SEATTLE — One might think of local southern resident orcas as sea pandas, playfully spy hopping through their day. The local porpoises will tell you different. Scientists for decades have observed fish-eating orcas mauling and killing Dall’s and harbor porpoises. But why do they do it? It’s not to eat them, the southern residents that frequent Puget Sound eat only fish, primarily Chinook salmon. Even when Chinook are scarce, they do not switch to eating the abundant sea mammals, including porpoises all around them, as transient, or Bigg’s killer whales, do. So what are they up to? In a study p...