Adult male humpback whale #117 purposely approached killer whales that were attacking a Steller sea lion in August 2012. NPS photo by Chris Gabriele, taken under authority of National Marine Fisheries Service scienfitic research permit #15844.
Mammals large and small defend their young from predators, so it is not surprising that humpback whales do the same when a killer whale attacks a calf. But humpback whales take it a step farther: they have a puzzling tendency to interfere when killer whales are attacking other marine mammals. We've seen it more than once in Icy Strait; when killer whales are attacking a Steller sea lion, one or more humpback whales come barging in, wheezing and thrashing and staying close to the beleaguered sea lion. Sometimes they'll even recruit other humpbacks to join the fray.
The purpose of the behavior is not clear: we've never seen the sea lion (or harbor seal, porpoise or minke whale) escape because of it. But it is similar to "mobbing" behavior displayed by many species (think of how a gull colony looks when an eagle flies by). However, similar behavior has had different results in the Antarctic, where humpback whale interference has been observed to tip the balance in favor of seals that were being attacked. Several observations from biologists who have observed this phenomenon in the Glacier Bay and Icy Strait area are included in a fascinating new article in Marine Mammal Science.
http://onlinelibrary.wiley.com/doi/10.1111/mms.12343/abstract?campaign=wolearlyview