Mtumbwi hauwezi kujua panapokuwa pamejaa maji. (Swahili)
The dugout canoe does not know the depth of the water. (English)
So believe the Hangaza, an ethnic group living along Lake Victoria near Tanzania’s Serengeti National Park. The lake has long been an object of contemplation for the Hangaza. They know that animals like crocodiles swim just beneath its surface, but, they say, there’s more to this vast waterbody than can be seen from a canoe.
Half a world away along the U.S. West Coast, marine scientists studying the California Current mirror the Hangaza.
The oceanographers are finding new ways of looking beneath research vessels that ply the seas. They’re getting a fish-eye-view of the deep by placing electronic tags on predators such as blue whales and California sea lions, yellowfin tuna and white sharks.
The project is called Global Tagging of Pelagic Ocean Predators (GTOPP). It focuses on vast open ocean areas, among them the California Current. This undersea river of water flows south along the west coast of North America, beginning off British Columbia and ending near Baja California. The current is a marine savanna along which whales, sharks, seals, seabirds, turtles and tunas migrate.
Savannas of the sea
Like the grasslands near Lake Victoria, the California Current and similar ocean expanses are major corridors for the movements of predators and prey. They’re the savannas of the sea.
Findings from GTOPP reveal, for example, that the migrations of at least 23 Pacific Ocean species overlap – a picture of critical marine habitats.
Scientists Steven Bograd and Elliott Hazen of the U.S. National Oceanic and Atmospheric Administration (NOAA)’s Southwest Fisheries Science Center (SWFC) in La Jolla, California, and colleagues at other organizations launched GTOPP. The project is the world’s largest “biologging” study.
“Advances in the miniaturization of biologging tags now allow collection of data at the scale of an individual,” state Bograd and Elliott in the journal Oceanography’s December, 2014, issue. By combining information from migratory marine species and overlaying it with oceanographic data, scientists are gaining clearer views of how open ocean ecosystems work.
Cheetahs and bluefin tuna, elephants and blue whales
Where are the African watering holes a zebra or a cheetah might frequent? Where are the deserts animals might avoid, and the migratory corridors species like wildebeest use to travel from place to place?
GTOPP biologists are asking similar questions about ocean animals from bluefin tuna to blue whales to leatherback turtles. They’re tracking more than two dozen species, and along the way recording such variables as water temperature, salinity and depth. Researchers have deployed more than 4,000 electronic tags, yielding a huge amount of data.
Now an iPhone and iPad app called Shark Net allows scientists and the public to follow the comings and goings of white sharks – the African lions of the sea. Shark Net, developed by biologists at Stanford University, brings users eye-to-eye with white sharks and notifies them when the sharks’ electronic tags are detected by underwater receivers.
Such biologging data have led to the discovery of intersecting hotspots and highways of ocean life.
White sharks, for example, spend their winters and springs at the “White Shark Café,” a spot in the open Pacific halfway between Hawaii and Mexico’s Baja Peninsula. To get there, they migrate long distances from North American coastal waters. What’s served at the café? Only the sharks know the answer.
Leatherback and loggerhead turtles have their own secrets.
“So little is understood about young leatherbacks that the decade after they hatch on beaches has been called the lost years,” says George Shillinger, executive director of The Leatherback Trust in Monterey, California.
Where do the turtles go? Shillinger and other GTOPP scientists have found that eddies – whirlpools within offshore currents – serve as turtle highways. Rides on eddies offer fast getaways from near-shore predators and refuges during growing-up years.
In late 2014, clues to loggerhead turtles’ whereabouts surfaced. NOAA biologists surveying waters 200 miles off Southern California chanced upon more than 70 young loggerheads. The researchers believe they’re “lost years” turtles from nesting beaches in Japan – loggerheads that will someday reappear off Baja California.
Grand march of the animals
The Serengeti, too, “is an ecosystem that’s synonymous with animal movements,” says ecologist Grant Hopcraft of the University of Glasgow, who studies herbivores such as wildebeest, zebras and Thomson’s gazelles. “Each year more than one and a half million wildebeest and other species cross its plains.”
Their seasonal migrations trail cyclical rains that fuel the growth of savanna grasses. Where the grasses spring up, wildebeest and other herbivores follow. Predators like lions pad closely behind (see National Geographic: The Serengeti Lion).
This grand march of the animals isn’t unique, Hopcraft says.
“The journeys of marine species in the California Current are similar to those in the Serengeti,” he says, “which begs the question: why? Research suggests that a changing food supply drives animal migrations. But recent wildebeest and zebra results from the Serengeti show a huge amount of variation in when individuals decide to move from one place to another.”
Peering beneath the surface
There’s a lot more going on, Hopcraft believes, beneath the surface.
“For the Serengeti – and the California Current – does an animal’s internal condition determine how it responds? Is it remembering previous routes and responding to the same cues?”
GTOPP results show that many marine creatures, like animals in the Serengeti grasslands, come back to the same regions each year. It’s akin, scientists say, to a student from London studying in Rome and returning each summer at the same moment, but doing it all in the dark without a map or compass.
“How or why a young bluefin tuna less than two years old wakes up in the light of the Japan Sea and decides to swim to Baja California is still unknown,” says biologist Barbara Block of Stanford University. But once it arrives, tagging data indicate that it lives there for years, taking advantage of rich “forage” along the coast.
Dozens of species – including black-footed albatrosses, bluefin tunas and salmon sharks – migrate hundreds to thousands of miles to reach the California Current’s food resources. GTOPP scientists are watching.
“The seasonal ebbs and flows of whales and fish in the oceans, and of antelope and elephant herds across the land, are one of nature’s most impressive spectacles,” says Stuart Pimm, an ecologist at Duke University who is also chair of the conservation organization SavingSpecies. “They require our special efforts to save them.”
African ‘game parks’ in the Pacific: Blue Serengetis
“Africa-like ‘game parks’ flourish in waters off the U.S. West Coast and other coastlines,” says Block. Without conservation of these ocean realms, she and others believe, bluefin tuna and blue whales, whale sharks and white sharks may vanish by our grandchildren’s time.
Hopcraft’s thoughts echo Block’s. He imagines animal migrations like those in the African savanna – and the Pacific Ocean – “as the inhalations and exhalations of our planet. They’re the tell-tale signs of a healthy world.”