Boating Tips

Black or Blue Marlin? Here’s How You Can ID Them

Jason Schratwieser of the IGFA shares some tips on how to ID a blue and black marlin.

blue marlin
Matt Santora’s Big Blue Marlin

This year’s marlin bite in southern California has been nothing short of insane. Not only are there a large number of fish being caught, but there are also a few BIG fish being caught. Whenever a big girl comes to the boat or dock in certain areas of the Pacific, there’s often the question of whether it’s a blue or a black marlin.

Distinguishing between the two species is the hardest marlin-call in the Pacific, but there are few ways that you can tell them apart.

Pectoral Fin

marlin idMost folks know that blacks and blues have pretty different pectoral fin morphology and behavior. Most notably, black marlin pectoral fins (in fish over 100-pounds or so) are unable to retract and this is a really informative characteristic. However, just because a fish’s fins don’t fold doesn’t mean they can’t fold, so you have to be careful about making a hasty decision boat side. Most of the time, when a blue marlin jumps, and especially when grey hounding, they will fold their pectoral fins so this is a good time to hazard a guess. Both species’ pectoral fins taper to a point, but black marlin are really, really sickle shaped and curve back towards the rear of the animal.

marlin fishing

Blue marlin really don’t have this characteristic. Sure their fins are pointed and curve a bit, but it’s nothing like a black’s.

Body Shape

marlin id

Blue and black marlin also have somewhat different overall body shapes. Blacks, regardless of size, have the appearance of carrying much more weight around the head and shoulders, while blue marlin seem to carry weight more uniformly through the body.

Bill Shape

marlin bill
Blue Marlin Bill

Black marlin bills appear a bit shorter and heaver- especially through the snout than a blue marlin. It’s a relative thing, but when you compare the two repeatedly, you get the picture.

Marlin Dorsal Fin

marlin comparisonThe dorsal fin is a pretty informative character for these species. Black marlin have the lowest dorsal relative to size than all other marlins. A blue marlin’s dorsal, on the other hand, is significantly taller and comes to a very acute point. A black’s dorsal is also pointed but it often has the appearance of being more curved than pointed.

Second Dorsal and Anal Fins

anal finNow this is definitive but it’s really only helpful if the fish is lying quietly on its side next to the boat or hanging at the dock. BUT… this is one way to really distinguish between the two species. Take a look where the second dorsal fin originates on a marlin’s back and trace an imaginary line to where the second anal fin originates.

blue marlin
Blue Marlin

If the origin or start of the second dorsal occurs at a point before where the second anal fin occurs, you have a black. Period. End of story. If you see the exact opposite, then it’s a blue.

Branchiostegals

fish id
Black Marlin

Brancioste-what? Branchiostegals refer to the boney elements in the umbrella-like membrane on a marlin’s “throat” below the gill plates or operculum. A black marlin’s branchiostegals are much longer and reach farther back to the rear edge of the operculum than a blue marlin’s. This gives the appearance of a bigger head and, in addition to the thicker shoulders, makes a black marlin look like it’s got way more weight forward than aft.

Putting It All Together

marlin id
Capt. Scott Goodwin with Black Marlin

Unfortunately, under most conditions, you have to rely on relative differences to distinguish between the two species. However, with practice, it’s completely doable and not that hard. And if you find yourself lucky enough to see one up close, with a good side view, the second dorsal/anal fin characteristic is a clincher.

Photo Credit: NOAA Billfish ID PDF and Capt. Scott Goodwin

Jason Schratwieser
Jason Schratwieser is Conservation Director for the International Game Fish Association, which allows him to marry his background in biology with his ...