Humpbacks’ Southern Pacific migration routes revealed

Humpback whales spend a great deal of time moving along coastlines, much to the delight of whale-watchers and the businesses who profit off of them. But the leviathans don’t spend all their time inside of shallow, coastal areas. As they migrate between the icy poles and the warm tropics every year, they invariably swim through the open ocean. But where? If long-distance migrants like humpback whales are going to be conserved, then it stands to reason that conservationists need a complete accounting of their annual movements – including when they’re a bit further from shore.

To find out where Southern Pacific humpback whales spend their time, cetacean researcher Claire Garrigue outfitted 47 adult whales—21 females and 26 males—with satellite tags over the course of four seasons in the waters off of New Caledonia. After removing data from faulty tags, she and her colleagues were left with 34 tagged animals, which gave them a total of 5,004 geolocated datapoints. Thirteen whales transmitted data while in their New Caledonia breeding grounds, but 11 females and 10 males continued to provide useful data after they left New Caledonia and headed south toward Antarctic waters.

The whales’ movements gave Garrigue and her team two surprises. First, the whales swam away from New Caledonia in a variety of directions, which is contrary to findings from other humpback populations, who tend to follow a fairly narrow route between their breeding and feeding grounds. That means that protecting the whales in only a narrow route will miss the whales that take alternative pathways.

The second discovery – which could in part explain the first – was that migrating humpback whales seemed to favor shallow topography, even when in the open ocean. They seemed to prefer hanging out around seamounts – underwater mountains that rise at least 100 meters from the seafloor. They didn’t just use the seamounts as navigational aids, though it’s possible they used them as landmarks. For one thing, they seemed to prefer only certain seamounts. Within New Caledonia’s exclusive economic zone there are many seamounts, but the whales seemed to prefer just two, which happen to be among the shallowest: 30 and 60 meters deep.

Indeed, the satellite data showed that the whales changed their behavior when in the vicinity of these shallower areas. They spent more time near the seamounts, slowing their rate of travel. While Garrigue isn’t sure just what they’re doing there, she has some ideas. Seamounts could serve as additional breeding spots, places where males can sing and females can pick their favorite males. Lots of marine life tend to cluster around seamounts, so they could also be additional feeding spots, places to add a bit more fuel to the tank to help the whales complete their long journeys. It’s also possible that the whales could simply be using the shallow waters as places to rest before continuing on to the frigid seas off of Antarctica.

As far as the researchers can tell, the South Pacific population of humpback whales that they study is the only one to take multiple pathways towards the poles from their breeding grounds.

This study is also the first to highlight the apparently important role that seamounts play in guiding the whales along their migration. “Since a substantial number of seamounts are shallow and in low latitudes, our results suggest that such remote features could represent previously overlooked cryptic habitats for humpback whales,” they write this week in the Royal Society’s Open Science journal. To protect the humpback whales in the area, it probably makes sense to consider creating marine protected areas around them – and, the researchers argue, the possible use of seamounts by other populations elsewhere in the world is worth investigating. If the results of this study can be generalized to other humpback whales, then conservationists can be better prepared to identify critical habitats necessary to protect for ensuring the continued existence of these ocean giants. – Jason G. Goldman | 27 November 2015

Source: Garrigue C, Clapham PJ, Geyer Y, Kennedy AS, Zerbini AN. (2015). Satellite tracking reveals novel migratory patterns and the importance of seamounts for endangered South Pacific humpback whales. Royal Society Open Science 2, 150489. DOI: 10.1098/rsos.150489.

Header image: Southern Pacific humpback whale off the coast of New Zealand, via shutterstock.com

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