By feeding just below the ocean's surface, North Atlantic right whales are at higher risk of colliding with ships.
Endangered right whales feed at depths just below the surface where they're vulnerable to collisions with ships.
This depth corresponds to high concentrations of their copepod prey.
Ship collisions are a leading threat to this species, which numbers only between 300-400 individuals.
Critically endangered North Atlantic right whales foraging in Cape Cod Bay in the spring spend most of their time just below the surface where they can't be seen but remain vulnerable to collisions with ships, according to a new study.
The whales appear to be following their food, because the researchers also found high levels of copepods, tiny crustaceans the size of a grain of rice, at the same depths.
"It makes sense that whales are spending time where their food is," said Susan Parks of Pennsylvania State University, lead author of the study published today in Biology Letters.
Still, the fact that the whales spent nearly all of their time just below the surface came as a surprise to the team.
"In the past I had known that right whales feed at the surface in Cape Cod Bay; you can see them swimming through the water with their head above the water and their mouth open," Parks said. "What was really surprising to me in this study was how much time the whales were spending just out of sight but at a really dangerous depth for a boat to run into them."
In fact, with the aid of trackers attached to the whales using suction cups, researchers knew that at times as many as 10 whales were within their visual range, yet they could see almost nothing at the surface. "We would know that they were right in front of us," Parks said.
While one part of the team tracked the whales' locations and sounds, another part of the team monitored the concentrations of copepods in the water column, collecting samples and using sonar techniques to determine copepod numbers at various depths.
Consistent with other studies of North Atlantic right whales, the results showed tight agreement between the depths of the whales and their prey, but the whales' early spring passage through Cape Cod Bay is special because the prey aggregate so near the surface.
In other locations the prey are found deeper in the water, drawing the whales on deeper dives to feed. "This habitat at this time of year makes them particularly vulnerable," Parks said.
A large proportion of the population faces this vulnerability: around 45 percent of the North Atlantic right whale population gathered in the area in early spring 2010.
The findings could aid efforts to conserve the species. Approximately one-third of all right whale deaths result from ship strikes or entanglement in fixed fishing gear according to the National Oceanic and Atmospheric Association. NOAA also reports that one to two deaths per year of reproducing females by human activities could lead to the species' extinction.
A number of conservation measures are already in place including aerial and ship surveys to spot whales, mandatory speed reductions for large ships in key locations at certain times of year, and underwater detectors to listen for the whales.
"This is a suggestion that another way we could improve our protection for the whales is being able to monitor where in the water column the food is," Parks said.
Meanwhile, the findings could also be helpful in ensuring that regulations enforcing slower ship speeds continue, said Doug Nowacek of Duke University in Durham, NC. The current speed restrictions mandated by NOAA are set to expire in December 2013.
"We need to get those regulations to be maintained in perpetuity, and not sunsetted," Nowacek said. "Ship strikes and entanglements in fishing gear are the two most significant conservation concerns for this species."