How to Identify Blue Marlin

To identify Blue Marlin (Makaira nigricans), look at these key features.

Physical Characteristics

Body shape

torpedo

Coloration

Dorsal side is dark cobalt blue, fading to a silvery-white on the belly. Often exhibits vertical bars or spots on the flanks, which may fade after death. Fins are typically dark blue to black.

Scale type

cycloid

Mouth position

terminal

Distinctive features

Long, rigid dorsal fin that is relatively low anteriorly and tapers posteriorly, fitting into a groove when depressed., Prominent, spear-like upper jaw (bill) that is round in cross-section., Two keels on each side of the caudal peduncle., Pelvic fins are slender and can be retracted into a groove.

Dorsal fin

First dorsal fin is long and low, with 39-43 rays, fitting into a groove. Second dorsal fin is small, with 6-7 rays.

Tail fin

Deeply forked, crescent-shaped caudal fin, highly adapted for speed.

Pectoral fin

Long and pointed, with 19-22 rays, relatively rigid and not easily folded against the body.

Distinctive fins

Pelvic fins are slender, shorter than pectoral fins, and can be retracted into a ventral groove.

Identification Tips

1. 1

   Coloration

   Dark cobalt blue dorsal, silvery-white ventral. May show faint vertical bars, especially when excited or just caught.

2. 2

   Dorsal Fin

   First dorsal fin is relatively low along its entire length, unlike the high, sail-like dorsal fin of the Sailfish or the very tall dorsal fin of the White Marlin anteriorly. It also retracts into a groove.

3. 3

   Pectoral Fins

   Relatively rigid and not easily folded flat against the body, distinguishing it from White Marlin (Kajikia albida) and Striped Marlin (Kajikia audax) which have more flexible pectorals.

4. 4

   Bill Shape

   The bill (upper jaw) is round in cross-section, in contrast to the flattened bill of the Swordfish (Xiphias gladius).

5. 5

   Lateral Line

   The lateral line is a complex, reticulated (net-like) pattern, which is often difficult to see in large specimens but is a key diagnostic feature.

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