Study Leads to New Understanding of Dive Behavior in Juvenile Loggerhead Turtles in the North Pacific

Most management regulations governing the Hawaii longline fishery were introduced to limit incidental interactions of the fishing gear with protected sea turtles while enabling a continued catch of tuna and swordfish for local and export markets. To devise effective regulations, NMFS must understand many factors, including the distribution and behavior of turtles and target fish species, configuration and behavior of longline gear, and oceanographic conditions that reduce or increase the likelihood of sea turtle interactions.

Recently, a team of scientists studied the influences of oceanography on the dive behavior of juvenile loggerhead turtles (Caretta caretta) in the North Pacific. The collaborators included Evan Howell, Jeff Polovina, and George Balazs of PIFSC; Peter Dutton of the NMFS Southwest Fisheries Science Center; Denise Parker of the University of Hawaii Joint Institute for Marine and Atmospheric Research; and NRC post-doc Helen Bailey. A manuscript describing their work has been accepted for publication in the journal Marine Biology.

Before it was returned to the sea, this juvenile 
            loggerhead turtle was outfitted with a satellite-transmitting data recorder, enabling the collection of 
            valuable data on the turtle's location and diving behavior.
Before it was returned to the sea, this juvenile loggerhead turtle was outfitted with a satellite-transmitting data recorder, enabling the collection of valuable data on the turtle's location and diving behavior.

Loggerheads in the North Pacific originate from nesting beaches in Japan and are the principal species of turtle taken in the Hawaii shallow-set longline fishery that targets swordfish. Data on diving behavior were collected by satellite-transmitting sensors attached to the backs of 17 juvenile turtles that had been captured by longline vessels based in Long Beach, California. Scientific technicians aboard the vessels, trained to attach the transmitters in a safe and secure manner, installed the Wildlife Computers sensor packages before releasing the turtles back to the ocean. As the turtles resumed their activities, the tracking devices transmitted the collected data via satellite to shore-based computers. Concurrent oceanographic data were compiled and analyzed along with the turtle location and depth data to determine the influence of regional and seasonal oceanography on loggerhead dive behavior in the North Pacific Ocean.

Pooled data for all turtles in the study showed that the turtles spent over 80% of their time at depths < 5 m, and more than 90% of their time at depths < 15 m. Multivariate classifications of dive data revealed four major types of dives, three representing deeper, longer dives, and one representing shallower dives shorter in duration. Turtles exhibited variability in their dive behavior across oceanographic regions, making longer, deeper dives in the Hawaii longline swordfish fishing grounds during the first quarter of the year and in the Kuroshio Extension Bifurcation Region and the region near Baja California. Dive behavior of turtles within the Kuroshio Extension Bifurcation Region also varied in association with mesoscale eddy features, with the turtles making deeper, longer dives in areas with the strongest total kinetic energy.

Turtles within the Central North Pacific exhibited seasonality in dive behavior that appeared to reflect synchronous latitudinal movements of the turtles with shifts in the North Pacific Subtropical Front and associated seasonal, large-scale oceanographic features. Turtles made longer, deeper dives during the first quarter of the year within this region (see graphic below). This time period and area coincide with the highest levels of loggerhead bycatch in the longline fishery.

This is the first comprehensive study of loggerhead dive behavior in this region. The increased understanding of juvenile loggerhead dive behavior and influences of oceanography on dive variability should provide further insights about factors leading to interactions with the longline fishery and suggest methods for reducing bycatch of this threatened species.

(a) Box plots of the monthly sea surface temperature (SST) values associated with 17 juvenile loggerhead turtles in the longitudinal region 180°-140°W during 2003-2005. The grey area represents the 17.5°-18.5°C (63.5°-65.5°F) SST region currently identified by the PIFSC TurtleWatch product as the region of higher expected loggerhead bycatch.
(b) Frequency (%) of dives of the longer, deeper dive type, by month.
(c) Frequency (%) of dives of the shorter, shallower dive type, by month.
In the (b) and (c) plots, the black line represents the median monthly SST value for the 17 turtles in the spatiotemporal range described above.