foraging behavior of Killer Whales

Michael Williamson (whe_william@flo.org)
Mon, 9 Dec 1994 15:12:46

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From: Michael Williamson <WHE_WILLIAM@flo.org>
Subject: foraging behavior of Killer Whales
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Subj:	ABSTRACT: Foraging behaviour and ecology of transient killer              whales
 
Date:         Fri, 9 Dec 1994 09:52:48 PST
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From:         Robin Baird <rbaird@calafia.uabcs.mx>
Subject:      ABSTRACT: Foraging behaviour and ecology of transient killer
              whales
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Baird, R.W. 1994. Foraging behaviour and ecology of transient
     killer whales (Orcinus orca). Ph.D. Thesis, Simon Fraser
     University, Burnaby, B.C. 157pp.
 
ABSTRACT
 
     The foraging behaviour and ecology of transient killer whales
(Orcinus orca) around southern Vancouver Island was studied from
1986 through 1993. Predation on marine mammals (mostly harbour
seals) was observed on 136 occasions, and no predation on fish was
observed. Transient killer whale occurrence and behaviour varied
seasonally and between pods; some pods foraged almost entirely in
open water and were seen throughout the year, while others spent
much of their time foraging around pinniped haul-outs and other
near-shore areas, and used the area primarily during the harbour
seal weaning and post-weaning period. Overall use of the area was
highest during that period, and energy intake at that time was
significantly greater than during the rest of the year. Energy
intake varied with group size, with groups of three having the
highest energy intake rate per individual, and the lowest risk of
an energy-shortfall. The typical size of groups comprised of adult
and sub-adult whales, engaged primarily in foraging and feeding,
was 3.29, implying that these individuals are found in groups
consistent with the maximization of energy intake hypothesis.
However, larger groups were also regularly seen.
 
     Near the end of this study, a time-depth recorder/VHF radio
tag was deployed on six residents and one transient, to look for
differences in diving behaviour between the two forms. While
detailed information was only obtained for 23 hours, the data
suggest that foraging-related differences in diving behaviour may
exist. The proportion of time spent at depth differed between the
two forms, with the residents spending the majority of their time
at shallower depths than the single transient individual.
 
      Utilizing information collected during this study and from
previous research, a model of indirect interactions between
transient and resident killer whales was formulated. The model
suggests that the evolution of foraging specializations in these
populations may have occurred through frequency-dependent indirect
interactions acting in concert with density-dependence within
populations and disruptive selection on prey-type specific foraging
characteristics. I suggest the two forms of killer whales may be in
the process of speciating, i.e, they may be incipient species.