WE LANDLUBBER HUMANS tend to think of the Pacific Ocean as one big aquarium where large animals such as whales wander freely, sometimes over large distances.
While this is partly true, in reality it is much more complicated than that.
Take the humpback whale, for example. These 40-tonne leviathans are returning to the British Columbia coast in greater and greater numbers. We’re seeing them sporadically all year-round now, but the real humpback invasion off Victoria happens in late summer and fall as they get ready to migrate for the winter to places such as Mexico and Hawaii.
A recent study casts new light on the makeup of the North Pacific humpback nation. It identifies five distinct populations: Hawaii; Mexico; Central America; Okinawa and the Philippines; and another West Pacific population with unknown breeding grounds.
THE SCIENTISTS EXAMINED nearly 2,200 tissue biopsy samples collected from humpback whales in 10 feeding regions and eight winter breeding regions during a three-year international study known as SPLASH (Structure of Populations, Levels of Abundance and Status of Humpbacks).
SPLASH involved more than 400 researchers in 10 countries, making it the largest study ever conducted on wild whales.
The study team analyzed DNA to describe genetic differences and outline migratory connections between several breeding and feeding grounds.
“Although humpback whales are found in all oceans of the world, North Pacific humpbacks should probably be considered a sub-species at an ocean-basin level, based on genetic isolation of these populations on an evolutionary time scale,” says the study’s lead author Scott Baker, a marine mammal scientist from Oregon State University.
The study shows there are multiple distinct populations of humpbacks in the North Pacific. They differ genetically and by geographical distribution, and members of each population usually return year after year to the same breeding and feeding areas.
The SPLASH program used photo identification records—based on the black and white colour pattern and shape of each whale’s tail flukes—to estimate North Pacific humpback numbers at about 22,000.
THIS IS A DRAMATIC COMEBACK from the paltry 1,400 remaining when—after more than a century of over-exploitation—commercial whaling was globally banned in 1966. (Soviet whalers continued to hunt them illegally until 1971.)
Humpback whales are listed as “threatened” under Canada’s Species at Risk Act. Until the 1980s, a humpback was a rare sight off British Columbia. Since then, more than 2,000 individual whales have been identified and catalogued along the BC coast, and that number is growing by four per cent a year.
The SPLASH study shows that most humpbacks feeding off BC migrate to either Mexico or Hawaii. But there’s an interesting geographical division. While most humpbacks north of Vancouver Island head to Hawaii, just over half of humpbacks off southern Vancouver Island seem to prefer Mexico. The rest go to Hawaii or Central America.
“Unlike most terrestrial species, populations of whales within oceans are not isolated by geographic barriers,” says Baker. “Instead, migration routes, feeding grounds and breeding areas are thought to be passed down from mother to calf, persisting throughout a lifetime and from one generation to the next.”
“We think this fidelity to migratory destinations is cultural, not genetic,” adds Baker. “It is this culture that isolates whales, leading to genetic differentiation—and ultimately, the five distinct populations identified in the North Pacific.”