Satellite tracking and stable isotope analysis highlight differential recruitment among foraging areas in green turtles

Abstract

Identifying links between breeding and non-breeding sites in migratory animals is an important step in understanding their ecology. Recognising the relative importance of foraging areas and ascertaining site-specific levels of recruitment can provide fundamental and applied insights. Here, satellite telemetry and the stable isotope ratios (δ 13 C, δ 15 N and δ 34 S) of 230 green turtles Chelonia mydas from a regionally important rookery in northern Cyprus were employed to evaluate the relative importance of 4 foraging areas. A preliminary analysis of stable isotope ratios suggested that a major foraging area had been missed through satellite telemetry as a large proportion of turtles had isotope ratios that did not correspond to sites previously identified. Stable isotope ratios were then employed to select 5 turtles to be fitted with platform terminal transmitters in 2015. All 5 turtles were subsequently tracked to the same location, Lake Bardawil in Egypt. Serially collected tissue samples from 45 females, ranging over 2 to 4 breeding seasons, suggested that foraging site fidelity was very common, with 82% of females exhibiting extremely high temporal consistency in isotope ratios. Quantifying fidelity allowed an evaluation of foraging areaspecific contributions to each breeding cohort over the past 2 decades and demonstrated that recruitment was unequal among sites, and dynamic over time, with Egypt now currently the major contributor to the nesting aggregation. This work demonstrates the utility of stable isotope analysis to elucidate the spatial ecology of cryptic taxa and illustrates how more robust baselines can be assembled against which to measure the success of future marine conservation initiatives.

Mitochondrial DNA short tandem repeats unveil hidden population structuring and migration routes of an endangered marine turtle

“…Finally, no telemetry data have been published regarding the foraging areas used by the turtles nesting in Akamas, although our results suggest that the Israeli coast is not one of them. Considering published data and our own results, it seems that there might be a contradiction, as stable isotope analysis suggested that a significant proportion of turtles (39%) from Alagadi approach the Israeli coast on the migration to Egypt (Bradshaw et al, ), but this was not detected when performing an MSA (present study).…”

“…Our results demonstrate that a significant proportion of Turkish individuals follow the Levantine coast and arrive in Israeli foraging areas. Additional studies incorporating stable isotopes are desirable to fully understand the habitat use of Turkish nesting turtles (Bradshaw et al, ). Five animals from Israel, three females and two males, were also recently tracked by telemetry showing that Israeli turtles remain in the vicinity of this area, and thus explaining why they were detected among the stranding animals (Levy et al, ).…”

“…The first possibility is that the Levantine shore is a migratory corridor but not a foraging area for Cypriot turtles. Although some individuals travelled across the Israeli coast on their migration to Egypt, satellite telemetry showed that the migration speed was rapid in this area, and not indicative of foraging (Bradshaw et al, ). The probability of a turtle being detected by the MSA is necessarily related to the time they spent in the area and the potential for them to interact with fishing gear and other threats, so fast migrating individuals may remain undetected.…”

Spatial ecology of loggerhead turtles: Insights from stable isotope markers and satellite telemetry

“…If isotope ratios of foraging grounds are distinct, this enables untracked females to be reliably assigned to putative foraging grounds from a single tissue sample, hence providing an understanding of the spatial ecology at a population level (e.g., Ceriani et al, ; Seminoff et al, ). In addition, the temporal consistency of isotope ratios has been used for confirming foraging ground fidelity in marine taxa (e.g., Newsome et al, ), including marine turtles (e.g., Bradshaw et al, ; Thomson et al, ). If foraging ground fidelity occurs, then long‐term studies enable the proportion of individuals in each annual nesting cohort using each foraging ground to be determined across multiple nesting seasons and therefore identifying potential shifts in population dynamics (e.g., Bradshaw et al, ; Ceriani et al, , ).…”

“…In addition, the temporal consistency of isotope ratios has been used for confirming foraging ground fidelity in marine taxa (e.g., Newsome et al, ), including marine turtles (e.g., Bradshaw et al, ; Thomson et al, ). If foraging ground fidelity occurs, then long‐term studies enable the proportion of individuals in each annual nesting cohort using each foraging ground to be determined across multiple nesting seasons and therefore identifying potential shifts in population dynamics (e.g., Bradshaw et al, ; Ceriani et al, , ). Temporal changes in the proportion of individuals using each foraging ground could be inferred as changes in the foraging ground dynamics, including changes in recruitment, survival of individuals or changes in foraging resources and environmental conditions.…”