Stanford University
Northern elephant seals

Tracking migration patterns of marine predators yields geopolitical challenges

Understanding the movements of migratory marine animals through different countries' waters and in the open ocean beyond is vital to their management and conservation.

BY Stanford School of Humanities and Sciences with Smithsonian Conservation Biology Institute and UC Santa Cruz
ClockSeptember 03, 2018

Tunas, leatherback turtles, white sharks and blue whales pay no heed to geopolitical boundaries as they crisscross the Pacific Ocean in their annual wanderings, but an improved understanding of migratory marine creatures’ movements through the territories of different countries will be crucial to their conservation.

In a new study coauthored by Stanford marine biologist Barbara Block, researchers used state-of-the-art animal tracking devices to reveal how much time 14 migratory marine predators spend in the waters of different countries and in the open ocean, or “global commons,” beyond national jurisdictions.

Elephant seal
Satellite tags, such as this one on a female elephant seal, have enabled unprecedented tracking of migratory marine species. ​​​​​(Photo credit: Dan Costa)

The project drew upon thousands of days of tracking data collected and analyzed by Block’s team at Stanford as part of the Census of Marine Life’s Tagging of Pacific Predators (TOPP) program. For over a decade, Block co-directed the program, which tracked the movements and behaviors of top ocean predators throughout the Pacific Ocean.

“Our electronic tag tracks demonstrate that highly migratory marine animals, such as bluefin tuna and leatherback sea turtles cross the open ocean for thousands of nautical miles annually, plying the waters of many nations. The high seas cover almost half our planet and we need more accountability for the catch of wildlife in these waters,” said Block, the Charles and Elizabeth Prothro Professor in Marine Sciences at Stanford’s Hopkins Marine Station and the co-principal investigator of the study.

The results, published online on September 3 in the journal Nature Ecology & Evolution, provide critical information for designing international cooperative agreements for the management of these species.

“Determining which stamps each of these species would have in their passports helps us understand which countries need to cooperate to ensure the animals’ are protected during the most important parts of their journey,” said the study’s lead author, Autumn-Lynn Harrison, a research ecologist at the Smithsonian Conservation Biology Institute in Washington, D.C.

Under international law, every coastal nation can establish an exclusive economic zone (EEZ) extending up to 200 nautical miles from shore, giving it exclusive rights to exploit resources and regulate fisheries within that zone. The high seas beyond the EEZs are a global commons and are among the least protected areas on Earth. Discussions have been under way at the United Nations (U.N.) since 2016 to negotiate a global treaty for conservation and management of the high seas, which accounts for nearly 50 percent of the planet’s surface.

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The high seas cover almost half our planet and we need more accountability for the catch of wildlife in these waters.

White shark
Mature sharks spend about 60 percent of their time in the high seas where they are vulnerable to global fishers. (Photo credit: Terry Gross / Wikimedia Commons)

Harrison will present the team’s key findings at a September session of the U.N. General Assembly’s Intergovernmental Conference, which will focus on securing an international agreement for the conservation of marine biological diversity in the global commons.

"These migratory species are a shared heritage, and this paper shows their international travels better than ever before," Harrison said. "The first step to protect them is knowing where they are over their annual cycle and promoting international agreements to manage the threats they may face across several countries."

The new study reveals that many of the ocean’s top predators spend time in different parts of the ocean, where they can face a variety of threats. For example, white sharks are protected in U.S. and Mexican exclusive economic zones, but the TOPP data show that mature sharks spend about 60 percent of their time in the high seas where they are vulnerable to global fishers.

Cumulatively, the animals in the study visited the jurisdiction of 37 countries and all of them spent some time in the high seas, where they are vulnerable to overfishing and incidental catch of non-target species (bycatch) and pollution.

leatherback turtle
Leatherback sea turtles, a critically endangered species, may visit over 30 countries during their migrations. (Photo credit: Smithsonian Conservation Biology Institute)

Some traveled only through a couple of countries, making their management relatively straightforward. For those species, this study shows which countries need to collaborate on management decisions and when during the year those measures need to be put into place.

But others, like the leatherback turtle, were found to travel through dozens of countries. Leatherbacks were among the most critically endangered of those included in the study and are a priority species for the National Oceanic and Atmospheric Administration, which contributed to the research. The population that the researchers tracked could face a 96 percent decline by 2040. Because they travel through more than 30 countries and the high seas, leatherback turtles highlight the need for multi-lateral, cooperative and international agreements to manage the species, according to the paper’s authors.

“Without a concerted effort for international fisheries to allow these migratory turtles to move through corridors of open ocean free from longlines and purse seines, we will not be able to save these ancient mariners,” Block said.

Other coauthors on the study include Steven Bograd at NOAA Fisheries; Arliss Winship at NOAA's National Centers for Coastal Ocean Science; Scott Benson, Heidi Dewar, Peter Dutton, and Suzanne Kohin at NOAA Fisheries; Michelle Antolos and Patrick Robinson at UCSC; Aaron Carlisle at University of Delaware, George Shillinger at Upwell; Sal Jorgensen at the Monterey Bay Aquarium; Bruce Mate at Oregon State University; Kurt Schaefer at the Inter-American Tropical Tuna Commission; Scott Shaffer at San Jose State University; Samantha Simmons at the Marine Mammal Commission; Kevin Weng at the Virginia Institute of Marine Science; and Kristina Gjerde at the IUCN Global Marine and Polar Program.

TOPP research was funded by the Alfred P. Sloan Foundation, David and Lucile Packard Foundation, and the Gordon and Betty Moore Foundation, with additional support from the Office of Naval Research, the National Oceanic and Atmospheric Administration, the E&P Sound and Marine Life Joint Industry Program, donors to the Oregon State University Marine Mammal Institute, and the Monterey Bay Aquarium Foundation.

Block is also an affiliate of the Stanford Woods Institute for the Environment.

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