Discussion
Long-term databases on the tagging and recapture of C. mydas on the east coast of Australia and in New Caledonia revealed multiple migrations across the Coral Sea showing heterogeneous patterns in connectivity between C. mydas nesting and feeding grounds across the Coral Sea. Despite low tag recovery, migratory paths spanned the entirety of the Coral Sea with considerable longevity between mark and recaptures periods.
Low Tag Recovery
Low tag recoveries could be explained by several factors. Firstly, a very high dispersal rate in the South Pacific. The individuals found nesting in New Caledonia are likely to be foraging on every reef and seagrass pasture within the Southwest Pacific region, thus making it difficult to recapture them (or be informed of their recapture) from areas with low human populations. Secondly, very high loss rate of tags. If a turtle loses its tag then when it is encountered again it is not seen as a recapture. However, there are much higher rates of tag recoveries at dedicated tagging–recapture study sites in Australia of breeding C. mydas returning for nesting in subsequent years and in foraging areas . Thirdly, high anthropogenic impact. The low proportion of recaptures could be explained by high numbers being killed for food consumption, as bycatch or by pollution; and finally low percentage of tag return. People are often reluctant to return tags to the appropriate authorities as it is prohibited to hunt turtles in New Caledonia since 2009 (although exceptions are made for traditional purposes).
Post–Nesting Trends
The different species of sea turtles are dissimilar in many ways, but one behavior they have in common is that they return to the area where they hatched in order to reproduce, a phase often referred to as “natal homing. A way to study this period is to tag turtles as they are leaving the beach after nesting and study their migration back to their feeding grounds. Many studies have looked at different hypothesis on how certain taxa migrate and many specifically on sea turtles but very little is known on why and what pathways do they use. Here we try to provide records to understand the underlying patterns of C. mydas dispersion in the South Pacific by looking at the tag recoveries geographically. Tag recoveries from females tagged at D’Entrecasteaux atolls were found all along the QLD coast, in PNG and also around the main island of New Caledonia. We found a very clear west-ward trend for post-nesting migration when nesting occurred in New Caledonia. This has been reported previously from the central South Pacific where 96% of individuals in the study migrated westward and more specifically to Fiji. The explanation given was that Fiji has large areas of seagrass and algae. No record has been found of C. mydas migrating from New Caledonia to Fiji but two individuals have also been recorded to travel from French Polynesia to New Caledonia, thus going past Fiji. Within the same study, five individuals were tracked from French Polynesia to Fiji. More factors are likely at play to explain the observed westward trend besides the abundance of food resources. Moreover, three females known to nest in New Caledonia were found feeding on Heron Reef, while 23 females known to nest on Heron Island were found in New Caledonia. Turtles born on Heron Island therefore seem to have enough forage near their nesting grounds so why travel 1300 km to feed, with increased energy demands associated with long migration. Taken together, these findings lend strength to the hypothesis that more cues are used by sea turtles to choose their feeding grounds than just the abundance and proximity of food sources. Several species from the family Salmonidae have a similar pattern of natal-homing but their migration seems to be explained by a feeding pattern and a trail of pheromones left by descending smolt that triggers the migration which is not proven in sea turtles. The composition of foraging aggregations seems to be also influenced by currents and the Earth’s geomagnetic field but not all individuals choose to settle in the exact same way, otherwise individuals coming from the same rookery and born in the same year at one rookery would all be found in one feeding ground, which is not the case. A recent hypothesis is that foraging site selection reflects passive drift experienced by hatchlings thus the adult’s movements seems to be directed by constant currents from breeding sites. The North Caledonian Jet (NJC) and the South Caledonian Jet (SCJ) both have a western direction thus possibly pushing hatchlings towards Australia. However, this does not explain the eastern trend of turtles known for nesting in Australia and found feeding in New Caledonian waters as these currents now act as restraints. This pattern of ocean crossing is also found in loggerhead turtles (Caretta caretta). This was proven genetically and by tag recoveries, showing individuals feeding in Australia and belon
DiscussionLong-term databases on the tagging and recapture of C. mydas on the east coast of Australia and in New Caledonia revealed multiple migrations across the Coral Sea showing heterogeneous patterns in connectivity between C. mydas nesting and feeding grounds across the Coral Sea. Despite low tag recovery, migratory paths spanned the entirety of the Coral Sea with considerable longevity between mark and recaptures periods.Low Tag RecoveryLow tag recoveries could be explained by several factors. Firstly, a very high dispersal rate in the South Pacific. The individuals found nesting in New Caledonia are likely to be foraging on every reef and seagrass pasture within the Southwest Pacific region, thus making it difficult to recapture them (or be informed of their recapture) from areas with low human populations. Secondly, very high loss rate of tags. If a turtle loses its tag then when it is encountered again it is not seen as a recapture. However, there are much higher rates of tag recoveries at dedicated tagging–recapture study sites in Australia of breeding C. mydas returning for nesting in subsequent years and in foraging areas . Thirdly, high anthropogenic impact. The low proportion of recaptures could be explained by high numbers being killed for food consumption, as bycatch or by pollution; and finally low percentage of tag return. People are often reluctant to return tags to the appropriate authorities as it is prohibited to hunt turtles in New Caledonia since 2009 (although exceptions are made for traditional purposes).Post–Nesting TrendsThe different species of sea turtles are dissimilar in many ways, but one behavior they have in common is that they return to the area where they hatched in order to reproduce, a phase often referred to as “natal homing. A way to study this period is to tag turtles as they are leaving the beach after nesting and study their migration back to their feeding grounds. Many studies have looked at different hypothesis on how certain taxa migrate and many specifically on sea turtles but very little is known on why and what pathways do they use. Here we try to provide records to understand the underlying patterns of C. mydas dispersion in the South Pacific by looking at the tag recoveries geographically. Tag recoveries from females tagged at D’Entrecasteaux atolls were found all along the QLD coast, in PNG and also around the main island of New Caledonia. We found a very clear west-ward trend for post-nesting migration when nesting occurred in New Caledonia. This has been reported previously from the central South Pacific where 96% of individuals in the study migrated westward and more specifically to Fiji. The explanation given was that Fiji has large areas of seagrass and algae. No record has been found of C. mydas migrating from New Caledonia to Fiji but two individuals have also been recorded to travel from French Polynesia to New Caledonia, thus going past Fiji. Within the same study, five individuals were tracked from French Polynesia to Fiji. More factors are likely at play to explain the observed westward trend besides the abundance of food resources. Moreover, three females known to nest in New Caledonia were found feeding on Heron Reef, while 23 females known to nest on Heron Island were found in New Caledonia. Turtles born on Heron Island therefore seem to have enough forage near their nesting grounds so why travel 1300 km to feed, with increased energy demands associated with long migration. Taken together, these findings lend strength to the hypothesis that more cues are used by sea turtles to choose their feeding grounds than just the abundance and proximity of food sources. Several species from the family Salmonidae have a similar pattern of natal-homing but their migration seems to be explained by a feeding pattern and a trail of pheromones left by descending smolt that triggers the migration which is not proven in sea turtles. The composition of foraging aggregations seems to be also influenced by currents and the Earth’s geomagnetic field but not all individuals choose to settle in the exact same way, otherwise individuals coming from the same rookery and born in the same year at one rookery would all be found in one feeding ground, which is not the case. A recent hypothesis is that foraging site selection reflects passive drift experienced by hatchlings thus the adult’s movements seems to be directed by constant currents from breeding sites. The North Caledonian Jet (NJC) and the South Caledonian Jet (SCJ) both have a western direction thus possibly pushing hatchlings towards Australia. However, this does not explain the eastern trend of turtles known for nesting in Australia and found feeding in New Caledonian waters as these currents now act as restraints. This pattern of ocean crossing is also found in loggerhead turtles (Caretta caretta). This was proven genetically and by tag recoveries, showing individuals feeding in Australia and belon
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