The research team, led by Bianca S. Santos of Stanford University’s Doerr School of Sustainability together with Larry B. Crowder and Dana K. Briscoe, drew on satellite telemetry from 53 juvenile loggerheads tracked between 2023 and 2025. These animals are part of a broader, decades-long effort known as Project STRETCH, which has involved raising and releasing loggerheads hatched from Japanese nesting beaches and tracking their trans-Pacific journeys. The newly published analysis adds a crucial layer to that legacy: a quantitative description of the ocean conditions the turtles select, and a jurisdictional accounting of where suitable habitat falls under national or international authority.

The technical core of the study rests on two satellite-derived oceanographic variables: sea surface temperature, or SST, and chlorophyll-a concentration, a satellite proxy for the abundance of phytoplankton at the base of the marine food web. By matching each turtle’s transmitted position with contemporaneous measurements of these variables, the researchers characterized the environmental envelope the juveniles actually occupy. The animals were found in waters with a mean sea surface temperature of 17.4 degrees Celsius, with the 10th to 90th percentile range spanning 15.4 to 19.9 degrees Celsius. Chlorophyll-a concentrations averaged 0.14 milligrams per cubic meter, with a 10th to 90th percentile range of 0.07 to 0.22 milligrams per cubic meter.

Those numbers may sound modest, but they encode a great deal of ecology. Loggerhead turtles are ectotherms, meaning their body temperature and activity levels track the surrounding water, so temperature effectively sets the thermal window within which they can forage, grow, and migrate. Chlorophyll-a, meanwhile, marks biologically productive water, the kind of frontal zones and nutrient-rich patches where the small prey items of juvenile loggerheads concentrate. Together, the two variables define a habitat signature: cool but not cold, productive but not eutrophic, characteristic of the transitional waters of the eastern North Pacific where warm subtropical and cooler subarctic water masses meet.

The team then applied these thresholds to a longer archive of oceanographic data spanning 2014 through 2025, a period representing warm ocean conditions in the region, including the influence of large-scale climate oscillations that periodically warm the northeastern Pacific. For each season and year, they mapped where conditions fell within the turtles’ observed environmental envelope, producing inferred patterns of suitable habitat across the entire eastern North Pacific. This approach allowed the researchers to move beyond the limited footprint of individual tracks and ask a broader question: across twelve years of varying ocean conditions, where does juvenile loggerhead habitat actually sit relative to the lines humans draw on the sea?

The answer was strikingly seasonal and strikingly international. Suitable habitat shifted through the year, with between 2 and 31 percent of the United States exclusive economic zone and between 1 and 13 percent of the Mexican exclusive economic zone containing suitable habitat annually. In other words, the amount of turtle-friendly water inside national waters varied enormously from season to season and year to year, expanding and contracting as oceanographic fronts moved and water masses shifted with the climate cycle. A juvenile loggerhead swimming through this dynamic seascape would cross jurisdictional boundaries not once but repeatedly over the course of its development.

The headline figure, however, concerns the high seas. Overall, 86 percent of the suitable habitat for juvenile loggerheads in the eastern North Pacific occurred in areas beyond national jurisdiction, compared with just 6 percent within the United States exclusive economic zone and 8 percent within the Mexican exclusive economic zone. For a population that spends years, possibly decades, in this developmental stage before returning toward western Pacific foraging grounds, the implication is unavoidable: the bulk of its juvenile life unfolds in waters where no single nation holds regulatory authority, and where conservation depends on international cooperation rather than any one country’s domestic protections.

This finding lands at a consequential moment for ocean governance. The high seas have long been a patchwork of sectoral management, with regional fisheries bodies, shipping regulations, and scattered agreements addressing individual activities but no comprehensive framework for biodiversity protection. The recent agreement under the United Nations Convention on the Law of the Sea on the conservation and sustainable use of marine biological diversity of areas beyond national jurisdiction, often called the BBNJ Agreement, aims to change that by enabling the establishment of marine protected areas on the high seas and requiring environmental impact assessments for activities there. The new loggerhead analysis provides exactly the kind of species-specific, jurisdictionally explicit evidence that such frameworks will need to prioritize particular regions for protection.

The study also underscores why migratory marine species pose such a persistent governance challenge. Unlike seabirds that return to fixed colonies or marine mammals tied to predictable coastal haul-outs, juvenile loggerheads drift through a moving mosaic of thermal fronts and productivity gradients that respect no maritime boundary. As climate change warms and reorganizes ocean conditions, the location and extent of suitable habitat will continue to shift, potentially drawing turtles into new areas, new fisheries interactions, and new management gaps. The authors’ use of a warm-conditions baseline for 2014 through 2025 is a deliberate acknowledgment of this reality: the habitat map of the recent past may not be the habitat map of the coming decades.

For the endangered North Pacific loggerhead population, the practical stakes are concrete. Where juveniles concentrate, they risk entanglement in fishing gear, and the effectiveness of any bycatch mitigation measure depends on knowing when and where turtles and fisheries overlap. The new environmental thresholds, a temperature window centered near 17 degrees Celsius and a chlorophyll-a range around 0.14 milligrams per cubic meter, offer fishery managers and conservation planners a transferable tool: satellite data, freely available in near real time, can now be used to anticipate where suitable juvenile habitat is likely to appear across both national waters and the high seas. Combined with the finding that the overwhelming share of that habitat lies beyond any single nation’s reach, the study makes a clear case that protecting this population’s youngest members is inherently a shared, international enterprise, one that the emerging high seas conservation architecture is only beginning to equip the world to undertake.

Subject of Research: Environmental preferences and jurisdictional distribution of juvenile loggerhead turtles in the eastern North Pacific

Article Title: Environmental preferences and jurisdictional distribution of juvenile loggerheads in the eastern North Pacific

Article References: Santos, B. S., Crowder, L. B., Balazs, G. H., Polovina, J. J., Abreu-Grobois, A., Gaspar, P., Lee Hing, C. A., Jim, L., Kurita, M., Mori, M., Parker, D. M., Rice, M. R., Saito, T., Yamaguchi, N., & Briscoe, D. K. (2026). Environmental preferences and jurisdictional distribution of juvenile loggerheads in the eastern North Pacific. npj Ocean Sustainability. https://doi.org/10.1038/s44183-026-00250-w

Image Credits: AI Generated

DOI: 10.1038/s44183-026-00250-w

Keywords: loggerhead turtles, satellite telemetry, eastern North Pacific, high seas, exclusive economic zone, sea surface temperature, chlorophyll-a, marine conservation, habitat modeling, BBNJ Agreement, endangered species, ocean governance