ECS. Though there were no correlations between biomass of pearlside and environmental indices, significant correlations between biomass of lanternfish and southern oscillation index (SOI) in March (positive correlation), arctic oscillation (AO) in March (negative) and October (positive), monsoon index (MOI) in February (positive), and Kuroshio flow mass in winter (positive) were observed. Weak AO and strong MOI would cool down the sea temperature and would lead to increased primary and secondary production in the ECS, thereby enhancing larval survival of lanternfish. The SOI would affect the Kuroshio meander in the ECS, and strong SOI and Kuroshio flow mass would transport larvae of lanternfish to the present survey area. This is the first report on the lanternfish standing stock and its fluctuation in the ECS.

Trophic positions and predator–prey mass ratio of the pelagic food web in the East China Sea and Sea of Japan

Size-based food webs analysis is essential for understanding food web structure and evaluating the effects of human exploitation on food webs. We estimated the predator–prey mass ratio (PPMR) of the pelagic food web in the East China Sea and Sea of Japan by using the relationships between body mass and trophic position. Trophic position was calculated by additive and scaled models based on nitrogen stable isotope ratios (δ15N). The PPMRs based on additive and scaled models were 5032 (95% confidence interval (CI) 2066–15506) and 3430 (95% CI 1463–10083) respectively. The comparatively high PPMRs could reflect low ecosystem transfer efficiency and high metabolic rate.

Using a multivariate auto-regressive state-space (MARSS) model to evaluate fishery resources abundance in the East China Sea, based on spatial distributional information

The abundance index (AI) is a representative indicator used to assess the state of fishery resources. Conventional AI is generally calculated by summing the catch per unit of effort (CPUE) weighted by the size of each fishing area. However, CPUE data has many missing values owing to annual changes in operational fishing areas, and this can lead to a considerable bias in the estimated AI. To obtain an unbiased AI, a multivariate auto-regressive state-space (MARSS) model was used to estimate and interpolate missing values in a spatially arranged, long-term bottom-trawl CPUE dataset for yellow seabream Dentex hypselosomus and largehead hairtail Trichiurus japonicus in the East China Sea. As expected, increasing the number of analyzed fishing grids improved interpolation accuracy, but remarkably increased the time required for the analysis. Reducing the maximum number of expectation–maximization (EM) iterations in the maximum likelihood procedure was an effective way to practically reduce analysis time, while keeping the accuracy of the estimation. Thus, this EM-reduction MARSS model was applied to the entire CPUE datasets of yellow seabream and largehead hairtail to address the annual shifts in their AIs and their seasonal migration.

Difference in Flipper Beating Frequency of Green Turtles in Water and on Land

Sea turtles spend most of their lives in marine habitats, but they require a terrestrial environment for oviposition. In both conditions, they use limbs for thrust production. We attached animal-borne data loggers on green turtle and calculated the stroke frequency during swimming in water and crawling on land from surging acceleration. Stroke frequency was compared during swimming and crawling. The results showed that stroke frequency during terrestrial crawling is significantly higher than during swimming. This contrasts with previous studies of animals performing drag-based swimming. Because green turtles are considered to be lift-based swimmers that produce thrust mainly by dorsoventral excursion, one hypothesis is that anteroposterior excursion may be restricted despite its importance in terrestrial crawling and drag-based swimming. Small anteroposterior excursion resulting in short stride length may be complemented by higher stroke frequency during crawling.

Seasonal changes in reproductive output of a year-round nesting population of the green turtle Chelonia mydas at Huyong Island, Thailand

For the vast majority of reptiles, low temperature is a major constraint on reproduction. However, continuous reproduction is not always observed even in equatorial climates. In the present study, we examined the reproductive output of green turtles in a year-round nesting population on Huyong Island, Thailand. From 1996 to 2007, 121 females were identified. Because individual turtles nest several times within a season, we defined the nesting season of an individual as the month during which the first nesting activity was detected. We found a significant difference in clutch frequency among months. However, there were no significant relationships during the nesting period between clutch frequency and environmental parameters such as mean daily air temperature, precipitation and maximum wind speed. Although we could not identify the specific factors, our findings suggest that environmental factors may regulate seasonal variation in reproductive output even in the tropics.