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Track the prey, miss the whale

By Martin Leggett - 02 Aug 2011 23:0:1 GMT
Track the prey, miss the whale

Image Caption: A North Atlantic right whale swimming just below the surface, out of sight from vessels that are not right on top of them. Credit: Susan Parks, Penn State

Whales and boats just don't mix - a truism that rings especially true for the tiny numbers of endangered North Atlantic right whales, found cruising along the east coast of the US. Ship strikes count for close to 40% of all deaths amongst these 50-foot long whales; the unlucky sea-mammals feed at depths that put them just out of sight of boat-pilots. Avoiding such carnage on the ocean's highways is made doubly difficult, because, unlike other whales, right whales don't call-out while feeding.

That means using auto-detection buoys - which pick up on whale-songs, so allowing warnings to be issued to marine traffic - a non-starter for protecting right whales. But a two-year study off of Cape Cod offers hope for a new method to flag up their presence to boats plying their feeding grounds - by tracking the dense swarms of copeods that are the right whale's prey. The research is published in the latest Biology Letters.

Magnified image of a copepod, the North Atlantic right whale's preferred food in Cape Cod bay

Image Caption: Magnified image of a copepod, the North Atlantic right whale's preferred food in Cape Cod bay. Credit: Joe Warren, Stonybrook Univeristy

Copepods key

The team - led by Susan Parks from Penn State Applied Research Laboratory - used special acoustic recording tags, from the Woods Hole Oceanographic Institution. These helped to track the whales as they fed in the shallow waters off of the Massachusetts peninsula. They also used echo-sounders to track blooms of the tiny copepod crustaceans, a nutritious main-course that right whales scoop up by the tonne.

Researchers place a suction cup, digital acoustic recording tag on the back of a North Atlantic right whale

Image Caption: Researchers place a suction cup, digital acoustic recording tag on the back of a North Atlantic right whale. Credit: C. Hotchkin

'We found that every whale spent a lot of time just below the surface, where they can't be seen while feeding,' said Parks. 'It is a good thing that the whales are in Cape Cod Bay in April when it is pretty cold and not a lot of recreational boating is going on, because any boat, even small recreational boats, could bump into them.'

By taking samples of the water, where copepod swarms were indicated by the echo-sounder, the team were able to confirm that they had zeroed in on the right whale's prey - and not just drifting matter. They were also able to show that the right whales homed in on the heart of these swarms _ the whales were typically found to be cruising right through those areas where the copepods were most dense.

Giving scientists the slip

That means that tracking the copepods offers an excellent way to follow the most likely path of the submerged right whales. The team suggests that an array of moored echo-sounders in the feeding grounds of the right whale could be used to track the copepods - and so warn shipping and pleasure boats of the predicted locations of the whales.

There still remains a gap in the knowledge of what right whales get up to, however - at night. Despite the best efforts of the research team, using the most advanced suction-based tracking devices, the right whales proved reluctant nocturnal-participants in the study. 'The daytime and nighttime behavior may be different, but we don't know the nighttime behavior because every whale we tagged with a suction cup recorder slipped out of it before evening,' said Parks.