PENN STATE (US)—North American right whales increase the volume of their calls as ocean noise grows, but it may become too costly to continue to shout, new research shows.

“The impacts of increases in ocean noise from human activities are a concern for the conservation of marine animals like right whales,” says Susan Parks, assistant professor of acoustics and research associate at Penn State. “The ability to change vocalizations to compensate for environmental noise is critical for successful communication in an increasingly noisy ocean.”

Whales produce upcalls, sometimes called contact calls, when they are alone or in the process of joining with other whales. An upcall begins low and rises in pitch. It is the most frequent call produced by right whales, which were nearly hunted to extinction and currently are monitored to determine the health and size of the population.

The researchers’ data came from right whales tagged with acoustic suction cup tags. The team looked at received level, duration, and fundamental frequency of the calls, and they compared background noise levels with the call-received levels of the individual calls. They reported their findings in a recent issue of Biology Letters.

Noise below 400 Hertz dominated the recorded background noise. These frequencies overlap with the frequencies of right whale upcalls. Much of the increase in background ocean noise in right whale habitat is believed to be due to commercial shipping.

It appears that right whales increase the amplitude, or the energy in their calls, directly as background noise levels increase without changing the frequency. This suggests that right whales can maintain the signal to noise ratio of their calls in moderate levels of ocean noise.

“To our knowledge, this is the first evidence for noise-dependent amplitude modification of calls produced by a baleen whale,” says Parks.

Changing calling patterns can, however, incur costs including increased energy expenditure, alteration of the signal and the information it contains, and increased predatory risks. With increased noise the effective communication range for feeding or mating will shrink and stress levels on individual animals may rise.

“Whether they can maintain their communication range in noisier environments still needs to be tested,” says Parks. “Ocean sound levels will probably continue to increase due to human activities and there is a physical limit to the maximum source level that an animal can produce.”

Another implication for potential changes in whale calls is that upcalls are the whale calls that conservationists use to monitor right whale populations. They do this using automated acoustic sensors that are looking for specific parameters to tease out the whale calls from other noises.

The research team cautions that “variability of call parameters also can reduce the effectiveness of detection algorithms and should be taken into account when calculating the probability of detection in different habitats.”

The National Oceanic and Atmospheric Administration and the Office of Naval Research supported this work. Researchers from the Woods Hole Oceanographic Institution and Duke University collaborated on the project.

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