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Migration Destinations

news_sealDo sea creatures have their own traffic rules?
Beneath the surface of the Pacific Ocean, fast-moving highways of migrating predators cross the hemisphere to feed and mate.

Researchers from UC Santa Cruz recently mapped these migratory routes, which intersect in two distinct hotspots—one off the West Coast in the California Current, and the other in the North Pacific Transition Zone between Hawaii and Alaska. Published June 22 by the academic journal Nature, the findings offer the first large-scale analysis of oceanic migratory systems, and provide a glimpse into the critical need for conservation in high-traffic areas.

“We knew that animals migrated to and from certain areas, but we didn’t know the details of their routes,” says Daniel Costa, of the Department of Ecology and Evolutionary Biology at UCSC. “Fishing can be better managed if we know where animals travel, and when they are breeding.”

Over the last decade, Costa and his colleagues placed 4,300 electronic tags on nearly 2,000 animals. The tags relay an animal’s location to satellites, allowing researchers to track species as they swim.

They found that 23 different marine predators cross paths in the two oceanic hotspots. Sometimes different types of predators prefer similar water conditions, and share locations with multiple species. This close predator proximity creates hotspots within biodiversity hotspots.

Researchers discovered that other predators prefer their own space—they feed at different levels or hang out in cooler or warmer waters than their neighbors. When the migratory routes of two species overlap, these habitat preferences help animals share the ocean. This good roommate behavior is important, as most animals return to the same locations year after year.

“Knowing that animals return to the same areas during their migration is very important,” says Costa. “This means any disturbance can potentially impact large numbers of animals from many different species.”

Costa co-founded the Tagging of Pacific Predators (TOPP) program in 2000 with colleagues from Stanford’s Hopkins Marine Station and the NOAA Southwest Fisheries Science Center. With the help of 70 researchers, TOPP employs a variety of state-of-the-art tags. Some measure temperature, salinity and depth, while others detect the light around an animal as it swims through the ocean.

TOPP also perfected methods for tracking tuna, which can’t swim well when tags are strapped to their fins. Tuna also lack hair, which helps scientists glue tags on seals and sea lions—the tags eventually fall off when the hair is shed. By implanting small tags into tuna, TOPP scientists tracked several different species, including Bluefin and yellowfin.

Lightweight tags similarly helped researchers track sooty shearwaters and other seabirds during long flights. As tags add additional weight, they are usually removed from birds after a month. TOPP scientists, in contrast, observed many birds for a full year.

Loggerhead turtles from Japan and Leatherback turtles from Papua New Guinea also pass through the California Current, according to the findings. Leatherbacks make their way to the Monterey Bay and then turn back—a trip that takes a few years to complete. Researchers previously tracked the turtles swimming to the southwest of the Monterey Bay, but they didn’t have a detailed map of the migratory route.

Along with the California Current, many animals cross paths in the North Pacific Transition Zone (NPTZ), says Costa. Here warm and cool currents mix, creating an east-west migration corridor that extends from Japan to Washington State.  The waters provide food for elephant seals, albatrosses, tuna, and other predators. Loggerheads also use the route as part of their migration from Japan to the California Current.

These findings may help policy makers design preserves in international waters, says Guy Oliver, a research fellow at the Long Marine Lab. “Preserves in state waters and coastal areas are helping fisheries come back, but international waters are less understood, and it’s been hard to say which areas are the most important for protection,” says Oliver, who has no affiliation with the study.

The hotspots identified by Costa are located in international waters, and the study details which areas are most important for migration.

Such findings may become increasingly important as climate change continues, forcing animals to travel longer distances in search of food and breeding grounds, says Oliver.

Costa’s findings show that animals are sensitive to changes in water temperature. In 2005, when the oceanographic conditions were warmer than usual, male California sea lions foraged much farther offshore. Female sea lions traveled much farther North. Female sea lions also traveled three times as far to get food, which took them away from their pups. “Usually a female sea lion goes out for two or three days to find food, but when waters were warmer they left for six to seven days,” says Costa.

Next up, Costa and his colleagues want to know how climate change will alter migratory routes and hotspot habitats, and how animals and their food sources might respond to long-term changes in water temperature.

“Animals from Southern California might move northward as temperatures increase in the South, but there aren’t as many island breeding grounds in Northern California,” says Costa. “The next step is to predict how climate change will alter the habitats we just characterized.” 

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