[4] They are sought after as a highly prized game fish by anglers and are taken by commercial fishermen, both as a directed catch and as bycatch in major industrial tuna fisheries.

Blue marlin are currently considered a threatened species by the IUCN due to overfishing,[1] particularly in the international waters off the coast of Portugal where they migrate to breed in the June/July months.

[11] A separate study by V. P. Buonaccorsi, J. R. Mcdowell, and Graves indicated that both Indo-Pacific and Atlantic show "striking phylogeographic partitioning" of mitochondrial and microsatellite loci.

[14] The largest blue marlin caught by IGFA angling rules is from Vitoria, Brazil, which weighed 1,402 lb (636 kg).

The pelvic fins are shorter than the pectorals, have a poorly developed membrane, and are depressible into ventral grooves.

Blue marlin, like other billfish, can rapidly change color, an effect created by pigment-containing iridophores and light-reflecting skin cells.

It has about 15 rows of pale, cobalt-colored stripes, each of which has round dots and/or thin bars, located on both sides of the fish.

The body is covered with thick, bony, elongated scales that have one, two, or three posterior points, with one being the most common form.

[20] Warm currents such as the Gulf Stream in the western Atlantic have a major influence on their seasonal distribution.

Being the most tropic of all the billfishes, they spend a lot of time in the water column shifting from the top to a depth of 100 meters.

[23] Several fish have been recaptured in the same general area where they were tagged, implying reverse migration after/over several years, but the data are insufficient to accurately determine seasonality.

[25] Blue marlin have many parasites, including from these groups: Digenea (flukes), Didymozoidea (tissue flukes), Monogenea (gillworms), Cestoda (tapeworms), Nematoda (roundworms), Acanthocephala (spiny-headed worms), copepods, barnacles, and cookiecutter sharks.

In adolescents, the first dorsal fin is large and concave, gradually reducing in proportion to body size with continued growth.

Squid and deep-sea fishes such as pomfret and snake mackerel are also important prey items in certain areas.

Scientists and fishermen have long debated the extent to which blue marlin and other billfish use their elongated upper jaw in feeding.

A 2007 Japanese study of stomach contents of fish captured in a commercial trolling fishery found that 130 undigested prey items obtained from 227 blue marlin had spearing, slashing, and other injuries that were judged to have been inflicted by the bill.

The sportfishing pursuit of marlin and other billfish has developed into a multimillion dollar industry that includes hundreds of companies and thousands of jobs for boat operators, boat builders, marinas, dealerships, and fishing tackle manufacturers and dealers.

The most established sport fisheries for blue marlin are found along the eastern seaboard and the Gulf Coast of the United States, Bermuda, the Bahamas, and several other Caribbean islands (notably St Thomas and Puerto Rico).

Recreational fishing for blue marlin also takes place in Hawaii, Brazil, Venezuela, and the Atlantic coast of Mexico, particularly the Yucatan peninsula.

The International Game Fish Association all-tackle world record for blue marlin currently stands at 1,402 lb 2 oz (636 kg).

The initial efforts to develop electronic tags for tracking highly migratory fish were carried out on marlin in Hawaii, in collaboration with anglers in the Hawaiian International Billfish Tournament.

[23] Both Zane Grey and Ernest Hemingway, who fished for blue marlin off the Florida Keys, The Bahamas, and most famously in Cuba, wrote extensively about their pursuit.

In Hemingway's novella The Old Man and the Sea, a fisherman named Santiago battles a blue marlin for three days off the coast of Cuba.