Killer Whale Change in Feeding

[Robin Leech]

Fellow Taxacomers,

I retired on 3 May, so I no longer have to account for my evening's activities with marking papers, preparing courses, etc.  So, last night, I was able to watch the last 20 minutes of a movie on the Aleutian Island, Attu Island and a few other islands.

During that last 20 minutes, there was discussion on the Sea Otter, and how its population numbers went from the 35,000 level to the 3500 level.  A study revealed that it was killer whale predation.  It stated that specialists and authorities had no clue as for the shift in feeding habits by killer whales.

I think I have a clue, and it comes by analysis and analogy from the relationship between the lynx and the varying hare.  The varying hare is the main prey of lynx.  At the peak of the boom-bust populations, the hare population is preyed upon by an increasing population of lynx.  When the hare population begins to crash, hungry lynx start feeding on everything and anything else - birds, squirrels, voles, etc.  When these run low, the lynx population begins to crash.

Killer whales normally feed on mammals, porpoises and seals in particular.   I do not know if there is a boom-bust population cycle between killer whales and their normal food.  The cycle, if there is one, may be such a long one that we have not been aware of it.  But, because of our marine activities such as fishing and toxic dumping, porpoise population levels have been dropping (being caught in nets by the thousands each year, and dying from toxins, underwater explosions, etc.).  With the porpoise population levels dropping, the killer whales are taking anything they can find, and sea otters and inshore seals happen to be vulnerable and present.  The populations of the inshore seals and sea otters are crashing.

The first question is this:  How long can large mammals (porpoises) lose thousands from their populations each year before they begin to crash?

And the second question is this: Will the populations of killer whale crash when their food runs low?

I note in Banfield's "Mammals of Canada," page 264, that the "...stomach of one killer whale contained thirteen porpoises and fourteen seals."  It takes a lot of sea otters to equal the food and energy value of even one porpoise, and it takes a lot of work by a killer whale to find the porpoise food equivalent in sea otters.  The killer whales are starting to deplete the last of an alternate food source.

In our headlong pillaging of the seas, we may have triggered a mess that is far greater than the fish depletions.  We are aware of the fish overfishing and depletions.  We do not have even the smallest inklings of the other problems farther up the food chain.  We like to think that we are, but we are not.

We are such an avaricious creature that even when we know we are causing a problem (such as the Japanese "scientific harvesting" of whales), we continue.  It's almost as if the collective thoughts are "If I don't get them, someone else will."  We do not seem to be able to help ourselves.  We know the ethics, we lack the morals, and so we continue.

Robin Leech