Swordfish have evolved to be formidable predators. They possess acute eyesight, with which they can locate prey, and their flesh consists primarily of ‘white’ muscle which provides energy for sudden bursts of activity, such as when in pursuit of their quarry (3). The fish then uses its bill to stun or impale its victim, slashes it into pieces or swallows it whole (3) (5). Swordfish feed during the day (3), primarily on squid, but also fish and occasionally crustaceans (6) (7) (8). They undertake vertical migrations in the ocean, following the movement of many small shrimp, fish and squid that move with the changing light intensity in a (somewhat unsuccessful) attempt to avoid predators (3) (7). Unlike some fish, swordfish are unable to maintain a body temperature higher than the temperature of the surrounding water. Instead, they have a unique muscle and brown tissue that warms blood flowing to the brain and eyes, enabling it to tolerate the extreme cold of the ocean depths (3).
Swordfish also undertake lengthy seasonal migrations, to temperate or cold waters in the summer where they feed, and back to warm waters in autumn for spawning (2). Unlike tuna, which have mostly ‘red’ muscle which is good for endurance activities, the mostly ‘white’ muscle of fish is not suited to swimming for long periods without fatigue (3). Therefore, swordfish undertake their long migrations by moving with prevailing currents (3).
Spawning occurs year-round in warm equatorial waters, while in cooler regions it occurs in the spring and summer (2) (4). The best known spawning grounds of the fish are found in the Mediterranean Sea, south of the Italian peninsula and Sicily (2). Swordfish eggs have been found here from June to September, and large numbers of juveniles occur throughout the Mediterranean from November to March (2). Fertilisation is external (3), whereby a female releases millions of buoyant eggs into the water, which are then fertilised by sperm secreted by the male. From the fertilised eggs hatch swordfish larvae. At only four millimetres long, with a short snout, and distinct, prickly scales (4), the larvae is vastly different to the great predator it will become. During the first year of life the larvae grow at a phenomenal rate, reaching a length of 90 centimetres (3). Female swordfish are thought to reach maturity at around 150 centimetres; whereas males are thought to mature at much smaller sizes, perhaps at around 100 centimetres (3).