Blue Marlin Facts

The blue marlin (Makaira nigricans) is the largest billfish. It is related to the black marlin, striped marlin, white marlin, spearfish, sailfish, and swordfish. The blue marlin is easily recognized by its cobalt blue-to-silver color, cylindrical body, and sword-like bill. Originally, two species of blue marlin were recognized: the Atlantic blue marlin (Makaira nigricans) and the Indo-Pacific blue marlin (Makaira mazara). However, most sources now classify both populations as Makaira nigricans.

Description

Like other billfish, the blue marlin has pigment and light-reflecting cells that allow it to change color. Most of the time, the fish is cobalt blue on top and silvery underneath with 15 rows of pale blue stripes. It has two dorsal fins with body structures called rays, two anal fins, and a crescent-shaped tail. The bill is round and pointed. Small teeth line the roof of the mouth as well as the jaws.

Females are up to four times heavier than males. Females may reach up to 16 feet in length and weight up to 1,800 pounds, while males rarely exceed 350 pounds.

Habitat and Range

Blue marlin range expands across the temperate, subtropical, and tropical waters of the Atlantic, Pacific, and Indian Oceans. In the warmer months, they migrate to the temperate zones, but return toward the equator during cooler months. They spend their lives out at sea, following ocean currents. While blue marlin usually live near the surface, they can dive to great depths to feed on squid.

Diet and Behavior

The blue marlin is a carnivore. The planktonic larvae feed on fish eggs, other larvae, and other zooplankton. As they grow, they feed on squid and a variety of fish, including tuna, mackerel, and smaller marlin. When fully grown, blue marlin are only preyed upon by large sharks, such as the great white and the shortfin mako.

The marlin's pointed bill is visible shortly after hatching. The fish darts through a school of prey, incapacitating its victims using a slashing motion. Larger targets may be stabbed with the bill. The blue marlin is among the fastest fish. It also frequently jumps out of the water.

Reproduction and Offspring

The blue marlin reaches sexual maturity between two and four years of age, when males weigh between 77 and 97 pounds and females weigh between 104 and 134 pounds. Breeding occurs in the summer and fall. Females spawn up to four times in a season, releasing up to seven million eggs at a time that are fertilized by the male's sperm in the water column. The tiny 1-millimeter (0.039 inch) eggs drift in the pelagic zone. Upon hatching, larvae grow over half an inch each day, but most eggs and larvae are eaten by other animals. Very few marlin reach maturity. Larvae are blue-black in color, fading to white on their bellies. They have blue iridescent patches on their heads and transparent caudal (tail) fins. The first dorsal fin is large and concave initially, but it becomes more proportional to the body size as the fish grows. Males live up to 18 years, while females may live 28 years.

Conservation Status

The International Union for Conservation of Nature (IUCN) classifies the blue marlin conservation status as "vulnerable." Estimates place the population reduction from 1990 to 2006 at approximately 64% in the Atlantic. Researchers conservatively estimate the population reduction of the blue marlin in the Pacific from 1992 to 2009 at 18%. In the Indian Ocean, the fish population has decreased around 70%, as of 2009.

Threats

By far, the greatest threat to blue marlin survival is death as bycatch, particularly from longline fishing for tuna and swordfish. Experts believe switching from J-hooks to circle hooks could increase catch-and-release survival, while removal of shallow hooks on longline sets could significantly decrease bycatch. Although the blue marlin is listed under Annex I of the 1982 Convention on the Law of the Sea, implementation of additional management measures will be necessary to protect this species.

Blue Marlins and Humans

The blue marlin is important both for commercial and sport fishing. The fish is prized for its meat, its beautiful appearance, and the challenge posed by catching it. Sports fishermen are leading efforts in blue marlin conservation, including tagging fish to track their migration and formulating sustainable fishing policies.

Sources

• Collette, B., Acero, A., Amorim, A.F., et al. Makaira nigricans. The IUCN Red List of Threatened Species 2011: e.T170314A6743776. doi:10.2305/IUCN.UK.2011-2.RLTS.T170314A6743776.en
• Nakamura, I. Billfishes of the world. An annotated and illustrated catalogue of marlins, sailfishes, spearfishes and swordfishes known to date. FAO Fish. Synop. 1985.
• Restrepo, V.; Prince, E.D.; Scott, G.B.; Uozumi, Y. "ICCAT stock assessments of Atlantic billfish." Australian Journal of Marine and Freshwater Research 54(361-367), 2003.
• Serafy, J.E., Kerstetter, D.W. and Rice, P.H. "Can circle hook use benefit billfishes?" Fish Fish. 10: 132-142, 2009.
• Wilson, C.A., Dean, J.M., Prince, E.D., Lee, D.W. "An examination of sexual dimorphism in Atlantic and Pacific blue marlin using body weight, sagittae weight, and age estimates." Journal of Experimental Marine Biology and Ecology 151: 209-225, 1991.