Nancy Black says orcas have killed four gray whale calves in eight days.
Black, who co-owns Monterey Bay Whale Watch, says a family of nine killer whales has taken part in all the attacks but the first killing involved 33 orcas.
Killer whales commonly feed on gray whale calves but Black calls the frequency of the kills unprecedented.
The calves and their mothers are migrating up the California coast from Mexico. The migration was a few weeks later than usual this year, so a lot of hungry killer whales were waiting.
Black also tells the Monterey Herald that adults in the orca family group may be teaching the youngsters how to hunt.
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