Combining stable isotopes and skeletal growth marks to detect habitat shifts in juvenile loggerhead sea turtles Caretta caretta 

Abstract

Understanding the phase and timing of ontogenetic habitat shifts underlies the study of a species' life history and population dynamics. This information is especially critical to the conservation and management of threatened and endangered species, such as the loggerhead sea turtle Caretta caretta. The early life of loggerheads consists of a terrestrial egg and hatchling stage, a posthatchling and juvenile oceanic, pelagic feeding stage, and a juvenile neritic, primarily benthic feeding stage. In the present study, novel approaches were applied to explore the timing of the loggerhead ontogenetic shift from pelagic to benthic habitats. The most recent years of somatic growth are recorded as annual marks in humerus cross sections. A consistent growth mark pattern in benthic juvenile loggerheads was identified, with narrow growth marks in the interior of the bone transitioning to wider growth marks at the exterior, indicative of a sharp increase in growth rates at the transitional growth mark. This increase in annual growth is hypothesized to correlate with the ontogenetic shift from pelagic to benthic habitats. Stable isotopes of carbon and nitrogen just interior and exterior to the transitional growth mark, as well as stable isotopes from pelagic and benthic flora, fauna and loggerhead stomach contents, were analyzed to determine whether this transition related to a diet shift. The results clearly indicate that a dietary shift from oceanic/pelagic to neritic/benthic feeding corresponds to a transitional growth mark. The combination of stable isotope analysis with skeletochronology can elucidate the ecology of cryptic life history stages during loggerhead ontogeny.

Age and size at maturation- and adult-stage duration for loggerhead sea turtles in the western North Atlantic

“…In cases where only curved carapace length (CCL) measurements were available, SCL was calculated using Eq. (1) from Snover et al (2010).…”

Growth rates of adult sea turtles

“…While movement between alternative habitats may incur physiological, morphological and behavioural costs as individuals adapt to their new environment, these may be outweighed by the benefits associated with more suitable environmental conditions (Werner & Gilliam 1984, Bolten 2003b. Juveniles that move between habitats might benefit from higher growth rates due to higher food availability and quality and thus larger SSM (Werner & Gilliam 1984, Bolten 2003a, Snover et al 2010. Although Gross (1984) argued that, for evolutionary strategies to be stable, fitness of alternative strategies should be equivalent, with reproductive output being positively correlated with SSM in sea turtles (Van Buskirk & Crowder 1994), life-history dichotomies may ultimately result in differential fecundity both within and between populations and species (Hatase et al 2013, Ceriani et al 2015.…”

“…Hatchlings and small juveniles from the latter species inhabit oceanic waters for an undetermined period of time, feeding on nutrient-poor epipelagic prey and experiencing relatively slow growth (Bjorndal et al 2000b, Bolten 2003a. Upon reaching a size threshold (Bjorndal et al 2000b, Bolten 2003b), large juveniles undergo what was long thought to be a marked, nonreversible ontogenetic shift to neritic waters, feeding on more abundant, nutrient-rich benthic prey (Hawkes et al 2006, Snover et al 2010. Although this change of environment may come at the cost of an increase in predation risk (Bolten 2003b), it could result in as much as a 30% increase in juvenile growth rates (Snover et al 2010), thus appearing highly advantageous.…”

“…Upon reaching a size threshold (Bjorndal et al 2000b, Bolten 2003b), large juveniles undergo what was long thought to be a marked, nonreversible ontogenetic shift to neritic waters, feeding on more abundant, nutrient-rich benthic prey (Hawkes et al 2006, Snover et al 2010. Although this change of environment may come at the cost of an increase in predation risk (Bolten 2003b), it could result in as much as a 30% increase in juvenile growth rates (Snover et al 2010), thus appearing highly advantageous. Recent studies, however, suggest that the ontogenetic shift is both facultative, with some individuals remaining in oceanic waters throughout their life cycle (Hatase et al 2002, Hawkes et al 2006, Ramirez et al 2015, and reversible, with some individuals returning to oceanic waters (McClellan & Read 2007, McClellan et al 2010, Ramirez et al 2015.…”

Comparing Acoustic Tag Attachments Designed for Mobile Tracking of Hatchling Sea Turtles

“…Information about sea turtle dispersal and behaviour during the "lost years" has been gained through modelling approaches (Hays et al, 2010;Shillinger G. L. et al, 2012;Putman et al, 2013;Casale and Mariani, 2014), telemetry (Nagelkerken et al, 2003;Witherington et al, 2012;Mansfield et al, 2014;Scott et al, 2014), and other emerging technologies, such as stable isotopes (Bowen and Karl, 2007;Reich et al, 2007;Snover et al, 2010;López-Castro et al, 2014). Due to a lack of information on active dispersal capacity, modelling efforts rely heavily on classifying young turtles as "passive drifters, " with little influence on their environment (Hays et al, 2010;Gaspar et al, 2012;Shillinger G. L. et al, 2012;Putman and Mansfield, 2015).…”