Echolocation in wild killer whales (Orcinus orca)

Echolocation by odontocete whales has been demonstrated in captive settings many times, yet little is known about its use and function in the wild. In this thesis, I describe echolocation sounds in pods of killer whales (Orcinus orca) off the coasts of British Columbia and Alaska. I examine the relationships between echolocation and water clarity, ambient noise, and behavioural activity. I compare echolocation by two populations with different feeding habits and foraging patterns. The study provides new insight into the functional role of odontocete echolocation, and into the relative contributions of vision and passive listening to spatial perception and prey detection. Underwater recordings were made during 111 encounters with killer whales. Known members of the resident (fish-eating) population were photographically identified 85 times, and transients (mammal eaters) 23 times. Click sounds characteristic of echolocation were identified in the recordings aurally, and by spectrographic and waveform analysis. Most clicks occurred in series (trains), but isolated clicks were occasionally detected. Echolocation behaviour differed strikingly between residents and transients. An echolocation index (EI) was defined as the average percentage of time that an individual produced click trains. The mean EI for residents was 4.24%, 27 times greater than for transients. The duration of resident click trains averaged 6.83 s, compared to 0.86 s for transients. Resident click trains were comprised of evenly spaced clicks, whereas transient trains had uneven click spacing. Transient individuals used isolated clicks once every five minutes, four times as often as residents. For resident killer whales, EI values were significantly higher during foraging and travelling than during other behavioural activities. In residents, EI declined with group size. This was also true of transients, for the recordings in which click trains were detected. Transient EI levels were significantly higher when close to shore than when offshore. No relationship between EI and ambient noise level was found for either residents or transients. Residents increased the amplitude of their clicks in response to increasing ambient noise levels. No relationship was found between water clarity and EI for either type of killer whale. I suggest that the differences in echolocation behaviour between residents and transients are accounted for by their different prey. Fish have little or no aural sensitivity in the frequency range of killer whale clicks. Marine mammals are able to detect clicks, and may use them to evade killer whale attacks. The use of isolated clicks and short, irregular, quiet click trains makes transient echolocation less detectable by marine mammals than the echolocation used by residents. Passive listening is probably the principal technique that transients use to locate prey, whereas residents use echolocation in combination with passive listening when foraging. Vision is not a major factor in locating prey, but may be used by either whale type during pursuits. Finally, I suggest that both residents and transients obtain much of their positional and orientational information using passive listening alone.Science, Faculty ofZoology, Department ofGraduat