Energy expenditure of freely swimming adult green turtles (Chelonia mydas) and its link with body acceleration

Marine turtles are globally threatened. Crucial for the conservation of these large ectotherms is a detailed knowledge of their energy relationships, especially their at-sea metabolic rates, which will ultimately define population structure and size. Measuring metabolic rates in free-ranging aquatic animals, however, remains a challenge. Hence, it is not surprising that for most marine turtle species we know little about the energetic requirements of adults at sea. Recently, accelerometry has emerged as a promising tool for estimating activity-specific metabolic rates of animals in the field. Accelerometry allows quantification of the movement of animals (ODBA/PDBA, overall/partial dynamic body acceleration), which, after calibration, might serve as a proxy for metabolic rate. We measured oxygen consumption rates ((V) over dot(O2)) of adult green turtles (Chelonia mydas; 142.1+/-26.9 kg) at rest and when swimming within a 13 m-long swim channel, using flow-through respirometry. We investigated the effect of water temperature (T-w) on turtle (V) over dot(O2) and tested the hypothesis that turtle body acceleration can be used as a proxy for (V) over dot(O2). Mean mass-specific (V) over dot(O2) (s(V) over dot(O2)) of six turtles when resting at a T-w of 25.8+/-1.0 degrees C was 0.50+/-0.09 ml min(-1) kg(-0.83) . s(V) over dot(O2) increased significantly with T-w and activity level. Changes in s(V) over dot(O2) were paralleled by changes in respiratory frequency (f(R)). Deploying bi-axial accelerometers in conjunction with respirometry, we found a significant positive relationship between s(V) over dot(O2) and PDBA that was modified by T-w. The resulting predictive equation was highly significant (r(2)=0.83, P<0.0001) and associated error estimates were small (mean algebraic error 3.3%), indicating that body acceleration is a good predictor of (V) over dot(O2) in green turtles. Our results suggest that accelerometry is a suitable method to investigate marine turtle energetics at sea.