Scientists solve mystery of missing basking sharks
Head south, young basking shark. Way, way south. And way deep, while you're at it.
That's the unexpected advice the world's second largest fish seem to be taking -- at least in the Western Atlantic. To some marine scientists, the newly discovered itineraries these gentle giants follow could have significant conservation implications.
The International Union for the Conservation of Nature and Natural Resources lists the fish as "vulnerable" to extinction. The new wrinkles in their migration patterns appear in a recent study in the journal Current Biology. The results suggest that efforts to conserve these creatures may not work if those efforts are limited to one or two ocean basins, notes Gregory Skomal, the lead scientist behind the shark-tracking project.
Other research, which focuses on basking-shark genetics, already has hinted that this approach might be needed, he adds. Now, the latest shark-tracking information gives that earlier work additional gravitas.
Until now, conventional wisdom held that tropical waters presented a barrier to basking sharks as they migrated north and south each year. As marine biologist Elliott Norse puts it during a phone chat about the results, you looked to the temperate oceans for basking sharks and to tropical oceans for whale sharks, the largest living fish.
But a team led by Dr. Skomal, a marine biologist with the Massachusetts Division of Marine Fisheries now has tracked basking sharks from the start of their journey in waters off southeastern Canada and the northeastern US all the way to the coasts of Brazil, well into the southern hemisphere. Until now, the farthest south the creatures had been tracked was to waters off the US Southeast.
So much for a barrier for basking sharks.
So big a fish, so little known
"Here it is May 2009, and we are still discovering really surprising new things about one of the biggest animals on Earth," enthuses Dr. Norse, who heads the Marine Conservation Biology Institute in Bellevue, Wash. "This is a beautiful example of things we had assumed we understood based on what what we could see, and we find out that they are not true."
Basking sharks get their name from their tendency to swim close to the surface along continental coastlines. That's where their food, plankton, hangs out.
In some respects, their migration patterns along the east coast of North America track those of the endangered Atlantic right whale. The sharks and whales hang out off southeastern Canada, in the Gulf of Maine, and off Massachusetts in the summer, then many head south in the fall.
Scientists have tracked many of the right whales to wintering grounds off the southeastern US, but the basking sharks just vanish. So do anywhere from 25 to 50 percent of the right whales.
"The winter disappearance of the basking shark has been a conspicuous topic in the scientific literature for a hundred years," Skomal explains. By finding the missing sharks, scientists suspected they might also find some of the missing whales.
Tag! You're tracked!
To track the sharks, Skomal and his team from the University of New England, the Woods Hole Oceanographic Institution, and state fisheries agencies in Massachusetts and Maine, applied pop-up tags to 25 sharks. The tags, essentially narrow tubes about eight inches long, measure light levels, water temperature, and depth every 10 seconds. They store the data, which is sent back to the lab via satellite after a tag releases itself from the fish and bobs to the surface.
Out of 25 tags, the team got data back from eight, spanning periods ranging from 12 to 234 days. The sharks migrated from New England to the Bahamas, Puerto Rico, and off the coast of Venezuela. Stunningly, two ended up below the equator in along the coast of Brazil. Moreover, as they traveled, the sharks often dove to depths of between 2,625 and 3,281 feet. Some stayed there for up to five months.
At such depths, the light sensors are useless. The sensors allow marine biologists to use sunlight to estimate an animal's position and reconstruct its migration path. So the research team used oceanographic maps of temperatures in different masses of water -- which at those depths remain remarkably stable over time -- to reconstruct the sharks' paths in more detail than merely saying they started here and finished there.
Little is known about the sharks' reproductive behavior, and the team was unable to ensure a good gender mix among the sharks it tagged. So it remains unclear how much of the migration is driven by the search for food an how much of it represents a migration to breeding grounds.
Eighteen tags may seem like a small number to use for shouting "Eureka!"
But Norse begs to differ.
"If a small sample like that missed a migration, I wouldn't be at all surprised," he says. "But that a small sample shows a migration like that, that tells me that in all probability this is a real phenomenon. This is not just one or two crazy sharks that lost their way."