Movement patterns and marine habitat associations of juvenile loggerhead sea turtles (Caretta caretta) in the southwestern Atlantic Ocean

Graduation date: 2012Bycatch, or the incidental capture of non-target species, has been implicated as one of the main factors leading to population declines of many large marine vertebrates, including sea turtles. To effectively manage and conserve these long-lived species, their marine distribution, high use areas, foraging habitats, and regions of highest likelihood of interaction with fisheries must be understood. I analyzed the movements and habitat use of satellite tracked juvenile loggerhead sea turtles (Caretta caretta) in the Southwestern Atlantic Ocean, a poorly studied region with high turtle-fisheries interactions. Between July 2006 and March 2010, 27 satellite transmitters were deployed at sea on juvenile loggerheads captured as bycatch in the Uruguayan and Brazilian pelagic longline fishing vessels. I characterized the broad-scale behavioral patterns, inter-seasonal variability, and general high use areas for 26 juvenile turtles, which were tracked for 259±159 days between latitudes of 25-45°S and longitudes 35-54°W. The high use areas for the tracked turtles were over the continental shelf and slope within the Uruguayan and Brazilian Economic Exclusive Zones, and in oceanic international waters between the Rio Grande Rise and the continental slope off of southern Brazil. Diving information was available for 5 of the tagged turtles; the maximum dive depth recorded varied between 100-300m depths, and two turtles demonstrated potential bottom-feeding behaviors by diving to depths that corresponded with the bathymetry at their location. The mean sea surface temperature encountered by turtles was 19.8±2.3°C (10.21°C-28.4°C) and turtles showed an affinity for mesotrophic waters (0.458±1.012 mg/m³ chlorophyll-a). Overall, broad scale latitudinal movements of juvenile loggerheads varied by season and sea surface temperature.\ud \ud Because recent studies on marine megafauna movements have highlighted that ocean currents can have an important effect on movement paths, I decoupled active foraging behavior from likely passive movement of tracked juvenile loggerheads in ocean currents. Using First Passage Time analysis; a method to measure changes in movement patterns along a pathway through the environment, and generalized additive mixed models, I quantified similarities in the movement patterns and habitat "affinities" of the turtles and surface drifters in the ocean. Turtles and drifters both exhibited movement patterns that could be classified as likely "foraging behavior" at a spatial scale of 80km. This corresponds to the identified scale of eddies in the Southwestern Atlantic Ocean, which may suggest that passive movement of turtles in ocean currents largely drives their scale of search. Current velocity and sea floor depth were the most important variables correlated with both turtle and drifter movement patterns at that scale. Both turtles and drifters generally showed a negative relationship between first passage time and current velocities. Some differences\ud between turtle and drifter behavior were evident, particularly on the continental shelf; deviations in turtle behavior from the patterns of drifters is likely indicative of active movement on the turtles part. There were no seasonal or annual effects on the fine scale movements of turtles or drifters. Interestingly, turtle search behavior was not correlated with temperature or chlorophyll a in this scale of analysis. I suggest that evaluation of drifter movements in the area of study is an important addition to satellite tracking work that attempts to identify foraging behavior in sea turtles or other large marine vertebrates that may take advantage of ocean currents for transport and feeding