Striped Marlin: How to Tell Billfish Apart, and Why They Light Up

Photographing striped marlin

By William Drumm, underwater photographer and ocean ambassador

A striped marlin comes into a bait ball at speed, and the first thing that happens is that it lights up. The bars along its flank go from a dull grey blue to a hard electric cobalt, in about a second, and then it swings its bill sideways through the sardines and turns away.

For a long time nobody knew why they did that. We do now, and the answer is better than the guesses.

Marlin vs Swordfish vs Sailfish

This is the thing everyone actually wants to know, and it is genuinely easy once you know what to look at. They are all billfish. They are not the same fish.

Marlin vs swordfish

• Pelvic fins. A marlin has them. A swordfish has none at all. If there are no fins under the belly, it is a swordfish. This is the single most reliable tell.
• Dorsal fin. A marlin’s is long and sail like, running well down the back. A swordfish’s is a short, stiff, crescent shaped blade near the head.
• The bill. A marlin’s is round in cross section, like a spear shaft. A swordfish’s is genuinely flat and broad, like a sword. The name is not decorative.
• Skin. Marlin have scales and, in most species, visible bars. Adult swordfish have neither.

Marlin vs sailfish

The sail. A sailfish has an enormous dorsal fin that runs nearly the whole length of its back and stands taller than the body is deep. It is unmistakable and it looks absurd. A marlin’s dorsal is tall at the front and then drops away along the back.

Size settles the rest. Sailfish are usually under 200 pounds. A blue marlin can go past a thousand.

Striped marlin vs blue marlin

Striped marlin are the slim ones, usually 200 to 300 pounds, with obvious pale blue vertical bars down the flank and a dorsal fin that stays tall along much of the back. Blue marlin are far heavier and deeper through the body, with a pointed front dorsal and faint or absent bars.

If you are watching a billfish work a bait ball in the eastern Pacific, and it is lit up like a neon sign, it is almost certainly a striped marlin.

The 80 mph Myth

Every article about marlin tells you they hit 80 mph. It is not true, and it is worth knowing where the number came from.

It traces back to a single fishing incident in which line stripped off a reel at a certain rate and somebody did some arithmetic. That is not a measurement of a fish. It is a measurement of a reel.

A peer reviewed study published in 2016 approached it properly, estimating maximum burst speed from muscle contraction time and stride length, and concluded that billfish are physically incapable of exceeding roughly 10 to 15 metres per second. That puts their true top speed far below the numbers you will find repeated everywhere, sailfish included.

Marlin are extremely fast. They are among the fastest fish in the ocean. They are not 80 mph fast, because nothing is, and the muscle physiology says so.

I would rather tell you the true number than the exciting one.

The Bill Is Not a Spear

The second thing everyone gets wrong.

A marlin does not impale fish on its bill. It has never done that. What it does is charge into the edge of a bait ball and swing the bill sideways through the school in a hard lateral slash, stunning and maiming fish as it goes, then turn and come back to pick up the ones that are no longer swimming straight.

Once you have watched it happen the design makes complete sense. The bill is not a skewer. It is a bat.

Why They Light Up

This is the good part, and it is new.

Striped marlin can brighten their stripes almost instantly, using cells in the skin that reflect light. It has been known for a long time that they do it. The explanations were guesses: mood, excitement, confusing the prey.

In 2024, researchers analysing drone footage of marlin hunting bait balls worked out what is actually going on. A marlin lights up as it commits to its attack run. The other marlin see it and hold off. Once it peels away, its stripes dim, and another fish lights up and takes its turn.

It is a turn signal. These are large, fast animals carrying rigid three foot spears, hunting the same tight ball of fish at the same time, and they have evolved a way to say I am going now, stay back. It stops them killing each other.

I have thousands of frames of this and never knew what I was looking at until that paper came out. Go back through the images and it is obvious: the fish attacking is bright, and the ones waiting are dull.

What Marlin Eat, and How They Hunt

Sardines, mackerel, anchovies, squid, small tuna. Anything that schools.

The hunt itself is the reason to be there. A bait ball forms when predators push a school of small fish up against the surface, which is the only barrier available in open ocean. Once the ball is up, it is a queue. Marlin come through in turns. Mahi mahi cut in from the side. Sea lions drive up from below, birds punch down from above, and the whole thing turns over on itself for as long as it lasts, which is rarely long.

The marlin work the outside. They almost never plunge into the middle of the ball. They peel fish off the edge, which is a more patient and far more effective way to eat than it looks.

How Big Do Marlin Get?

Striped marlin, the species in these photographs, run typically 200 to 300 pounds and up to around 14 feet including the bill.

Blue marlin are a different proposition entirely. The all tackle world record is a 1,805 pound fish, and females routinely pass a thousand pounds. In every billfish species the females are the giants and the males stay comparatively small.

Where to See Striped Marlin Underwater

Magdalena Bay, on the Pacific side of Baja California Sur, from roughly October into December. It is the most reliable striped marlin bait ball action on Earth, and it is where these photographs were made.

It is a snorkel and freedive operation, not a scuba one. Everything happens near the surface, the action moves constantly, and bubbles are no help to anybody. Boats run out of San Carlos and Puerto Adolfo Lopez Mateos, you spend the day looking for working birds, and when the birds go down you go in.

You do not chase the ball. You get dropped ahead of it and you wait, and if you have read it right, the whole thing comes to you.

Are Marlin Dangerous?

No, and yes, and the distinction matters.

No marlin has any interest in a person. On a bait ball they are completely locked onto the sardines and they know precisely where you are. They go around you, at speed, and it is one of the more remarkable things about being in the water with them: an animal that big, moving that fast, carrying a rigid spear, and it never touches you.

That said, a 250 pound fish with a three foot bill is a serious object, and people have been badly hurt by hooked marlin thrashing next to a boat. The danger is not the animal hunting. It is the animal panicking.

Under Pressure

Striped marlin are listed as Near Threatened. Blue marlin are Vulnerable.

The causes are the familiar ones: longline bycatch, targeted commercial fishing, and the slow arithmetic of a large, late maturing predator that cannot replace itself quickly. A marlin population that gets hit does not bounce back in a season.

The one thing genuinely working in their favour is that a living marlin is worth more than a dead one. Catch and release sport fishing, done properly, and the small but growing business of putting people in the water with them, both make the fish worth keeping alive, repeatedly. That is a blunt argument and it happens to be the one that works.

Photographing Striped Marlin

It is the hardest thing I photograph, and the reason is simple: everything worth having happens in about four seconds, in open blue water, with no bottom, no reference, and no second chance.

• Do not chase. A marlin outruns you without trying. Get in ahead of the ball and let it come.
• Shoot wide and get close. Water eats contrast. If the fish is more than a couple of metres away it is a grey smudge.
• Watch for the lights. Now that we know what the brightening means, it is a tell. A marlin that lights up is committing. That is your frame, and it is about a second away.
• Get the bill and the eye in the same picture. Without the eye it is a fish. With the eye it is a predator making a decision.
• Expect to fail. Most of the day is looking at birds. Then it is over in seconds and you find out whether you were ready.

If you want the other predators on the same bait ball, there is the mahi mahi, and the craft side is in how to become an underwater photographer. Prints are in the shop, and a portion of every sale supports marine conservation.

Related Images

The Hidden Hunter – Mahi Mahi Bait BallFrom $45.00Sanctuary in Bloom – Damselfish in TongaFrom $45.00Sapphire Dancer – Striped Marlin in Magdalena BayFrom $45.00

Browse all prints

Frequently Asked Questions About Marlin

What is the difference between a marlin and a swordfish?

The dorsal fin settles it. A marlin has a long, tall, sail like dorsal fin that runs a good way down its back, and it has pelvic fins under the belly. A swordfish has a short, stiff, crescent shaped dorsal fin and no pelvic fins at all. The bill is different too: a marlin’s is round in cross section, like a spear shaft. A swordfish’s is flat and broad, like an actual sword. And marlins have scales and stripes. Adult swordfish have neither.

What is the difference between a marlin and a sailfish?

Size and sail. A sailfish has an enormous dorsal fin that runs almost the entire length of its back and stands far taller than its body is deep, which is where the name comes from. A marlin’s dorsal is tall at the front and then drops away. Sailfish are also much smaller, typically under 200 pounds, where a blue marlin can pass 1,000. If the fin looks like a sail, it is a sailfish.

What is the difference between a striped marlin and a blue marlin?

Stripes and bulk. Striped marlin carry obvious pale blue vertical bars along the flank and are the slimmest of the marlins, usually 200 to 300 pounds. Blue marlin are far heavier and deeper bodied, with a pointed front dorsal and faint or absent bars. Striped marlin also have a dorsal fin that stays tall along much of the back, closer to a sailfish than a blue marlin does.

How fast can a marlin actually swim?

Slower than you have been told, and this is worth getting right. The famous figure of 80 mph is a myth. It traces back to a single incident where line stripped off a fishing reel, which is not a measurement of anything. A peer reviewed 2016 study estimated maximum burst speeds for billfish using muscle contraction time and stride length and concluded that these fish cannot exceed roughly 10 to 15 metres per second, putting realistic top speeds far below the popular claims. Marlin are genuinely fast. They are not 80 mph fast, and nothing in the ocean is.

What is a marlin’s bill for?

Slashing, not spearing. This is the other thing almost every article gets wrong. A marlin does not impale fish on its bill like a skewer. It charges into a bait ball and swings the bill sideways through the school in a fast lateral slash, stunning and injuring fish, then turns and comes back to eat the ones that are no longer swimming straight. Watch it happen once and the whole design makes sense.

Why do striped marlin light up and change colour?

Because they are talking to each other. Striped marlin can brighten their stripes in an instant using cells in the skin that reflect light. It was long assumed this was about mood, or confusing prey. Research published in 2024, using drone footage of marlin hunting bait balls, found something better: the marlin lights up as it commits to an attack, and the other marlin hold back. Once it peels away, the stripes dim and another fish takes its turn. It is a signal that stops several very fast animals carrying very long bills from hitting each other.

What do marlin eat?

Whatever is schooling. Sardines, mackerel, anchovies, squid, and smaller tuna. They hunt bait balls, often cooperatively and often alongside completely different predators, and they work the outside of the school rather than diving into the middle of it.

How big do marlin get?

It depends entirely on the species. Striped marlin typically run 200 to 300 pounds and up to about 14 feet including the bill. Blue marlin are in another category: the all tackle record is a 1,805 pound fish, and females routinely pass 1,000 pounds. Female marlin are far larger than males in every species.

Are marlin dangerous to humans?

Not deliberately, and there is no such thing as a marlin attacking a swimmer. But a 200 pound animal moving fast with a rigid three foot bill is a serious object, and people have been badly injured by hooked marlin thrashing beside a boat. In the water, on a bait ball, they are entirely focused on the sardines and are aware of exactly where you are. They will go around you.

Are marlin endangered?

Under pressure, and it varies by species. Striped marlin are listed as Near Threatened, and blue marlin as Vulnerable. The problems are longline bycatch, targeted commercial fishing, and the sheer slowness of recovery in a large, slow breeding, late maturing fish. Catch and release sport fishing, done properly, is one of the few forces on their side, because it makes a living marlin worth more than a dead one.

Where can you see striped marlin underwater?

Magdalena Bay, on the Pacific coast of Baja California Sur, from roughly October into December. It is the most reliable striped marlin bait ball action anywhere, and it is done as a snorkel and freedive, not scuba, because the whole event happens near the surface and bubbles do not help. Boats run out of San Carlos and Puerto Adolfo Lopez Mateos.

Can you eat striped marlin?

Yes, and it is common in Mexico and Japan, where it is smoked or served raw. But it is a large, long lived predator and it carries mercury accordingly. It is also a species under real pressure, which is worth thinking about before ordering it.