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Seasonal Distribution and Behavior of Loggerhead Sea Turtles in the North Pacific : statistical analysis in relation to environmental oceanographic parameters

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Title: Seasonal Distribution and Behavior of Loggerhead Sea Turtles in the North Pacific : statistical analysis in relation to environmental oceanographic parameters
Other Titles: 北太平洋におけるアカウミガメの季節分布と行動 : 海洋環境要因に関する統計解析
Authors: Ikeda, Takayoshi1 Browse this author
Authors(alt): 池田, 隆美1
Issue Date: 25-Mar-2008
Abstract: Loggerhead sea turtles Caretta caretta nesting off coasts of Japan are known to travel an immense distance throughout their life, however much of their behavior in the open ocean is still not clear. Argos transmitters were attached to 30 turtles, which were either postnesting, headstarted, or bycaught, and their tracks were recorded spanning several months to over a year. Location data were low in quality with nearly 70% of the points having error radii that could not be estimated. As an initial step, the noisy data were smoothed under a systematic set of criteria to remove redundant information and obtain the most reasonable paths taken, controlling both location and average velocity. Smoothed turtle tracks were obtained for all individuals with average velocities no greater than 250 cm/s. Behavior was categorized into three groups; 1) remaining, 2) returning and 3) departing, among which significant differences were found in straight carapace lengths (SCLs), being largest for remaining turtles and smallest for departing turtles. Logistic regression estimated the pivotal range of SCL to be 725 to 783 mm dividing coastal (remaining and returning) and non-coastal (departing) turtles. Longitudinal distributions were clearly different between behaviors and also within the returning turtles being west of 135°E from May to July due to mating and nesting, which took place near the coast. During the same season, departing turtles were in regions as far as 170°E. In latitudinal distribution, all turtles were seasonally variable, being in latitudes higher than 30°N during warmer months. Behavioral differences were examined based on relative velocity of the turtle with respect to ocean current, where geostrophic current velocity was obtained from optimally interpolated satellite data (J-OFURO). Regression analysis was undergone comparing relative velocity to current velocity, along with other oceanographic parameters, such as sea surface temperature (SST) from AVHRR satellite data and nutrient concentrations from WOA01 climatological data. Turtle paths were divided into specific stages and comparisons were made between returning and departing turtles. In regions with strong Kuroshio currents, all turtles were drifted, however returning turtles were swimming in the opposite direction of the current more often than the departing turtles, which were reacting more to cooler SST. This indicates that returning turtles were unintentionally drifted into the direction of the strong currents. Frequently, turtles made roaming or circular movements in the open ocean, just off of the Kuroshio extension, possibly being drifted into circular currents or eddies, however in all cases, turtles confronted regions with plenty of prey during the roaming stage. After this stage, turtles either moved farther east to the Kuroshio extension bifurcation region, a “hotspot” for juvenile turtles, or changed their directions and headed back to nesting grounds. Deciding moments were characterized by a difference in current velocity magnitude and direction, which was mainly due to the turtles being at different latitudes, in which returning turtles were located south of the Kuroshio mainstream, being closer to cyclonic currents and nutrient-high cold rings, whereas departing turtles were closer to the center of the mainstream. When returning turtles initiated their long journeys back to the coast spanning several months, they headed southward into regions of relatively weaker currents and with a lower chance of finding prey. Initially, they were swimming more actively in the opposite direction of the current, but were less active as they headed south towards the end of their returning path. Hence, they seemed to have been extremely cautious concerning energy consumption, by swimming slower even in warmer SST, only swimming opposite of weak currents to head towards high silicate regions. They may have also been making frequent shallow dives in order to sense subsurface water temperatures as a cue to returning back to familiar waters near Japan. Departing turtles in the bifurcation region were continuously influenced by the currents and constantly headed towards prey abundant regions, remaining in this area until transmissions ended. It has been verified that ocean currents and other environmental factors influenced the horizontal movement and behavior of loggerhead sea turtles in the North Pacific at different stages of their tracks. Abrupt environmental changes causing SST increase, distributional change in plankton and intensification of the Kuroshio and its countercurrent could have an impact on their behavior, however the degree of the impact would depend on the adaptability of the turtles. These effects as well as those related to human activity are the determinant factors for their survival to endangerment.
Conffering University: 北海道大学
Degree Report Number: 甲第8717号
Degree Level: 博士
Degree Discipline: 地球環境科学
Type: theses (doctoral)
Appears in Collections:学位論文 (Theses) > 博士 (地球環境科学)

Submitter: 池田 隆美

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