The Atlantic blue marlin (Makaira nigricans) is a species of marlin endemic to the Atlantic Ocean. The Atlantic blue marlin (hereafter, marlin) feeds on a wide variety of organisms near the surface. By using its bill, it can stun, injure, or kill while knifing through a school of prey and then return later at its leisure to eat. Marlin is a popular game fish and has commercial value because its meat has a relatively high fat content.
Marlin are distributed throughout the Atlantic's tropical and temperate waters; they are more populous in the western parts. It is a blue water fish that spends the majority of its life in the open sea, far away from land. Females can grow up to four times the weight of males. The maximum published weight is 820 kilograms (1,800 lb). Marlin have few predators apart from man; the IUCN currently consider it a threatened species due to overfishing.
The biggest females are more than four times as heavy as the biggest males. Males rarely exceed 160 kilograms (350 lb) in weight, and females commonly weigh over 540 kilograms (1,200 lb).The longest females can reach a length of 5 m (16 ft) with the bill, from eye to tip, constituting about 20% of the total body length. Body mass in the largest female specimens has been reported from 540 to 820 kg (1,200 to 1,800 lb) or even up to 1,800 kg (4,000 lb) depending on the source (few large specimens are scientifically verified). The all-tackle record weight verified for the species was 636 kg (1,400 lb). Both sexes have twenty-four vertebrae, of which eleven are precaudal and thirteen are caudal.
This marlin has two dorsal fins and two anal fins. The fins are supported by bony spines known as rays. Its first dorsal fin has 39 to 43 rays from front to back. Its second dorsal fin has 6 to 7 rays. Its first anal fin, which is similar in shape and size to the second dorsal fin, has 13 to 16 rays, and the second anal fin has 6 to 7 rays. The pectoral fins, which have 19 to 22 rays,are long and narrow and can be drawn in to the sides of the body. The pelvic fins are shorter than the pectorals, have a poorly developed membrane, and are depressible into ventral grooves. Its first anal fin, along with its pectoral and caudal fins, can be folded into grooves. This streamlines the fish and thereby reduces drag.
The body is blue-black on top with a silvery white underside. It has about fifteen rows of pale, cobalt-colored stripes, each of which has round dots and/or thin bars, located on both sides of the fish. The first dorsal fin membrane is dark blue or almost black and has no dots or marks. Other fins are normally brownish-black, sometimes with a hint of dark blue. The bases of the first and second anal fins have a hint of silvery white. Marlin can rapidly change color and usually appear bright blue when hunting. The coloration results from pigment-containing iridophores and light-reflecting cells. The body is covered with thick, bony, elongated scales that have one, two, or three posterior points, with one being the most common form.
The bill is long and stout. Both the jaws and the palatines (the roof of the mouth) are covered with small, file-like teeth. The lateral line system is a group of neuromasts rooted in lateral line canals that can sense weak water motions and large changes in pressure. It has the appearance of a net. It is obvious in immature specimens but unclear in adults, becoming progressively embedded in the skin. The anus is just in front of the origin of the first anal fin.