Decade of tagging has mapped predatorial pathways in the north Pacific Ocean.
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 synthesizes and compares the seasonal migration patterns of 23 species of predators. The findings are published today in Nature1.
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 265,386 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 east–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, North Carolina, 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 behavioural 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, rivalling 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.
Block, B. A. et al. Nature doi:10.1038/nature10082 (2011).
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Corbyn, Z. Migration tracking reveals marine Serengeti. Nature (2011). https://doi.org/10.1038/news.2011.379