Beaked whales avoiding predators, sonar?

Small whales around the Islands, which dive to nearly a mile deep and stay down for as much as an hour and a half, may sometimes make fast "bounce" dives in response to predators and also sonar.

That's a conclusion in a new paper by whale researcher Robin Baird and his team.

The paper "Diel variation in beaked whale diving behavior," was published in the July 2008 issue of the journal Marine Mammal Science, by Baird and Gregory Schorr of the Cascadia Research Collective, Daniel Webster of the Bridger Consulting Group, Daniel McSweeney of the Wild Whale Research Foundation, and Jay Barlow of NOAA's Southwest Fisheries Science Center.

They studied both Cuvier's and Blainville's beaked whales, and determined that both species feed extremely deep—close to 1,500 meters down—and stay down for as long as 94 minutes (Cuvier's) and 83 minutes, (Blainville's).

It's not clear what they're eating down there, but one likelihood is deepwater squid.

Baird's team attached recording devices to whales to track their movements, and found one distinct difference between whale day and night activity. After returning to the surface from day dives, the whales often "bounce" down to deep water. At night, they tend not to do so.

Baird said that suggests the bounces are not required for a physical need, like reducing the threat of "bends."

They also tend to stay in shallower water at night than during the day.

His team's conclusion is that the deeper depths maintained during the day and the bounces are done to avoid predators like big sharks, which may be foraging near the surface during the day. Previous research indicates, for instance, that white sharks are primarily daytime hunters.

Both species also avoid vocalizing in surface waters—restricting their calls to when they're considerably more than 1,000 feet deep. That might also be a technique to prevent them from alerting predators to their presence, Baird wrote.

The paper also cites previous studies that indicate that beaked whales move away and alter their diving behavior when they hear killer whale calls, and also that they seem to exhibit similar behavior when they hear mid-frequency sonar.

If the whales make panic dives when they hear such sonar, that might help explain some whale strandings that occur in areas where sonar is being used, they said.

"Mid-frequency sonar associated with naval exercises may continue for an extended period and propagate for a longer period than the higher frequency (and lower intensity) calls of killer whales, thus potentially eliciting a response for an extended period," wrote Baird and his associates.

It could put the animals at risk for the kinds of decompression-looking symptoms found in some strandings, they wrote.

© 2008 Jan TenBruggencate