Two vast areas of the north Pacific Ocean, one off the west coast of the United States and the other between Hawaii and Alaska, have been revealed as marine counterparts of East Africa’s Serengeti plain. Teeming with life, these oceanic “hotspots” provide major migration corridors for large marine predators ranging from tuna to whales. The discovery comes from a huge data set that synthesises and compares the seasonal migration patterns of 23 species of predators.
The findings are published in Nature. Between 2000 and 2009, the species were tracked under the Tagging of Pacific Predators (TOPP) programme, part of the Census of Marine Life international collaboration. Electronic tags attached to the animals recorded their movements and the water conditions around them, including temperature, salinity and depth. In total, the programme deployed 4,306 electronic tags, yielding 1,791 individual animal tracks and resulting in 2,65,386 days’ worth of tracking data.
The data derived over the course of the project have now been combined for the first time. “It is like asking, ‘How do lions, zebras and cheetahs use Africa as a whole continent?,’ only we have done it for a vast ocean,” says Barbara Block, a marine scientist at Stanford University in California and lead author of the paper. “We have had single-species papers before on a lot of the migration patterns, but they have never been put together as a whole.”
In the zone
The combined data from the tagged species, which carefully removes any bias introduced from where the animals had been tagged, shows two ‘hotspot’ regions where the predators’ migration routes concentrate in the north Pacific. These are the south-flowing California Current off the United States, and the North Pacific transition zone (NPTZ), which runs eastâ (euro) “west between Hawaii and Alaska along a boundary between cold sub-Arctic waters and warmer subtropical waters, and which acts like a trans-oceanic migration highway. These are the oceanic locations where food is most abundant, and that’s driven by high primary productivity at the base of the food chain. These areas are the savannah grasslands of the sea,” says Block.
Combining movement and physical data from so many tags can help to explain the behaviour patterns observed. For example, populations of salmon sharks, white sharks and mako sharks can be seen to “split the turf of the central and eastern Pacific,” says Block. Records from the tags show that slightly different preferences for water temperature prevent the closely related species from treading on one another’s fins.The work also shows that many species with long migratory paths – including yellowfin tuna, bluefin tuna, white sharks, elephant seals and salmon sharks – return faithfully from their migration to the same region every season. “For me, this homing capacity has been the biggest surprise,” says Block. “We didn’t really know these creatures had neighbourhoods.”
Pinning down predators
The TOPP data suggest that water temperature and the amount of ocean productivity from upwelling (where nutrient-rich water from the depths comes to the surface) could drive the seasonal migration of many species, with the effect particularly evident in the California Current. “Using satellite observations of temperature and chlorophyll concentrations alone, we can now predict when and where individual species are likely to be,” said Daniel Costa, an ecologist at the University of California, Santa Cruz, and a co-author of the paper.
Patrick Halpin, a marine geospatial ecologist at Duke University in Durham, N.C., who is a member of the Census of Marine Life but not of TOPP, says that the study is groundbreaking, providing not only a comprehensive picture of patterns of marine-predator behaviour in the region, but also a methodological framework for further broad-scale studies. “Future analyses originating from other regions will likely fill in a more comprehensive picture of the entire Pacific basin and identify additional hotspots,” he says.
David Sims, a behavioral ecologist at the Marine Biological Association in Plymouth, UK, also praises the study, noting its “unprecedented number” of electronic tags. “They have launched marine animal behaviour as a big science, rivaling in ambition, perhaps, some large projects in astronomy or physics,” he says. Block says that information from the study could aid efforts to protect and conserve the biodiversity of the hotspots. Knowing where and when species overlap is valuable information for efforts to manage and protect critical species and ecosystems, she says.
Nature News
Top-order predator
Sharks can be worth far more when they are swimming around the reef than when they are in a bowl of soup – as much as nearly $2 million each, in fact, according to the results of a recent study. For the study, researchers from the Australian Institute of Marine Science considered the expenditures of divers who travel from around the world to the tiny Pacific nation of Palau to dive with the mainly gray reef and reef whitetip sharks that inhabit its waters, which were declared a shark sanctuary in 2009.
As a remote country of more than 300 islands – Manila, 530 miles away, is the closest city of consequence – Palau does not have many attractions beyond diving, so spending by international tourists on airfare, lodging and diving makes up an important part of the nation’s economy.
Millions of sharks are killed each year for their fins or as bycatch, but there has been halting political progress toward reducing the pressure, like the creation of Palau’s shark sanctuary, the world’s first. Just a few years earlier, Palau had been considering opening a shark fishery with the aim of exporting fins. “People understand that when you take all the wolves out of Yellowstone or the lions out of the Serengeti, that there’s going to be quite an effect on the ecosystem,” he said. “It’s the same with the oceans, where sharks are the top-order predators.” Sharks grow slowly and give birth to few young, Rand noted. “Once they lose these species, they won’t recover like other fish,” he said, adding that quick action was therefore needed by governments.
The United States, the European Union and many other countries have banned the most widely condemned type of shark fishing, which involves cutting the fins from live sharks and dumping their bleeding carcasses back into the sea. Some countries have banned the shark fin trade altogether, but Japan, China and many Southeast Asian nations continue to allow, if not encourage, finning. “Fishermen see sharks as a commodity,” Keegan said. “We see them as an important part of the ecosystem. That’s a very hard argument to make to them. But make an economic argument, that it’s worth more letting people look at it than it is taking it.”
David Jolly
New York Times News Service