I thought I’d try something different today than the usual fish identification post.

Instead, we’re going to talk about telling the difference between two fish: the marlin and the swordfish. You’re not likely to run across either of these fish on a scuba trip, but they are commonplace on television (fishing shows, woohoo!) and on our dinner plates. For those that already know the difference, I congratulate you. The rest of us should read on.

First things first, both fish are members of the billfish family. Billfish encompass fish that have sword-like bills. This basically means marlins, swordfish, and sailfish.

Let’s get the easy one out of the way. A sailfish can be recognized by the large dorsal fin that looks like a sail that has accordion pleats. That’s the first test. If the top of the fish has a sail, it’s a sailfish. Done. For fun, let’s look at a picture. Oooh, pretty.

Marlins have a single dorsal fin that connects along the fish’s back to a short, soft-looking ridge. Swordfish, on the other hand, have a dorsal fin more like a shark, and even extends far up, looking somewhat like a feather.

Here is a picture of a marlin:

Compare that to this picture of a swordfish. Notice also that swordfish have pectoral fins that extend below its body, whereas a marlin’s pectoral fins are small and barely visible.

If you can’t get a good look at the fins, you may have to rely on body shape to tell the difference. Marlins have a long, tubular body that varies only a little in size along its length. A swordfish, while still elongated, has a more rounded body.

Check out the two pictures below. From what you’ve learned, which is a swordfish, and which is a marlin? Hover your mouse pointer over the image to see the answer.


Once you know a few differences, I think it’s easy to tell the difference between marlins and swordfish. Even if you get them confused with each other, I doubt anyone will think you're stupid. As long as you don’t call one of them a sailfish. :)