Finding Killer Whales: Easier to See or to Hear?

Is it easier to find killer whales by looking for them or by listening for them? Answering this question would help biologists to locate killer whales to better monitor population health, as well as to determine the impact of predation on endangered marine mammals such as Steller sea lions in western Alaska.

Traditionally researchers have relied on ship-based sighting surveys to estimate killer whale abundance. However, such surveys are expensive, limited to daylight hours, and are adversely affected by inclement weather and high seas. By contrast, the underwater calls and echolocation clicks of killer whales travel over many kilometers and allow researchers to detect pods at night and during periods of bad weather.

This summer, Volker Deecke and his assistants Nic Dedeluk and Nicola Brabyn set out to test just how effective acoustic surveys for killer whales are compared to sighting surveys. Deecke and his team looked for a site that was accessible and offered good observation and underwater listening conditions in an area frequented by both fish-eating and mammal-eating groups of killer whales. Looking at a map of the waterways of Southeast Alaska, their eyes were immediately drawn to Pt. Retreat on the northern tip of Admiralty Island where one of Alaska’s most scenic lighthouses is located.

The strategic position of the Pt. Retreat lighthouse at the confluence of Lynn Canal and Saginaw Channel meant that any killer whale groups traveling between Icy Strait, Lynn Canal and Stephens Passage would pass within visual and acoustic range of the lighthouse. A brief e-mail to Dave Benton, President of the Alaska Lighthouse Association in charge of restoring the lighthouse was met with an enthusiastic reply.

Spectogram (a visual representation of sound) from AF and AG killer whale pods off of Pt. Retreat

Deecke and his team installed hydrophones (underwater microphones) on both sides of the Point to acoustically monitor a 280º area of Lynn Canal and Saginaw Channel. For a 3-week period in June, the researchers listened to and recorded the hydrophone signal for 10 minutes every hour and scanned the same area with high-powered binoculars to see if they would first hear or see groups of killer whales that passed by.

In this first of two field seasons, the research team detected a total of 8 groups of killer whales. Four of these groups were identified as fish-eating resident killer whales, and four were mammal-hunting transients. All but one of the groups of residents were first detected acoustically, and calls could be detected up to an hour before the animals were sighted.

	Previous research had shown that transients vocalize less frequently than residents. This is presumably an adaptation to avoid detection by their acoustically alert marine mammal prey. Consequently, most transient groups were detected visually and not acoustically. However one transient group did give itself away by calling first.
This group of 7 animals produced loud vocalizations as they attacked a Steller sea lion. Since transient killer whales typically become vocal after a successful attack, acoustic monitoring has the potential to yield valuable information on predation rates.

The research team would like to thank Dave Benton, Lori Telfer and the other members of the Alaska Lighthouse Association for their help during the field research. Stay tuned for the results from next summer's field season.

4 October 2004