Blue marlin

The blue marlin, Makaira nigricans, is an oceanic billfish of the surface layer in warm seas, found in the Atlantic, Pacific and Indian Oceans ^[1]. In the Atlantic it is the most tropical billfish and can exceed 580 kg; in the Pacific fish of over 906 kg are reported, and in both females reach much larger sizes than males ^[2]. The IUCN lists it as Vulnerable ^[3]. In Peru its commercial catch and sale have been banned since 2008 ^[4].

It has a slightly compressed body, a strongly raised nape, a long stout bill that is round in cross section, and two dorsal fins; the first is lower than the greatest body depth and has 39 to 45 rays depending on the population. It has 24 vertebrae (11 precaudal and 13 caudal) ^[2].

The back is blue-black and the belly silvery white, with about 15 rows of pale cobalt bands made of dots or short bars. The pectoral fins are long, narrow and fold flat against the side, and the lateral line, visible in juveniles and nearly hidden in adults, forms a network pattern in the Atlantic and simple loops in the Pacific ^[2].

Similar species

• Istiompax indica: In the black marlin the pectoral fins are rigid and do not fold against the side, they have 12 to 20 rays, and the front lobe of the first dorsal fin is smaller than the body depth ^[2].

It lives in open water across the three warm oceans, with a single genetic stock in the Atlantic and a single genetic population across the whole Pacific ^[1]. Nakamura treated the Indo-Pacific form as a separate species (Makaira mazara), and the Pacific data cited here come from that catalogue. Judging by Japanese longline catches, its range reaches about 35° to 40° N in the North Pacific and 30° to 35° S in the eastern Pacific, and it becomes less abundant toward the eastern Pacific ^[2].

In the Atlantic it occupies water with a surface temperature of 22 to 31 °C, and in the Pacific it is mostly confined to the warm side of the 24 °C surface isotherm ^[2]. Satellite tags on three fish off Madeira show they spent 71% of their time in the top 5 m and 89% in the top 50 m, and most of the time at 20 to 26 °C ^[1].

Using 14 years of purse-seine catch data from the eastern Pacific, its suitable habitat was modelled: it shifts between coastal waters in winter and spring and oceanic waters in summer and fall, and moves closer to shore during El Niño ^[5].

Females reach much larger sizes. In the Atlantic fish over 136 kg are usually females, and the species exceeds 375 cm in length and 580 kg ^[2]. In the Pacific, males are thought to mature at 130 to 140 cm eye-fork length, and fish over 200 cm of that measure around the Bonin Islands were all females ^[2].

Ripe Pacific eggs measure 0.8 to 0.9 mm. Otolith readings put a 52.1 kg male at 6 years and a 336.5 kg female at 17 years ^[2]. A 270 kg fish followed with a transmitter swam from the surface to 73 m, mostly in the top 37 m ^[2].

Fish tagged off Madeira dived deeper than 200 m (maximum 336 m). One recovered tag showed that at night it mostly uses the surface and by day it makes dives, mostly deeper than 50 m. Because it lacks heat exchangers in its vascular system, it may need to return to the surface to rewarm between deep dives ^[1].

It feeds near the surface, though it sometimes takes food in deeper water ^[2]. For the Pacific, Nakamura reports that off Hawaii tuna-like fishes made up over 85% of the volume of its diet, and that in the northeastern Pacific it ate mainly squids and fishes of the families Bramidae, Carangidae and Gempylidae, plus Auxis and Xiphias gladius ^[2].

It uses its bill to strike prey. Fishes in its stomachs show deep slashes, and off Baja California a fish of about 3 m was seen stunning Dosidicus gigas squids with its bill before eating them ^[2].

The IUCN lists it as Vulnerable globally ^[3]. The Atlantic stock is assessed as overfished, with a biomass 40% below the level that would give maximum sustainable yield, and most of its fishing mortality is bycatch from tuna and swordfish longlines ^[1].

In Peru, Supreme Decree 009-2008-PRODUCE bans the commercial catch, trade and sale of the blue marlin (which the decree names Makaira mazara) and allows sport fishing only as catch and release ^[4]. Even so, marlins, without separating species, are still recorded as bycatch and in artisanal landings in the north, with a peak of 1419.9 t in 2014 ^[6].

Between 1978 and 1982 recorded world catches of the Atlantic form were 1842 to 2448 t a year and those of the Pacific form 18193 to 21413 t a year; the Pacific form is mostly taken incidentally in fisheries aimed at other marlins and tunas ^[2].

Off Madeira, recreational fishers have targeted it since the 1970s, and post-release mortality is generally low, about 5% ^[1]. In Peru the use of the harpoon goes back about 10000 years BC; for billfishes it seems to have been used mainly on the north coast and to have declined after 1970 ^[6].

It is an oceanic surface fish that the cited sources document through satellite tags and landings, not through dive observations. In the sampling of harpoon landings at El Ñuro (Piura) during 2019, swordfish made up 95% of the 369 fish and landings of blue marlin and black marlin were minimal ^[6].