Robin W. Baird, Lawrence M. Dill and M. Bradley Hanson
Page 9 in Abstracts of the World Marine Mammal Science Conference, Monaco, January 1998.
Like all cetaceans, killer whales spend the vast majority of their time beneath the water's surface, where they are invisible to observers. However, behavioural studies on killer whales have routinely described and quantified their activities based only on surface behaviours, with no discussion of whether such behaviours are indicative of subsurface activities. Similarly, studies of behaviour have taken place almost exclusively during daylight hours, with no examination of diurnal patterns. This study investigated the subsurface behaviour and night-time activities of killer whales in southern British Columbia (Canada) and in northwestern Washington state (USA) from 1993 through 1997, using suction-cup attached time-depth recorder (TDR)/VHF radio tags. Through June 1997 these tags have been deployed on 13 occasions, 12 on so-called "resident" killer whales and one on a so-called "transient" killer whale, with a total of 77 hours of TDR data. Depth was recorded once per second, and for 6 deployments (in 1996 and 1997) velocity was recorded every 5 seconds. Tag attachment ranged from 15 minutes to 22 hours. Although the sample size is small, several consistent behavioural features have been observed. All "resident" killer whales spent the vast majority of their time (>70%) in the upper 20 m of the water column, where salmon (thought to be their primary prey) are concentrated. However, during periods when they were considered to be foraging, all "residents" dove occasionally to 100 m or more (maximum recorded dive depth of 201 m), and we suspect these deep dives reflect regular feeding on bottom and mid-water fish. An examination of night-time diving parameters (maximum depth, variability in depth, dive shape/profile, rate of ascent and descent, and velocity) suggest that resting may occur much more frequently at night than during the day. Diving parameters from the single "transient" differed from those of all "residents", possibly due to differences in prey species, since "transients" feed primarily on harbour seals. The combined data suggest that studies of killer whale behaviour must take into account subsurface activities in order to provide an accurate representation of habitat use and diurnal activity patterns.