Investigating Loggerhead sea turtle in the western North Pacific

Dr. Kobayashi held a juvenile green sea turtle. Dr. Kobayashi and Dr. Balazs also studied sea turtles taken as bycatch off Muroto Point, Shikoku in Japan. Takashi Ishihara (back) from the Sea Turtle Association of Japan was organizer of their research on the Pound Net fishing boat at Muroto Point.

Loggerhead sea turtles are long-lived, wide-ranging species. They inhabit vast open ocean, therefore it is very difficult to survey their behaviour and habitat. However the satellite data provides special opportunity to observe their pelagic life.

Until now satellite tags revealed many aspect of loggerheads’ life in the World Oceans such as the Mediterranean Sea and the North Pacific. In the North Pacific loggerheads are born at their nesting sites in Japan and they move from there to off the coast of Baja California. During this travel they live and forage in open sea. Despite all the North Pacific loggerheads population is originating from Japan, they do not directly cross the Pacific Ocean and some are found in the western North Pacific.

In the western North Pacific loggerhead turtles are often caught by Taiwanese fishery. Currently the population of loggerheads in this region is in a healthy condition in spite of accidental catch. Since this part of population is very valuable in the world wide, it is important to know the relation between loggerheads and oceanic features in this region. For that reason NOAA investigated the movement of loggerhead turtles and the mesoscale eddies (circular motion of ocean current).

Prior to this study 34 non-reproductive turtles were caught as bycatch in the Taiwanese fishery. In this study all those turtles were tagged with satellite transmitters and released at the northeast corner of Taiwan.

Tracking started from May 2002 and ended in June 2009. Data from the satellite tags provided daily turtle positions, while these data were also compared with eddy tracks on the study area.

The satellite also sent the oceanographic data to indicate each eddy character such as size, rotation speed, strength and direction, either cyclonic or anticyclonic.

Dr. I-Jiunn Cheng (center) and his assistants with one of the studied turtles. Dr. Cheng was the most important NOAA partner in Taiwan. He worked with local fishermen to tag and release loggerhead turtles during this study.

A specific example of eddy size and location on 25 May 2005. Blue represents cyclonic eddies and red for anticyclonic. Thicker circles characterize strong eddy. Black stars indicate the location of turtles.

The photo of eddy, taken by astronauts. Two eddies, one quite large and the other small, can be seen forming along the boundary of the Kuroshio Current. (The image provided by NOAA and NASA)

The East China Sea

Several features of turtle movement were demonstrated by examining data from satellite. First, most of the tagged turtles stay in a region between Taiwan, China, Japan, and South Korea. Especially many turtles remained in the East China Sea continental shelf.

Dr. Donald R. Kobayashi, NOAA Fisheries Pacific Islands Fisheries Science Center explains this result.
“The East China Sea continental shelf is biological hotspot. I think it relates to the major river, the Yangtze River, nearby. The freshwater outflow from that river makes the water of that region highly productive. Also shallowness helps turtles because turtles probably forage at the bottom of water. There may also be many fisheries in the area, which may be attractive to the turtles, either going after the same items or scavenging.
In any case more work is needed and it is very important to cooperate with oceanography expert in that region.”

Second, Eddy activity was apparent over the entire region during the study period. Then some turtles outside of hotspot likely stay close to eddies.

“There are differences between the hotspot turtles and the offshore turtles. I think eddies offer more food opportunities for items like jellyfish, which may be more important for turtles far from shore, away from the hotspot. But more analyses are needed to know this pattern” said prudently Dr. Kobayashi.