After being hunted out of the northern Pacific waters off Canada and Alaska in the mid 1960s, Blue Whales began reappearing there in the late 1990s. But now, for the first time, there's photographic evidence that some of these whales are migrating up from California, rediscovering migratory routes thought forgotten. At the same time, Blue Whale populations off California have been declining. New Scientist sums up the speculation as to why this might be happening:1 & 2. Krill Population Changes Off California Forcing Migration
The optimistic scenario is that blue whales have multiplied since the whaling ban, and some are now migrating north because there is not now enough krill to sustain them in California alone.
The pessimistic scenario is that numbers have scarcely increased, but factors including climate change are reducing the amount of krill available to whales off California, so they are forced to forage elsewhere.
There's some evidence, though not conclusive by any means, that the pessimistic scenario is playing out. Then there's this possibility:
3. Whales Following Cool Water & Krill
The whales may be moving with the Pacific Decadal Oscillation—a cycle in which the temperature of the north and south Pacific alternate between warm and cool every 20-30 years. Since krill do better in the cooler waters, the Blue Whales may simply be following them.
Past hunting records coincide with this theory. Of the 1300 whales killed between 1908 and 1965, more were caught in the parts of the Pacific undergoing the cool phase of the oscillation.
4. Lost Cultural Memory of Migration Routes Rediscovered
A fourth possibility is that as the northern area was hunted out, the whales lost the "cultural memory" of the migratory routes and the richness of the amount of food to be found in the northern waters was lost. Because blue whales reproduce so slowly, it's taken this long for the knowledge to be rediscovered and the migration routes to be reestablished.
The research behind this has been published in Marine Mammal Science
via: New Scientist
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