How Loggerhead Turtles Swim Relative to Ocean Currents in the South Pacific

In the past decade, research on satellite-tagged sea turtles has greatly increased our knowledge of the turtles' distribution and movements in the open ocean, information vital to minimizing fishery interactions with these protected pelagic species. Recent collaborative studies are shedding light on the habits of loggerhead turtles in the South Pacific. PIFSC researcher Donald Kobayashi and colleagues Richard Farman, Jeffrey Polovina, Denise Parker, Marc Rice, and George Balazs are examining fine-scale movement patterns of oceanic juvenile loggerheads in the South Pacific. One aspect of this study is an attempt to document rheotaxis in this species. Rheotaxis in the aquatic environment is defined as the response orientation of an organism to a current flow. Rheotaxis has been documented in many laboratory studies, but the experimental apparatus used often confounds laboratory results in that multiple cues may be available to an organism, making it difficult to isolate and identify true rheotaxis. Field studies with oceanic organisms offer an excellent opportunity to document true rheotaxis because the primary cue available to the organism is the current flow, with few other cues available to assist in orientation. To examine the possibility of rheotaxis in oceanic juvenile loggerhead turtles, an analysis of movement dynamics was undertaken using location data from satellite-tagged turtles in the South Pacific and data on current flows from 5 sources: NOAA drifter buoys, NOAA OSCAR, and 3 vertical subsets of HYCOM.

On 9 September 2008, 42 juvenile loggerhead turtles were satellite tagged and released simultaneously off of the coast of New Caledonia at 170.86 E longitude and 29.803 S latitude. All turtles were captive-reared from hatchling stage at the Aquarium des Lagons (Aquarium of the Lagoons) in Noumea, New Caledonia. They originated from eggs collected from multiple excavated beach nests in New Caledonia. After release, the juvenile turtles were tracked using Wildlife Computers Smart Position or Temperature (SPOT5) satellite transmitting tags. Raw position data from the satellite tags were processed with Bayesian State Space Modeling (SSM). The daily SSM satellite track data were matched in time and space to estimates of current flow from each of the 5 data sources. These currents were used to estimate the drift component of the daily track displacement and to infer the swimming vector on a daily basis using simple vector geometry.

The results indicate the tag movements cover a broad range of directions but with a slightly higher proportion of movements in a southward direction (upper left in figure below). Likewise, the directions of currents the turtle encounters are broadly ranging with a slightly higher proportion of eastward flow (upper right below). The turtle's swimming direction is computed as the difference between the tag movement and the current directions (lower left below.) This also exhibits a wide range of directions. However, when the swimming direction is compared to the current direction, a very high proportion of the swimming directions are exactly opposite to the current direction (lower right below).

In summary, turtles and tags are moving in a wide range of directions but on at the fine-scale turtles appear to be orienting their swimming so that it is against the current, whatever the current direction. The marked directionality in the difference between the turtles' swimming directions and the prevailing current directions is strong evidence of positive rheotaxis; oceanic juvenile loggerhead turtles are capable of detecting current direction and orienting toward the current. Future studies are needed to investigate potential mechanisms of this rheotaxis and the adaptive significance of such a response.

Polar histograms of tag movement direction (Tag) current direction from a shallow subset of HYCOM (HYCOM), turtle swimming direction 
               (Swim) and the difference between the turtle swimming direction and current direction (Swim-HYCOM). In the Swim-HYCOM plot, north 
               indicates no difference between swimming direction and current direction and south indicates swimming and currents are in exactly 
               opposite directions, i.e., swimming is directly against the current.
Polar histograms of tag movement direction (Tag) current direction from a shallow subset of HYCOM (HYCOM), turtle swimming direction (Swim) and the difference between the turtle swimming direction and current direction (Swim-HYCOM). In the Swim-HYCOM plot, north indicates no difference between swimming direction and current direction and south indicates swimming and currents are in exactly opposite directions, i.e., swimming is directly against the current.