As the tag technology improves, researchers will be able to determine spatial consistency in migration paths for a greater sample size of turtles, and during remigration. One previously tracked loggerhead followed the same migration path post-nesting and during its remigration back to nesting grounds Foley et al.
Therefore, the corridors we identified may also be important during remigration to nesting grounds, which would occur at a different time of year when threat levels may be different. Tracking turtles from foraging grounds to their nesting beaches would help determine how consistently they use corridors for both directions of migration. The cues that drive the phenology of migration for sea turtles are not well understood, and more research on the timing of migration from foraging grounds and associated environmental conditions could help with understanding what drives this behavior for individuals in some years and not others.
One turtle migrated out of the GoM south to Nicaragua, for a total distance moved of 4, km. The next longest migration in this study was 2, km, a difference of around 1, km.
The upper distance limit for adult Cheloniid sea turtles undertaking breeding migrations is thought to be around 3, km, because of limits on available fat stores Hays and Scott, Interestingly, the turtle migrating to Nicaragua paused along the coast of Cuba from late August until early October after traveling for about 1, km. While the SSM identified this time as migration, it is possible that this represents a type of stopover, where the turtle may have been seeking resources for refueling.
There are anthropogenic threats to sea turtles which we were not able to quantify and thus did not include in our analysis, such as plastic pollution, effects from climate change, and direct harvest. Plastic pollution presents a serious threat to sea turtles, with over half of sea turtles in the world predicted to have ingested plastic debris and a relatively high-risk of ingestion predicted specifically in the GoM for hard-shelled sea turtles Schuyler et al. Additionally, changes to ocean currents and sea surface temperatures due to climate change were not considered.
Theoretically, sea turtles could be affected by these changes during migration, as changing temperatures and currents could alter the energetic costs of migration, however, how these changes would affect sea turtle migration is not well understood Southwood and Avens, Direct harvest is considered the third highest threat to sea turtles based on expert opinion Donlan et al. Lastly, based on the small percentage of HABs we found occurring during the migration period, these blooms are likely of higher concern for loggerheads on foraging grounds than those migrating in the summer.
Therefore, while this may not be a primary, consistent threat to migrating sea turtles of those we examined, it can still have important impacts in some years. For commercial line fishing, the data represents a minimum estimate of possible impacts. This is because we included only reported line fishing trips with known gear. We also did not include trips where the gear was simply reported as combined multiple gear types.
Commercial line fishing is known to have cumulatively high sea turtle bycatch, but other forms of fishing with nets may also have impacts Lewison and Crowder, Not all trips will have the same impact, as that depends on the effort of each trip. Here we use only the number of trips as a metric, assuming that more trips mean more impact. Even with the threat from line fishing possibly being underestimated, the potential threat to sea turtles appears relatively high across most of the GoM, as many thousands of trips were reported for just one summer.
Shipping density is most problematic for migrating loggerheads that are traveling through the Florida Straits, and a lower level of this threat occurs across all other areas considered. We mapped this layer to demonstrate areas with more potential for ship strikes, which have been shown as a common cause of sea turtle mortality in the Mediterranean Casale et al. Lastly, shrimp trawling effort is not as high across most of the western Florida shelf as in waters surrounding Louisiana, however, this threat is persistent across neritic areas of the GoM.
Here we show that shipping density, commercial line fishing, and shrimp trawling can affect the mortality of loggerhead sea turtles in the GoM. However, we did not weigh these threats in relation to each other and were unable to consider all possible threats, therefore we consider our threats index to be a minimum estimate. Importantly, our identified corridor in the GoM overlaps with migration areas for other species of concern. Cumulative Effects Assessments CEAs , sometimes referred to as Cumulative Impact Assessments, are procedures that identify and evaluate the collective impact of multiple human activities and natural processes on the environment Jones, CEAs are considered critically important for informing effective marine policy, however, the use of CEAs in real-world management processes remains a challenge largely due to the wide variation in approaches.
CEAs are complex and have been criticized for a lack of measurable and clearly defined sustainability goals, being poorly aligned with institutional frameworks, and a lack of objective criteria Jones, Nevertheless, attempts have been made to improve on CEAs by re-evaluating the structure and intent, reducing ambiguity, and orienting toward a common objective across regions Willsteed et al.
As we did not weight threats with additional quantitative data on mortalities and injuries caused, our analysis may be considered a Cumulative Pressure Assessment CPA , and a step toward a fully parameterized CEA.
By overlaying anthropogenic threats onto the migration corridors, we were able to identify that hotspots of high values occurred around the northwest Florida coast, off of Tampa Bay, and in the Florida Straits. In our study, our conservation target is clear: the survival of migrating adult female loggerheads.
In the Loggerhead Recovery Plan, managing migratory pathways and minimizing vessel strike mortality are listed as Recovery Objectives NMFS and USFWS, , and therefore our results directly provide scientific information needed for designing management strategies for this threatened species. In order to inform what threshold is acceptable for each threat, a future CEA would benefit from an understanding of what mortality level during migration is deemed sustainable for population recovery.
As new information becomes available on the spatial intensity of threats, this estimated corridor can be used to inform adaptive management of threats during the migratory period. Overall, it is imperative to understand migration patterns and threats for these highly mobile species, and our conservative estimate of threats provides valuable information for the management and recovery of loggerhead sea turtles.
The datasets generated for this study will not be made publicly available. Restrictions apply to the datasets. Raw data is exempt from publication due to the sensitivity of endangered species location information. Requests to access the datasets should be directed to the corresponding author. All other data used for analyses are presented in the manuscript.
AI and KH contributed to the conception and design of the study. KH acquired funding. KH and ML managed tag deployment and data collection. AI wrote the first draft of the manuscript. All authors contributed to manuscript revision, read and approved the submitted version. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We acknowledge assistance from D. Ingram, J. Isaacs, A. Lauritsen, S. MacPherson, and J. Phillips from the U. Reynolds for field assistance in Alabama. We also thank many U. Geological Survey volunteers and employees for assistance in the field. We acknowledge the use of the satellite-tracking and analysis tool STAT and telemetry data generated as part of the Deepwater Horizon Natural Resource Damage Assessment publicly available from www.
Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U. Alerstam, T. Long-distance migration: evolution and determinants. Oikos 2, — Almpanidou, V. Thermal heterogeneity along the migration corridors of sea turtles: implications for climate change ecology.
Marine Biol. Berger, J. Connecting the dots: an invariant migration corridor links the Holocene to the present.
Block, B. Tracking apex marine predator movements in a dynamic ocean. Nature , 86— During nesting season, research conducted in the southeast United States helped discovered that loggerheads follow regular patterns between the nesting beach itself and offshore reefs and other rocky structures.
When it is not nesting season, sea turtles may migrate hundreds or even thousands of miles. Sea turtles can sleep at the surface while in deep water or on the bottom wedged under rocks in nearshore waters. Many divers have seen green turtles sleeping under ledges in reefs and rocks. Hatchlings typically sleep floating on the surface, and they usually have their front flippers folded back over the top of their backs.
Afterwards, only females come ashore to nest; males almost never return to land once they leave the sand of their natal beach. During mating season, males may court a female by nuzzling her head or by gently biting the back of her neck and rear flippers. He then folds his long tail under her shell to copulate.
Copulation can take place either on the surface or under water. Sometimes several males will compete for females and may even fight each other. Observers of sea turtle mating have reported very aggressive behavior by both the males and females. Females may mate with several males just prior to nesting season and store the sperm for several months.
When she finally lays her eggs, they will have been fertilized by a variety of males. This behavior may help keep genetic diversity high in the population. Very little is known about why sea turtles nest on some beaches and not on others. In Florida, loggerheads nest by the thousands on the central east coast, while identical looking beaches to the north see far fewer loggerheads. This nesting distribution may reflect conditions that existed centuries ago, when temperature, beach profiles or the lack of predation made some areas preferable to sea turtles.
Today, humans are affecting the places where sea turtles nest. Beach erosion caused by coastal armoring and navigational inlets, artificial lighting and beach renourishment are all impacting once pristine beaches. These changes will likely have lasting effects on future nesting patterns. The more we understand about how, where and when sea turtles nest, the better we will be able to protect their nesting habitat. Most females return faithfully to the same beach each time they are ready to nest.
Not only do they appear on the same beach, they often emerge within a few hundred yards of where they last nested. Only the females nest, and it occurs most often at night. The female crawls out of the ocean, pausing frequently as if carefully scoping out her spot. Sometimes she will crawl out of the ocean, but for unknown reasons decide not to nest. Most females nest at least twice during the nesting season, although individuals of some species may nest only once and others more than ten times.
Sea turtles are generally slow and awkward on land, and nesting is exhausting work. The female turtle crawls to a dry part of the beach and begins to fling away loose sand with her flippers. After the body pit is complete, she digs an egg cavity using her cupped rear flippers as shovels. The new research may provide insights that are helpful in conservation, Lohmann said. For example, different populations of loggerheads around the world are likely to have different magnetic maps, Lohmann explained, with each map specific to a particular migratory pathway in one part of the world.
If loggerheads in one geographic area go extinct, it will probably be impossible to replace them with turtles from another area, because the new arrivals will lack the inherited instructions needed to navigate within and from their transplanted homes. In addition, conditions that impair the functioning of turtles' magnetic sense may jeopardize survival. Lohmann says that in Florida and elsewhere, a common conservation practice is to surround turtle nests on the beach with wire cages to protect the turtle eggs from raccoons.
But such cages also distort the local magnetic field, and may thereby compromise the ability of hatchlings to navigate after they emerge from their nests. Approximate migratory route of Florida loggerheads around the Sargasso Sea.
Credit and Larger Version. The U. National Science Foundation propels the nation forward by advancing fundamental research in all fields of science and engineering. NSF supports research and people by providing facilities, instruments and funding to support their ingenuity and sustain the U.
Each year, NSF receives more than 40, competitive proposals and makes about 11, new awards. Those awards include support for cooperative research with industry, Arctic and Antarctic research and operations, and U. They then embark once again on their journey, taking advantage of the Gulf Stream to head towards the Azores and, from there, to the European continent.
The majority follow the warmer waters that take them to the coasts of Spain and Portugal. Many enter the Mediterranean where they congregate in numbers that exceed several hundred thousand turtles.
Here, too, they encounter the less-numerous colonies of loggerhead turtles Caretta caretta that do their laying in this sea, mainly on a small number of beaches in Greece, Turkey and Cyprus.
From the west of the Iberian Peninsula, the Gulf Stream, aided by the trade winds, pushes them in a southerly direction, traversing the western coasts of Africa until they pass by the Canary Islands and Cape Verde. To the south of this archipelago, the current changes direction and heads back out to the Atlantic Ocean in a westerly direction towards the waters of the Caribbean and the east coast of the Americas, where, once they are adults, they will lay their own eggs.
Oceana, on the turtle trail In , a North American biologist specialising in sea turtles, Archie Carr, presented the hypothesis that the loggerhead turtles that laid their eggs on North American beaches made a lengthy round-trip migration across the entire Atlantic, taking advantage of the Gulf Stream.
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