Predators migrate en masse to Pacific hotspots
Huge numbers of sharks, seals, turtles, whales and other ocean predators converge on two regions touching on B.C.'s coast at certain times of year, an animal tracking study shows.
After analyzing data collected over a decade from 4,306 tracking tags placed on 23 species in the North Pacific Ocean, scientists likened the mass migrations to the seasonal journey of vast herds of zebras and antelopes across the African savannah. They published their results Wednesday in Nature.
Ian Jonsen, a researcher at Dalhousie University in Halifax who co-led the study, said he was surprised by the huge scale of the migrations.
"In a lot of cases, these are spanning almost the entire Pacific Ocean," he said. "You see species after species doing something similar, coming from different locations, but all of them to some extent arriving in the California current … and timing their arrival with oceanographic conditions."
For example, leatherback turtles tagged in Indonesia travel across the Pacific Ocean to Monterey Bay, Calif., and back every year.
"It's just incredible," he said. "Obviously, they've got to do it without a map and a compass or a GPS."
The turtles and other predators arrive in the California Current, which stretches from the southern tip of Vancouver Island down to California and Mexico, in the spring and fall, just as cold, nutrient-rich water is rising to the surface, the study found. The upwelling causes phytoplankton to bloom, kicking off a nutritional bonanza that cascades all the way up the food chain.
Blue whales and some species of tuna and shark stay within the California Current year-round, but make very predictable north-south migrations within the current linked to seasonal changes in temperature and food availability.
Jonsen said the detailed information will help fisheries managers understand where and how marine predators move into the North Pacific hotspots. That will help them time fishery closures to minimize the impact on marine predators, which are often caught accidentally as bycatch during fishing for other species.
But there is still a lot researchers don't know, Jonsen said.
Climate change effects unknown
"We don't know how the animals are timing their migrations," he said, noting that climate change has the potential to alter the timing of upwelling in the California Current and may affect species that make long distance migrations. "What we don't know is: 'Do these species have the capacity to change their migration timing and patterns or not?' "
The tracking data was collected by Stanford University biologist Barbara Block and her team at the Tagging of Pacific Predators Project. It was one of 17 projects of the Census of Marine Life conducted by 80 researchers around the world from 2000 to 2009.
Jonsen's team at Dalhousie University was involved in analyzing the data using tools they had developed specifically for electronic tracking data.
The analysis was a challenge because some of the tags measured location far more precisely than others. That meant it took nearly two years to get the data "into a form where we could compare different data sets and start mapping it," Jonsen said.