If Hans has interpreted the inventor correctly, a captured Killer Whale
does not demote to Porpoise. Only two special rules about drops are given
(for Porpoise and Dolphin), and the general rule is that the taken piece
becomes a reinforcemnt to be dropped.
This is probably the best for play balance as well--the Killer Whale is
enormously powerful in this game and it costs less to lose it if it
demotes when captured.