Predation
Killer whales have been observed to attack
sea lions. The stomach of a dead killer whale that washed ashore
in Prince William Sound in the summer of 1992 contained flipper
tags from 14 Steller sea lions. A study led by Lance Barrett-Lennard
(UBC) sought to determine whether killer whale predation could
significantly affect sea lion numbers.
Using a number of techniques, they developed
a mathematical computer simulation that concluded that predation
by 'transient' killer whales may account for a significant portion
of the total annual mortality of sea lions in Alaska and British
Columbia. The effects of predation on a robust sea lion population
(consisting of over 100,000 animals) are minimal, but with smaller
populations (<50,000) the effects are more significant, and may
even be sufficient to drive a population decline. The authors
conclude that killer whales did not cause the sea lion decline,
but may now be a significant contributing factor. The model suggests
that as many as 18% of the sea lions that die each year in Alaska
are taken by killer whales. What is killing the other 82% of
the missing Steller sea lions remains unanswered.
Researchers are continuing to examine
the predation hypothesis using mathematical models. In addition,
field studies are underway in Alaska to document predation by
Killer whales, as well as sharks.
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