American Oceans

Black Marlin

The black marlin (Istiompax Indica) is a billfish that spends most of its life in warm, shallow waters, though it migrates thousands of miles each year.

Black Marlin or Istiompax Indica swimming in the ocean

It is a predator and has little to fear from any ocean creature. Scientists aren’t sure how many there are in the ocean and can’t agree on how long they generally live. There’s a lot we don’t know about them.

We do know that black marlins are also called Pacific black marlins, silver marlins, swordfish (even though they are not swordfish), and short-nosed swordfish.

They are among the fastest swimmers in the ocean, and they are prized as trophies by sport fishers around the world.

Characteristics & Appearance

Perhaps the most distinctive thing about a black marlin’s appearance (and all the other billfish in the ocean) is its sword-like upper jaw, which is sharp-edged. The lower jaw is similarly pointed but not nearly as long.

Weight & Length

Like many large fish, the female of the species grows much larger than the males. While male black marlins rarely weigh more than 400 lbs, a female black marlin can tip the scales at nearly 1000 lbs. Males grow to about 9 feet long, while females can be more than 15 feet long.

Physical Characteristics & Color

While black is in the name, black marlins aren’t actually black. They’re dark blue on the dorsal side (the top of the fish) and a whitish ventral (down-facing).

Black Marlin with dark blue dorsal side

Between those two colors is a series of faint blue stripes that run the length of their bodies. Their fins are brown except for the dark blue dorsal fin.

The dorsal fin of the black marlin stands as the shortest of all the billfish’s dorsals. While the sailfish has a dorsal fin that runs along almost its entire back and stands several times taller than the sailfish’s body, a black marlin’s dorsal fin only stands at about half the height of its body.

Black marlins have fins on the side of their bodies that do not retract. These fins make them different from every other billfish, all of whom sport retractable fins.

Black Marlin sport retractable fins on the side

The presence of these fins confounds scientists when it comes to learning how they swim so fast. Fins create drag, so retractable fins would seem to make a fish able to swim faster, but in this case, that isn’t so.

Lifespan & Reproduction

As long as people have been fishing for marlin, it may seem surprising that scientists and oceanographers don’t know how long marlins live, though some estimates center around the 25-year mark.

Black Marlin live in the ocean for reproduction

They reproduce via external fertilization. The females lay millions of eggs spawning ground. The males then fertilize them, and as soon as a day or two later, larval fish emerge and begin fending for themselves, as mom and dad don’t ever have anything to do with them.

The number of eggs released does not translate into millions of marlins. The sheer volume of eggs released is rather nature’s way of making up for lots of obstacles in the way of reproduction:

  • The sperm released into the water is subject to the current, so not every egg gets fertilized.
  • With all the plankton eaters and filter feeders in the ocean,  many eggs get eaten, and many of the resultant larvae do, too.

Black marlins can reach sexual maturity in about five years. At that point, during spawning season, females can spawn at a rate of about once every three days.


Black marlins prefer warmer waters, so they tend to stick to tropical waters for the most part and spend much time near land or coral reefs.

While they can dive to nearly 3,000 feet, they don’t do that often since the deeper the water gets, the colder it is. It’s unusual for a black marlin to dive deeper than 100 feet.

They migrate long distances, with some tagged black marlins traveling up to 5,000 miles, and while their journey can take them close to the South Pole, they do not live in those cooler water but merely pass through.

Where Do Black Marlin Live?

Black marlin are most commonly found in tropical and subtropical waters, such as the Pacific and Indian Ocean.

Black marlin are considered to be migratory fish. Because they migrate so far, they can be found in the warmer parts of the Atlantic Ocean and off the coast of Australia.

You could conceivably find the same fish in both places. They can also spend time in gulfs and bays since they stick close to the shores.

Food & Diet

The black marlin is a carnivorous predator, so it spends its day eating. While the ocean’s apex predator is the killer whale, the black marlin is pretty high on that same list.

The “sword” that the black marlin swims with is part of its hunting strategy, but not for stabbing. Instead, they slash with it or wield it like a club to stun their prey.

What Do Black Marlin Eat?

Mostly, black marlin will eat just about any fish, though they seem to prefer smaller tuna when given a choice.

They also eat squids and octopi and have been known to eat shellfish. Various other prey they have been known to eat include dolphinfish, cuttlefish, mackerel and trevallies.

Because they don’t spend much time far below the surface, neither do their prey. The fish they feed on prefer the same warmer waters the black marlin does.

The “sword” mentioned above helps the black marlin grab food from a school of prey fish, as the predator will swim through the school swinging it.

The fish that get hit by it fall stunned from the school, at which point the black marlin can have a nosh.

Black marlins typically hunt by themselves and spend very little time with others of their species.

Younger black marlins sometimes swim in pairs, but with the exception of spawning season, when they gather together at spawning grounds, they are rather solitary creatures, and you won’t find a school of black marlins.

Threats & Predators

The black marlin is not an endangered species, but Individual black marlins face threats as they swim in the sea.

Human Threats

Humans and human activity present the only real threats to black marlins. The sportfishing industry finds anglers chasing after it and other billfish, though more and more, they’re hunting for the trophy and not for food because of the high mercury content in their bodies.

Climate Change & Global Warming

As climate change drives changes in the ocean, the black marlin must endure those changes.

The fish move farther south and north in the corresponding hemispheres as the ocean warms.

Scientific studies also show that during La Niña, they move as much as 200 miles further south in the southern hemisphere than they otherwise would travel.


Adult black marlins have no predators in the ocean other than human fishers. Larval black marlins face threats from filter feeders, and juveniles can fall prey to larger predators until they reach maturity.

Other Threats

Humans and human activity constitute the main threat to these fish. Black marlin are typically at the top of the food chain in their natural habitat and are rarely put in a situation where they are completely threatened.

Conservation Status

Black marlins are not expressly considered threatened or endangered, but this may have more to do with a lack of data than with the current population size.

The “Encyclopedia of Life” lists their endangered status as “data deficient.” Still, they (and most other billfish) fall under the protection of the Billfish Conservation Act of 2012.

Fun Facts About Black Marlin

  • Every black marlin is born as a female.
  • The black marlin is one of the fastest fish, reaching speeds of up to 80mph— making it in the top ten fastest creatures in the world.
  • They have growth rings on a spine on their dorsal fins, which scientists can count to determine their age.
  • In the spawning season, females can deposit up to 40 million eggs at a time.
  • While there is sparse evidence so far, some scientists believe black marlins communicate using mind waves to each other.
  • A black marlin is worth more than $30,000 per pound.
  • It’s one of the largest bony fish in the world.

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