Billfish
Found in temperate and tropical waters around the world, these large migratory, predatory fish are characterised by long flat sword-like, or rounded spear-like, upper jaws (also called a beak, snout or rostrum), which they sometimes use to slash their prey, wounding and stunning them and rendering them easier to catch. Billfish (along with tuna and some sharks) have an uncommon ability to maintain their body temperature slightly above that of the surrounding water, which improves the efficiency of their swimming muscles and warms their eyes and brain, improving their speed and vision and making them excellent predators. They are among the fastest fish in the ocean.
While Swordfish are in a family of their own (Xiphiidae), the remaining billfish are members of the Marlin family (Istiophoridae), which is further divided into three subgroups: ‘true’ Marlin, Spearfish and Sailfish. Like Swordfish and Spearfish, Marlin are named for their beak, which is thought to resemble a sailor’s marlinspike, a pointed iron tool used for splicing rope.
Billfish are popular with game fishermen, but some are also increasingly popular as table fish. The two most commonly seen commercially in Australia are:
Swordfish (Xiphias gladius) are by far the most common. They have the longest bills (about 30% of their body length), which are smooth, flat, pointed and sharp (like a sword) and, unlike Marlin, their skin is scaleless. They are typically caught at 70-150kg and 1.5-3 metres (from the tip of the lower jaw to tail fin), but can grow to over 500kg and 4.5 metres. They are highly migratory, having the widest range among billfish, moving to cooler waters in summer and warmer in winter; those found in the Pacific reach a greater size than those in the Atlantic and Mediterranean. In Australia, they’re caught mainly by Tuna longliners off the east and west coasts, with peak supply in winter.
Striped Marlin (Tetrapturus audax) are far less commonly seen commercially in Australia, though they are the most common billfish worldwide. They have a shorter, rounded bill and small scales. The smallest member of the marlin family found in Australian waters, despite their name they are a member of the spearfish subgroup not the ‘true’ marlin one. They are only caught as bycatch, mainly by tuna longliners, so supply is limited, with peaks from October to April. They are generally caught at 30-120kg and 1.6-2.5 metres (tip of lower jaw to tail fin), but can grow to at least 260kg and 420cm.
Other Billfish occasionally seen in retail shops or caught recreationally in Australia are:
Sailfish (Istiophorus platypterus), distinguished by their large, long majestic sail-like dorsal fin, are often referred to as the fastest of all fish. They may be referred to as Indo-Pacific sailfish to distinguish them from Atlantic Sailfish (Istiophorus albicans), which some consider to be the same species.
Black Marlin (Makaira indica) and Blue Marlin (Makaira nigricans) are protected in Australia and so aren’t caught commercially, only by game fishermen who tag and release them. Some authorities consider Makaira nigricans an Atlantic fish and the Indo-Pacific Blue Marlin a separate species (Makaira mazara) though recent DNA analysis indicates that they are the same species. Members of the ‘true’ Marlin subgroup have a dorsal fin that peaks at the front then tapers very sharply along the back and is only half to three-quarters of their body depth.
Shortbill Spearfish (Tetrapturus angustirostris), like all members of the Spearfish subgroup, is distinguished by a higher dorsal fin (at least as high as the body depth), that tapers less sharply. Other spearfish found around the world are Atlantic White Marlin (Tetrapturus albidus), Roundscale Spearfish (Tetrapturus georgii), Mediterranean Spearfish (Tetrapturus belone) and Longbill Spearfish (Tetrapturus pfluegeri).
Buying
Large fish such as Swordfish and Striped Marlin aren’t filleted in the same way as smaller fish; instead their body is cut into four boneless loins, two from the top half of the fish and two from the bottom half. These loins are then sliced vertically into steaks. Swordfish is sometimes sold as sashimi and the tail section of smaller fish may be sliced into cutlets. Look for firm, lustrous, moist flesh without any brown markings or oozing water, cream-pale pink for swordfish and reddish-pink for marlin, with a pleasant fresh sea smell.
Storing
Lay fish in a single layer on a plate and cover with plastic wrap or place in an airtight container. Refrigerate for up to 3 days or freeze for up to 3 months below -18ºC.
Cooking
Swordfish has been caught since ancient times and is a classic ingredient in Mediterranean cooking. Its firm, moist flesh has a slightly sweet flavour, high oiliness and fine flakes. Marlin has a slightly stronger flavour, medium-high oiliness and dry, firm flesh with large flakes. Both easily become dry if overcooked, wrapping in foil or banana leaves before baking or barbecuing helps prevent this. The firm flesh holds together well in soups, curries and casseroles and can be cubed for kebabs. The thick skin should always be removed, but any dark bloodline can either be trimmed off or eaten, depending on personal taste. Both Swordfish and Marlin can be pan-fried, baked, braised, grilled, barbecued, smoked or eaten raw (sashimi) and swordfish can be pickled.
Swordfish & Mercury
Mercury occurs naturally in the environment and accumulates in the food chain (including fish), with larger predatory fish generally having higher levels than smaller fish. Most fish in Australian waters have very low mercury levels and mercury from fish is not a health concern for most people. Women who are pregnant, breastfeeding, or planning pregnancy, and children under 6 years old are however advised by the to limit their intake of billfish to no more than 150g per fortnight with no other fish that fortnight. Alternatively, they are recommended to have 2-3 x 150g serves of seafood per week (except for Shark, Billfish, Orange Roughy and Catfish). The above advice also applies to children under 6, although the recommended serving size for them is 75g.