Sea Wonder: Blue Marlin

Pacific Blue Marlin Photo Credit: NOAA

Known as one of the ocean’s largest fish, the blue marlin (Makaira nigricans) is also one of the most sought-after big game fishes in the entire world! People often confuse them for swordfish and sailfish (though all three are part of the billfish family) due to their similarities, but the blue marlin is one of the ocean’s strongest and fastest predators.

Appearance

The blue marlin is one of the most recognizable fish in the ocean. They are known for their silver underside and striking cobalt blue along their backs. The spear-shaped bill that grows from the front of its head is one of the blue marlin’s most distinguishing features and what makes people confuse them for swordfish. The fin atop the marlin’s back is taller at the front and slopes downward, though it resembles the tall, sail-like fins of sailfish, another member of the billfish family. 

This species experiences sexual dimorphism, which means one sex has different characteristics than the other, features that can include size, coloration, or presence/absence of features. For the blue marlin, females are much larger than males (we’re talking three to four times the size of males!), reaching lengths of up to 16 feet and weighing nearly 2,000 pounds. Males generally grow slower than females and do not weigh more than 400 pounds when fully grown.

Diet & Habitat

Blue marlin are open ocean fish native to the Atlantic, Indian, and Pacific Oceans. They are highly migratory and will follow warm waters (and their prey that rely on those warm waters) for hundreds or even thousands of miles. U.S. sanctuaries in which you might see blue marlin include: Florida Keys, Flower Garden Banks, any of the west coast sanctuaries, and National Marine Sanctuary of American Samoa. 

These predators are highly effective due to their speed, accuracy, and strength (which makes them challenging to catch and so desirable for sport fishermen). Near the ocean’s surface, blue marlin feed on smaller fish, mainly mackerel and tuna, and will eat squid when they are swimming at greater depths. To catch their prey, they will move their bill side to side, stunning their target before quickly swimming back around to catch and swallow their victim whole. 

Life History

The breeding season depends on where in the ocean they live, but it usually occurs when waters are a comfortable temperature and food is plentiful, though there may be other environmental or social cues that signal these fish to reproduce. Blue marlin breed via spawning, an external process of fertilization in which males and females release gametes (eggs and sperm) into the water column. Females release up to seven million eggs at one time to increase the chances of fertilization and avoid predators long enough to hatch. Once hatched, few larvae will survive into adulthood. Males have a maximum life span of about 18 years and females can live as long as 27 years. Throughout their lives, blue marlin will spend time alone and in schools. 

Nearly all fishes have cold blood, but billfishes like the blue marlin have specialized blood vessels that allow them to warm targeted parts of their body, primarily their brain and eyes. This physiological ability enhances their vision and thinking abilities and provides these hunters a huge advantage over their prey. 

Threats & Conservation

Blue marlin are considered vulnerable by the IUCN red list but are not considered threatened or endangered by the Endangered Species Act. Populations are showing decline in several locations, mostly due to these fish being accidentally caught when they are not being targeted – a phenomenon called bycatch. In areas where blue marlin are targeted as a commercial species (for example, the U.S. Pacific Blue Marlin fishery), it is generally considered sustainable. Due to their desirability as a game fish species due to their size and the challenge they pose to fishermen, blue marlin fishing tournaments are popular with sports fishers. Tournament hosts usually regulate what is permissible to ensure participants enjoy themselves but the species is also not overexploited. Climate change, pollution, and other human activities also pose threats to the health of blue marlin populations throughout the world