Activity of loggerhead turtles during the U-shaped dive: insights using angular velocity metrics

Rm Gunner,P Hopkins,Md Holton,R Scott,A Arkwright,A Fahlman,Rp Wilson,C Eizaguirre,C Duarte,M Ulrich

doi:10.3354/esr01125

Abstract

Understanding the behavioural ecology of endangered taxa can inform conservation strategies. The activity budgets of the loggerhead turtle Caretta caretta are still poorly understood because many tracking methods show only horizontal displacement and ignore dives and associated behaviours. However, time-depth recorders have enabled researchers to identify flat, U-shaped dives (or type 1a dives) and these are conventionally labelled as resting dives on the seabed because they involve no vertical displacement of the animal. Video- and acceleration-based studies have demonstrated this is not always true. Focusing on sea turtles nesting on the Cabo Verde archipelago, we describe a new metric derived from magnetometer data, absolute angular velocity, that integrates indices of angular rotation in the horizontal plane to infer activity. Using this metric, we evaluated the variation in putative resting behaviours during the bottom phase of type 1a dives for 5 individuals over 13 to 17 d at sea during a single inter-nesting interval (over 75 turtle d in total). We defined absolute resting within the bottom phase of type 1a dives as periods with no discernible acceleration or angular movement. Whilst absolute resting constituted a significant proportion of each turtle’s time budget for this 1a dive type, turtles allocated 16-38% of their bottom time to activity, with many dives being episodic, comprised of intermittent bouts of rest and rotational activity. This implies that previously considered resting behaviours are complex and need to be accounted for in energy budgets, particularly since energy budgets may impact conservation strategies.

Highlights

Sea turtles spend almost all of their lives at sea, which creates difficulties collecting data that may be relevant to their conservation (Gilman et al 2007, Hochscheid 2014, Wallace et al 2015, Butt et al 2016, Mingozzi et al 2016)
Focusing on sea turtles nesting on the Cabo Verde archipelago, we describe a new metric derived from magnetometer data, absolute angular velocity, that integrates indices of angular rotation in the horizontal plane to infer activity
The extremely low vectorial dynamic body acceleration (VeDBA) estimates in our turtles generally, mean values of VeDBA did increase over the 3 categories of resting, episodic-resting and active dives for each turtle (Fig. S3)

Summary

Introduction

Sea turtles spend almost all of their lives at sea, which creates difficulties collecting data that may be relevant to their conservation (Gilman et al 2007, Hochscheid 2014, Wallace et al 2015, Butt et al 2016, Mingozzi et al 2016) This is true with respect to behaviours associated with space use and en-. ‘type 1a’ dives, flat U-shaped dives, are very common among sea turtles (Hochscheid et al 2007, Cheng 2009), and are typically associated with resting (Hays et al 2000, Seminoff et al 2006, Hays 2008, Cheng 2009, Okuyama et al 2012) These dives are more prevalent between nesting events, at times when turtles are reported to be minimizing energy expenditure so as to allocate resources to developing eggs (Houghton et al 2002, Schofield et al 2009). Such information may be important for conservation initiatives, which may have to consider protecting the space used by inter-nesting females rather than considering it energetically barren

Methods

Results

Conclusion