Marlin: superfish

Swifter than a shark, more dashing than a dolphin and wielding a sword-like bill, the marlin is a swashbuckling superfish. Yet little is known about it and its billfish relatives.

Marlin: superfish

Swifter than a shark, more dashing than a dolphin and wielding a sword-like bill, the marlin is a swashbuckling superfish. Yet little is known about it and its billfish relatives. Kate Youngdhal discovers a family of thrilling predators.

Hurtling through the water like a massive torpedo, the black marlin’s silver and blue sides suddenly flush black. She is a giant – up to 900kg and nearly 5m of supercharged muscle – the biggest, fastest, most dangerous gamefish in the ocean. And though this fish has been hunted since ancient times, much of its story remains a mystery.
The very thing that defines this fish – its bill – puzzles experts. They know it’s a toothless, elongated upper jaw, but they can’t say exactly why the species has it. Many claim it’s a weapon, used to stun or spear prey. Yet marlin that have lost their bills still manage to feed, leading others to argue that the appendage is primarily a defensive tool.
Many maintain it is just a showy vestige from prehistoric times. It kept the fish competitive when primordial seas teemed with other huge predators. Today, it is not the spear but the animal’s versatility that makes it a consummate predator.
In a world where most animals must choose between warm and cold, light and dark, shallow and deep, the marlin rules all.
Roaming the open ocean
All four species of marlin – striped, white and the larger blue and black – roam the deep ocean of temperate and tropical regions. Though there’s plenty of room here for a big fish, there’s very little prey. So the marlin must seek out the ephemeral places where food congregates.
One of the most productive of these occurs where cold ‘green’ currents from the deep collide with warmer ‘blue’ waters. Mackerel and other fish get pulled along in the fast-moving current line, and rays, turtles, tuna and sharks swim in and out, picking off prey.
When the dolphins arrive, all hell breaks loose. Creating a wall of bubbles and sound, the cetaceans herd the fish into a dense pack known as a ‘baitball’, triggering a violent feeding frenzy.
Alerted by the commotion, more predators swoop in to reap the bounty. But nothing compares to a marlin on the prowl. Dwarfing its rivals, it circles the baitball. Then, just before it charges in, its shimmering sides turn black.
Changing colour

Colour change is a fairly common fish trick. Star-shaped colour cells between the scales and skin contain pigment. When the pigment shoots into the arms of the star, the cell colours. When the pigment withdraws to the star’s centre, the cell becomes invisible again.
Most colour-changers use this adaptation to escape detection by enemies, but the marlin doesn’t need to hide from anything. Perhaps its flashy wardrobe-change is a warning to other predators: the big fish is coming.
It takes a lot of fuel to power a 900kg muscle machine. A grown marlin can swallow a fish the length of a man’s arm in a single gulp. In a matter of minutes, a marlin can decimate a baitball. This fish can reach 97kmph in bursts, and satellite-tags have tracked individuals covering more than 14,000km in one season.
Its hydrodynamic body shape explains its speed – but how the marlin navigates across the vast oceans remains unknown. Like an elephant crossing the desert, it may have an instinctual map of the seafloor terrain inside its head; salinity and temperature may help it to judge depth and proximity of land, while the direction of the sun or the pull of the moon may also guide the marlin on its way.
The voyage is long, wide – and deep. Though the marlin spends most of its time in the light-filled top layer of water, it can dive down to 350 metres to stalk squid.
Its extraordinary eyes enable it to cope with those cold, dark waters. Huge and unblinking, they accommodate myriad photosensitive cells that can pierce the depths. More astonishing, a muscle attached to the eye heats the brain cavity. Cold water turns most fish sluggish, but with warm eyes and brain, the marlin stays alert and deadly.
Fishing with a torch
On a balmy night, a small boat rocks gently off Costa Rica’s Pacific coast. On board is underwater cameraman Rick Rosenthal, who has spent 18 months documenting the marlin’s life for a BBC Natural World film. Tonight, armed only with a large light and a small net, he is searching for a billfish nursery. His luck is in and he captures rare footage of wild billfish babies.
Where they go from here, no one knows. After infancy, billfish seem to vanish, leaving their juvenile years a blank in the scientific record.
Marlin, though, reappear with one of their biggest mysteries intact: the most impressive marlin are all female. Males only reach about a quarter of their size. Biologists believe that males just stop growing at about 160kg, while females continue to develop – probably so they can carry more eggs and are less likely to be predated.
Sadly, these are the fish most coveted by big game fishermen. What irony: anglers end up killing the mothers of the next generation – and with them the future of their own sport.
Still, whatever harm they do pales in comparison to the damage done by commercial fisheries. Marlin is now hot on the menu, and stocks are now down to five or ten per cent of its historic populations. Unless the massacre stops, many of the mysteries of the marlin will go forever unsolved.   

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