Underwater behaviour of green turtles monitored with video-time-depth recorders: what’s missing from dive profiles?

Abstract

We used a turtle-mounted video and data-logging system (Crittercam; National Geographic Society, USA) to study underwater behaviour and dive patterns of green turtles, Chelonia mydas, at a coastal foraging area in the Gulf of California, Mexico. Between August 1997 and June 2002, units were deployed 36 times on 34 green turtles ranging from 64.1 to 96.7 cm in straight carapace length and 38.6 to 120.5 kg in weight. A total of 89.5 h of video was recorded with corresponding dive data (1065 total dives). Foraging was observed during 8 deployments (28 events) at depths of 3.0 to 32.0 m and occurred while turtles were swimming in the midwater column and stationary on the seafloor; 4 marine algae and 5 invertebrate prey species were identified. Resting behaviour was seen during 9 deployments (33 dives) as turtles set on the seafloor at depths of 7.0 to 26.5 m. Overall, 6 dive types were observed and labeled Type 1 to Type 6 dives. Green turtles foraged during Type 1, Type 3, and Type 5 dives, whereas they rested only during Type 1 dives. In addition to elucidating the importance of specific habitats and resources in neritic foraging areas, our results confirm that a variety of underwater behaviours can be reflected by 1 specific dive profile. These data indicate caution should be exercised when ascertaining in-water activity solely based on the appearance of dive profiles.

Convergent Foraging Tactics of Marine Predators with Different Feeding Strategies across Heterogeneous Ocean Environments

“…All dive shape classes observed here have been previously identified for several diving species (e.g., Tremblay and Cherel, 2000;Baechler et al, 2002;Seminoff et al, 2006;Dragon et al, 2012;Viviant et al, 2014). However, most studies, and particularly the few conducted thoroughly on pelagic fish (Horodysky et al, 2007;Wilson and Block, 2009), have focused mainly on the two most frequent U and V-shaped dive types.…”

“…Moreover, a positive correlation between prey intake and duration of the bottom phase has also been found for harbor seals (Phoca vitulina; Baechler et al, 2002) and for penguins (Carroll et al, 2014). Although other functions, such as resting and traveling, have been proposed for U-shaped dives in marine turtles and seals (Hochscheid et al, 1999;Baechler et al, 2002;Seminoff et al, 2006), such dive functions seem to be less likely to apply to large predatory fish. U-shaped dives confined to a certain depth for extended periods of time have been observed in white marlin Kajikia albida (Horodysky et al, 2007) and Atlantic bluefin tuna Thunnus thynnus Wilson and Block (2009), associated with exploiting aggregated prey.…”

“…From a primary visual examination of the two-dimensional dive time series, along with a literature review (e.g., Seminoff et al, 2006;Wilson and Block, 2009;Cook et al, 2011;Thomson et al, 2011), standard dive shape classes consistently performed by the sharks were defined (Figure 1) and were afterwards visually assigned to every dive. Briefly, U-shaped dives were square or parabolic, with well-defined descent and ascent phases and a distinct, relatively flat bottom phase.…”

“…Two fundamental dive types have been identified in several species based on their distinctive "V" and "U" shapes, and have been observed for pinnipeds (e.g., Beck et al, 2003;Halsey et al, 2007), marine turtles (e.g., Hochscheid et al, 1999;Seminoff et al, 2006) and seabirds (e.g., Tremblay and Cherel, 2000;Cook et al, 2011) to fish (e.g., Horodysky et al, 2007;Houghton et al, 2009). For these predators, V-shaped dives are thought to correspond to transiting or prey searching behaviors: by swimming up and down through the water column, thus crossing between different depth layers, animals may increase the probability of detecting olfactory cues, since odor trails essentially propagate in the ocean horizontally (Carey and Scharold, 1990;Pade et al, 2009).…”

“…Linking movement patterns to habitat use remains, however, a challenging task. Detailed records of prey abundance and distribution and accurate indices of feeding are difficult to obtain for the majority of species and although visual assessment of prey capture is possible for some species (Seminoff et al, 2006;Elliott et al, 2008), in most cases, indirect parameters have been used as a proxy (e.g., gastric or visceral temperature changes, mouth/beak opening or head/jaw movement, accelerometer signatures; Sepulveda et al, 2004;Gleiss et al, 2011aGleiss et al, , 2013Nakamura et al, 2011Nakamura et al, , 2015Carroll et al, 2014;Nakamura and Sato, 2014). For efficient foraging by predators, patterns of habitat use are assumed to reflect the distribution, density and quality of prey resources (Stephens and Krebs, 1986;Austin et al, 2006;Carroll et al, 2017).…”