When surfacers do not dive: multiple significance of extended surface times in marine turtles

Abstract

SUMMARYMarine turtles spend more than 90% of their life underwater and have been termed surfacers as opposed to divers. Nonetheless turtles have been reported occasionally to float motionless at the surface but the reasons for this behaviour are not clear. We investigated the location, timing and duration of extended surface times (ESTs) in 10 free-ranging loggerhead turtles (Caretta caretta) and the possible relationship to water temperature and diving activity recorded via satellite relay data loggers for 101-450 days. For one turtle that dived only in offshore areas, ESTs contributed 12% of the time whereas for the other turtles ESTs contributed 0.4-1.8% of the time. ESTs lasted on average 90 min but were mostly infrequent and irregular, excluding the involvement of a fundamental regulatory function. However, 82% of the ESTs occurred during daylight, mostly around noon, suggesting a dependence on solar radiation. For three turtles, there was an appreciable (7°C to 10.5°C) temperature decrease with depth for dives during periods when ESTs occurred frequently, suggesting a re-warming function of EST to compensate for decreased body temperatures, possibly to enhance digestive efficiency. A positive correlation between body mass and EST duration supported this explanation. By contrast, night-active turtles that exceeded their calculated aerobic dive limits in 7.6-16% of the dives engaged in nocturnal ESTs, probably for lactate clearance. This is the first evidence that loggerhead turtles may refrain from diving for at least two reasons, either to absorb solar radiation or to recover from anaerobic activity.

Foraging grounds, movement patterns and habitat connectivity of juvenile loggerhead turtles (Caretta caretta) tracked from the Adriatic Sea

“…Our Wndings extend those reported for juvenile loggerhead turtles tracked from the Bay of Naples, one of which entered the Adriatic (Hochscheid et al 2010). In this respect, the case of turtle G is particularly informative, since it performed seasonal migrations shuttling between two distant foraging grounds and showed Wdelity to a speciWc north Adriatic foraging ground.…”

Movement patterns of marine turtles in the Mediterranean Sea: a review

“…In general, resident areas of juveniles in oceanic zones are much wider than in neritic zones (Revelles et al 2007b;Cardona et al 2009;Casale et al 2012aCasale et al , 2012b (Figure 4), as also observed for adults in the Mediterranean and elsewhere (Hawkes et al 2006;Schofield et al 2010a). Long-term residence of juveniles in neritic foraging grounds seems more common than in oceanic areas and it has been observed in various locations through tag returns (Casale et al 2007a;Revelles et al 2008) and satellite tracking Cardona et al 2009;Hochscheid et al 2010;Casale et al 2012aCasale et al , 2012bHochscheid et al 2013) (Figure 3). The juvenile neritic foraging sites are, however, usually larger than the adult neritic sites, with juveniles showing a propensity to wander over quite large areas (e.g.…”

“…Some just wandered across wide areas, with no sign of residence at any specific site (Bentivegna 2002;Casale et al 2007a;Eckert et al 2008;Cardona et al 2009;Hochscheid et al 2010) (Figure 3). Others frequented somewhat smaller areas (Cardona et al 2005;Revelles et al 2007aRevelles et al , 2007bEckert et al 2008;Cardona et al 2009;Hochscheid et al 2010;Casale et al 2012aCasale et al , 2012b (Figure 3), in some cases remaining for a long time in very restricted sites (Hochscheid et al 2010;Mencacci et al 2011). The areas frequented and the routes followed are the result of a combination of surface circulation patterns and the turtles' active swimming Revelles et al 2007b).…”

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

“…Apart from behavioural means (Hochscheid et al, 2010), chelonid sea turtles generally do not thermoregulate and, hence, do not spend energy for thermoregulatory purposes. Leatherback turtles, on the other hand, might use the metabolic heat generated by vigorous swimming activity to maintain an elevated and stable body temperature (Bostrom and Jones, 2007;Bostrom et al, 2011).…”

“…One possible complication in this context is the basking behaviour occasionally observed in marine turtles. For loggerhead turtles, it was suggested that periods spent basking at the surface have a re-warming function to compensate for decreased body temperatures when diving in cooler water, possibly to enhance digestive function (Hochscheid et al, 2010). While ambient temperature is monitored by the acceleration data logger and was included in our model, it is not clear how well such temperature measurements will represent turtle body temperature (affecting turtle metabolism) under these circumstances.…”