Right whales are not ordinarily hunted by respectable whalers. They are thought to be "inferior creatures," and most whalers "disdain" putting forth effort to capture them. The Pequod specifially has not been commissioned to cruise for right whales, and though a number of them have been passed to this point of the journey, the captain has not shown any interest in them. Now that the crew has captured a sperm whale, however, and lashed its head to the side of the sailing vessel, the captain makes the decision that "a Right Whale should be captured that day, if opportunity offer(s)."
The men, following orders, surprisingly run into a pair of right whales rather quickly, which is odd because they are not generally found in the area in which the ship is sailing. The crew capture and kill one of the huge Leviathans, but after the deed is done, Stubb wonders "what the old man wants with this lump of foul lard...not without some disgust at the thought of having to do with so ignible a leviathan." Flask has a conjecture about the captain's motives, pointing out that the Fedallah, who "seems to know all about ships' charms," has said that a ship which can boast of having hoisted "a Sperm Whale's head on her starboard side, and at the same time a Right Whale's head on the larboard...can never afterwards capsize." Apparently, the captain's unusual decision to catch a right whale is motivated by superstition, and the desire to assure his ship's safe passage (Chapter 73).