Fine-scale foraging ecology of leatherback turtles

Abstract

Remote tracking of migratory species and statistical modeling of behaviors have enabled identification of areas that are of high ecological value to these widely distributed taxa. However, direct observations at fine spatio-temporal scales are often needed to correctly interpret behaviors. In this study, we combined GPS-derived locations and archival dive records (1 s sampling rate) with animal-borne video footage from foraging leatherback turtles (Dermochelys coriacea) off Nova Scotia, Canada (Northwest Atlantic Ocean) to generate the most highly-detailed description of natural leatherback behavior presented to date. Turtles traveled shorter distances at slower rates and increased diving rates in areas of high prey abundance, which resulted in higher prey capture rates. Increased foraging effort (e.g., dive rate, dive duration, prey handling time, number of bites) was not associated with increased time at the surface breathing to replenish oxygen stores. Instead, leatherbacks generally performed short, shallow dives in the photic zone to or above the thermocline, where they disproportionately captured prey at bottoms of dives and during ascents. This foraging strategy supports visual prey detection, allows leatherbacks to exploit physically structured prey at relatively shallow depths (typically <30 m), and increases time turtles spend in warmer water temperatures, thus optimizing net energy acquisition. Our results demonstrate that leatherbacks appear to be continuously foraging during daylight hours while in continental shelf waters off Nova Scotia, and that leatherback foraging behavior is driven by prey availability, not by whether or not a turtle is in a resource patch characterized by a particular size or particular prey density. Our study demonstrates the fundamental importance of obtaining field-based, direct observations of true behaviors at fine spatial and temporal scales to enhance our efforts to both study and manage migratory species.

TurtleCam: A “Smart” Autonomous Underwater Vehicle for Investigating Behaviors and Habitats of Sea Turtles

“…In the shallow well-mixed habitat off Cape Cod, leatherback turtles are likely to feed throughout the entire water column and can encounter fishing gear anywhere from the surface to the sea floor, making depth-specific fishing gear modifications to reduce sea turtle interactions ineffective. The tagged turtle also exhibited prey-silhouetting behavior, which has been documented in great white sharks (Klimley et al, 1996;Strong, 1996;Skomal et al, 2015), and in leatherback turtles off Atlantic Canada (Wallace et al, 2015). While sharks silhouette their evasive pinniped prey as an ambush tactic, visual predators like leatherbacks may silhouette jellyfish with surface light in response to the murky, turbid conditions in our study area.…”

“…The transponder was attached to the turtle's carapace with two suction cups, eliminating the need to capture or handle the turtle and potentially impact its behavior (Heaslip et al, 2012). Turtles in our study dove immediately post-tagging, but they appear to quickly resume feeding dives (usually within minutes based on video footage), suggesting that surface application of suction cup tags has a minimal impact on the turtles' natural behavior (Heaslip et al, 2012;Wallace et al, 2015). The total weight of the transponder, camera and attachment materials was less than 1% of the turtle's minimum estimated body weight of 200 kg.…”

Using a remotely operated vehicle (ROV) to observe loggerhead sea turtle (Caretta caretta) behavior on foraging grounds off the mid-Atlantic United States

“…Yet despite the utility of these approaches, often it remains equivocal exactly what animals are doing and their specific behaviors (e.g., prey types, social interactions) may be missed in these electronic records. Hence direct observation of individuals also has great utility and can provide information not available from other approaches (Reina et al, 2005;Schofield et al, 2006;Seminoff et al, 2006;van Dam and Diez, 2000;Wallace et al, 2015). Together synergistic use of electronic logging devices and validation of events seen in the electronics records, has great utility for a range of taxa (e.g., Fossette et al, 2012Fossette et al, , 2015.…”

“…More recently, researchers began deploying animal-borne video and environmental data collection systems (AVEDs) to make observations from the perspective of the animal (Moll et al, 2007;Reina et al, 2005;Wallace et al, 2015). This technique allows for the observation of behaviors and sections of the marine environment otherwise very difficult to access.…”

“…However, similar to biologging devices, animal-borne video equipment is cumbersome and has the potential to impact the behavior of the animal (Jones et al, 2011;Ponganis et al, 2000). Loggerheads are not well suited for this type of device, as attachment would require capturing the animal to clean the carapace due to the variability of the condition of the carapace (common presence of barnacles and algae); unlike leatherback turtles, which do not require capture for AVED deployment (Wallace et al, 2015). Capture typically causes loggerheads to exhibit a temporary post-release reaction, thus limiting the value of a short term AVED deployment system to film natural at-sea behavior.…”