Blue marlins eat a variety of fish, including tuna and tuna-like fish, mackerel and younger members of the Balistidae and Acanthuridae families. They also eat squid and crustaceans. Blue marlins take prey from the open seas and the bottom of the ocean.
Balistidae contain the often vividly colored triggerfish. These fish can grow to nearly 20 inches long and protect themselves by erecting their first two dorsal spines. Some species also have powerful teeth. However, this does not seem to deter the blue marlin.
Acanthuridae include types of surgeon fish, which also have a dangerous spine near their tail.
Tuna and mackerels are some of the fastest fish in the world and are very efficient predators themselves. However, the blue marlin is an apex predator and is also an exceptionally fast swimmer. They have been seen to swim at least 7 feet per second. Though tuna fish can grow quite large, some can still be taken by a blue marlin.
The blue marlin is a billfish, which means it has a spearlike bill. It sometimes uses the bill to hunt. It will charge full speed into a school of fish and slash away at it. Invariably, some of the fish are stunned or killed. The marlin will then return to eat.