Predation on Gray whales by Orcas appears to have increased in the last decade with mortality as high as 30% per year in bad years.
No-one knows why, but some scientists believe that transient orcas, animals responsible for the devastating attacks on Gray whales, are also starving because of the diminishing food supply in the ocean. What we do know is that transient Orcas can cause havoc in populations on which they predate. The Orca is the only known natural enemy of the Gray whale – and what an enemy!
Orcas prey on Gray whales as they migrate north to their feeding grounds. Mothers and calves are the primary target of these orcas although juveniles are taken as well. Monterey Bay and Unimak Pass are two of the critical areas.
Whale watching captains in Monterey Bay tell pitiful stories of Gray whale mothers trying to protect their calves from packs of Orcas. Many whales travel close to the coastline so they can hide in the kelp and swim in shallow waters which Orcas tend to avoid. Others cut straight across the canyon where the waters are deeper. Some whale researchers believe younger whales and first time mothers may lack the knowledge and experience to take the safer route along the coastline.
Once a mother/calf Gray Whale pair are detected, Orcas group up and pursue them until the Grays are slowed down and surrounded by the Orca pod. Up to six hours may pass from the initial attack to the kill which includes ramming, biting, pulling on the pectoral fins and making attempts to separate mothers and calves. Mothers will often try to dash to safety with her calf, or roll on her belly with her calf on top as a respite from the brutal
onslaught. Once the mother and calf are separated, the Orcas drown the calf. All that the Orcas take for food are the tongue and blubber from around the lower
jaw. Sometimes they take all the blubber.
The California Gray Whale Coalition can find no evidence to suggest that the US Government takes into account the very high mortality rates caused by Orca predation.
Omitting Orca caused mortality from the complex Potential Biological Removal (PBR) formula – a mathematical hypothesis which is set out under the provisions of the US Marine Mammal Protection Act, is cause for considerable alarm. As with so many issues impacting the Gray Whale, Orca predation is in the too hard basket. But if the recent
scientific research which suggests cascading extinctions caused by over-fishing, climate change and pollution is right, we can expect to see an increase in Orca predation caused by increasing starvation.
We know from Russian scientists that Orca predation on Gray Whales on the Russian side of the Chukchi Sea is substantial. But the US government does not take these
statistics into account.