Olive ridley turtles are constantly on the move, so protected areas should keep up with them

Many of the ocean’s most charismatic animals spend their lives swimming, flying or gliding thousands of miles from shore to the open sea. Arctic terns, humpback whales and sea turtles are examples. Scientists have spent many years documenting and studying these magnificent journeys.

Chronicling where these species go is just the beginning. The next steps are to understand when and how far each animal travels and what causes it to wander.

We are a marine biologist and evolutionary ecologist and have worked together to study the nesting and migration habits of endangered olive ridley sea turtles (Lepidochelys olivaceae). This information is essential for managing turtle recovery, but our research shows that two identical-looking olive ridleys can follow very different paths.

Approximate range of olive ridley sea turtles.

Protect animals on the move

Mapping the spatial distribution and movement patterns of endangered or threatened marine animals is key to defining critical habitat – the areas these species need to recover, such as key breeding or breeding grounds. feed.

Once scientists have identified critical habitats, governments can incorporate them into marine protected areas. These are usually defined areas with fixed borders. They benefit marine animals that stay in one place, such as sea anemones; have small litters; and require specific habitats, such as coral reefs or sea grass beds.

But highly migratory marine animals have large ranges and can travel several kilometers a day. They may prefer a certain location one year and another the next. And their movements are driven by changing ocean circulation patterns. Marine protected areas are not effective in protecting highly mobile species – and olive ridley sea turtles are incredibly mobile.

ocean nomads

Olive ridley sea turtles are among the world’s smallest sea turtles and are found in the tropical Atlantic, Pacific and Indian oceans. They are best known for their synchronized mass nestings on beaches in early summer, called arribadas – Spanish for “arrival”.

Thousands of female olive ridley turtles nest in an arribada in Costa Rica.

Fishing in the eastern Pacific Ocean decimated olive ridley nesting colonies before commercial exploitation ceased in the 1980s. The species has begun to recover but remains listed as vulnerable by the Union international for the conservation of nature. The United States classifies olive ridley sea turtles as threatened, with the exception of a group that nests on the Pacific coast of Mexico that is listed as endangered. Threats include fishing, egg and turtle hunting on nesting beaches, coastal development, boat strikes and water pollution.

One of us, Pamela Plotkin, began studying olive ridleys in 1990, when satellite telemetry first emerged as a viable tool for tracking them at sea. Plotkin initially expected document groups of turtles migrating from their massive nesting beach in Costa Rica to an undiscovered feeding ground in the eastern Pacific Ocean, far from land.

Adult female sea turtles usually have a predetermined end point where they go to feed after they finish nesting on beaches. It was easy to imagine throngs of turtles migrating in “turtle beds” between the beach and their feeding grounds.

Plotkin envisioned creating a simple conservation plan to protect the flyway connecting these two critical habitats. But she found no such thing.

Instead, she learned, the olive groves’ journey has no endpoint. They swim hundreds to thousands of miles from their nesting range, continuously moving between multiple areas, following unpredictable and widely dispersed pathways that vary from year to year.

Clearly, there was a need to better define the critical habitat of this highly mobile turtle in a huge expanse of ocean.

In search of the sweet spot

In 2015, with the arrival of PhD student Christine Figgener, our research group picked up where previous studies left off. Part of Figgener’s thesis research focused on characterizing critical habitat for olive ridley sea turtles in the eastern Pacific Ocean and understanding their habitat preference based on changing sea turtles. environmental variables. They included sea surface temperature and concentrations of chlorophyll-a, which is found in seaweed, an essential food source for the diverse plankton that olive ridleys eat.

Figgener has captured olive ridley sea turtles on several beaches in Costa Rica, including turtles nesting alone rather than in large groups. She stuck satellite trackers on 23 turtle shells and tracked them from their nesting beaches. We also acquired satellite data on conditions including sea surface temperature, chlorophyll-a concentrations, and depth of sunlight penetration into the upper ocean in areas where turtles have moved.

By combining old and new data, with a total of 43 different flyways and 1,553 turtle locations in the water, we identified areas of high use for olive ridley turtles between Mexico and Peru and developed a model of habitat preference to understand what attracts turtles to specific areas. .

Predictably unpredictable

The female olive ridley sea turtles we tracked swam long distances north, west, and south of Costa Rica. Their routes did not overlap, and their movements revealed no well-defined migratory corridors, such as those followed by many species.

We mapped the areas where these nomads congregated and found that their areas of high use spanned the Exclusive Economic Zones – areas extending up to 200 miles offshore – of six countries: Mexico, Guatemala, El Salvador, Nicaragua, Costa Rica and Panama.

Map showing the distribution of the olive ridley sea turtle in the eastern Pacific
These images show areas of high olive ridden use in the Eastern Pacific. The black dots are the locations of the satellite trackers. Important areas are yellow (figure A), critical areas are red (figures A and B) and exclusive economic zones are marked with yellow lines (figure B).
Figgener et al., 2022., CC BY-ND

When we compared high-use areas for mass-nesting turtles with solitary-nesting turtles, we found some striking differences. Mass-nesting turtles clustered in a large connected area that was smaller and closer to shore. Areas of high use by nesting solitary turtles were disconnected, widely dispersed, and farther from the coast.

Our model indicated that the turtles preferred habitat where the water was warmer than about 77 degrees Fahrenheit (25 degrees Celsius) and deeper than 13,000 feet (4,000 meters), and where phytoplankton were abundant.

A conservation challenge

Current sea turtle conservation strategies generally emphasize the protection of static migratory corridors. But this approach will not benefit nomadic olive groves. Instead, the large-scale migrations of these turtles and the changing use of space require a dynamic ocean management strategy. This approach uses real-time data to track target animals where they are and creates mobile protected areas in a changing environment.

Dynamic management has been used successfully in developed countries to reduce threats to whales, fish and sea turtles from capture in fisheries and ship strikes. It incorporates many types of data, including satellite tracking, voluntary catch reports from fishermen, and modeling of habitat preferences of target species. Information is quickly shared via mobile apps so that, for example, ship captains are alerted to reduce ship speed when whales are likely to be nearby.

Extending this approach to developing countries is challenging, but within reach. A dynamic olive ridley management system should predict where turtles are likely to occur in an ever-changing environment and address threats in these critical spaces. It would also require nations to work together to regulate fisheries that capture and threaten turtles in their territorial waters.

Another important angle of our research is the contrast we found between habitat use by mass-nesting turtles and solitary-nesting turtles. It supports previous studies that found two distinct groups of turtles within the same population.

Conservation efforts at nesting beaches have primarily focused on protecting a small number of beaches in Mexico and Costa Rica that olive ridley turtles use for mass nesting. But hundreds of lonely nesting beaches stretching from Mexico to Ecuador are largely unprotected. These turtles have drastically decreased in number in recent years. We argue that conserving olive ridleys will require actions both on land and at sea.

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