Skipjack tuna surfs the swell of the ocean next to the Odyssey.
Photo: Chris Johnson
June 7, 2002
The Wandering Tuna
This is Genevieve Johnson speaking to you from the Odyssey as we sail the southwest trade winds toward the Chagos Archipelago.
Last night we had a small group of 10-15 Pan-tropical spotted dolphins, rolling and tumbling around our bow, hitching a free ride. However at sunrise this morning, the guests at our bow were no longer the cajoling, whistling marine mammals. Instead, from bow to stern, just below the surface of the ocean, was a large school of sleek, swell-surfing skipjack tuna.
Tuna have no country, nor lasting domicile.
All the sea is their native country.
They are a wandering fish.
- Fray Martin Sarmiento, 1757.
For nearly two days now, we have been able to count more than 20 skipjacks, swimming alongside Odyssey. Sporting the beautiful iridescent blue and violet camouflage of the high seas, they surf the swell and with an occasional flick of their tails keep pace-apparently with minimal effort. Like the school of rainbow runners that followed the Odyssey's path across 900 miles of the tropical Pacific Ocean last year, they are fast becoming our constant, steadfast companions as we cross the central Indian Ocean.
These fish are one of some 24,000 presently known species of fish-collectively the most successful vertebrates ever to have lived on this planet. They range in size from the tiny 10mm Pygmy Goby to the 12 meter long Whale Shark. However these skipjacks are open ocean predators and together with their cousins, the yellow and blue fin tuna, are among the most impressive hunters on the planet.
Gliding through the water on an endless quest for food, every feature of their bodies has evolved to maximise the chase. These fish are perfectly streamlined, even their gill cases are immoveable-something that reduces drag, and forces the water over their gills as they swim forward with their mouths open. Their fins are perfectly hydrodynamic, and provide much lift and manoeuvrability, similar to a bird's wings in the air. If you have ever seen a freshly caught tuna you may have noticed the grooves into which all of its fins slot when they are held against its body, something that increases streamlining still further. The shape of the tuna's body is the most hydrodynamic shape possible, one that humans have relentlessly sought to emulate in submarines and torpedoes. People often refer to these fish as torpedo-shaped, but it might be more appropriate if we referred to our subs and torpedos as tuna shaped!
Sylvia Earle once stated -
"If we were to design the optimum form of life on this ocean planet,
it might look remarkably like the majestic bluefin tuna."
At times the fish appear to huddle close to the hull, then whenever a school of flying fish bursts through the surface, the skipjacks spread out abreast of one another, about 25 meters from the boat in what looks like an attack formation. Like all tuna, even the enormous 500-kilogram Blue fin tuna, these open ocean, or 'pelagic' fish hunt in squadrons. There is accumulating evidence that they work as a team, the sensory abilities of a large group dramatically increasing the area they can search at any one time, thus greatly increasing their hunting efficiency!
Tuna often aggregate around comparatively small floating objects at sea. The explanation for this behavior is unclear, though it has been suggested that such flotsam serves as a meeting point in an environment lacking fixed surface points around which predators can gather when they aren't hunting. To a visual species like tuna such features would presumably be of great value as collecting points prior to their setting off on a communal hunt each morning.
First mate Jacob Kasper, attempts to catch the skipjack tuna
riding the bow-wave in order to take samples to be later analysed for contaminant concentrations.
Photo: Chris Johnson
Another impressive feature these fish exhibit is a counter-current blood system-in which veins carrying cold blood from the fish's body surface return to the heart parallel and closely pressed against warm arteries carrying warm blood on its way to the skin. In this way the venous blood is progressively warmed as it flows back to the heart by the heat from the arterial blood that is on the way to the skin. In this way the heat is not lost to the cold sea, but stays inside the fish. This physiological adaptation serves to ensure that the core body temperature of tuna always remains above that of the surrounding water, which allows tuna to extend their hunts into the cooler waters found at depth and to higher latitudes where the surface waters are cold.
Tuna are also open ocean predators, yet they are also prey for even larger natural predators like sharks. So perhaps, given the lack of hiding places out here, any object, even a relatively slow-moving vessel like the Odyssey, is a temporary hiding place from the watchful eyes of predators.
We have experienced the companionship of pelagic fish species before and have heard several stories of them accompanying sailboats and old Yankee whaling ships.
Frank T. Bullen describes in his famous book "Cruise of the Cachalot", how the crew would often take advantage of the fresh food bonanza afforded by the fish riding their bow wave and in their wake.
Fresh fish always makes a pleasant change from the usual dried or frozen fare that becomes the mainstay of life onboard any boat during a long ocean crossing. Although the thought of consuming our unflagging oceanic companions sits uneasily with us, samples from their kind are necessary if we are ever to find out what poisons are accumulating in the livers, gills, hearts, and kidneys of the oceanic fish species we humans consume. Samples that can be analysed for contaminant concentrations are an important addition to our global data set. So we have collected (hesitantly, but gratefully), two specimens from the school about us. And in addition to having valuable data, the Odyssey crew will be feasting on fresh tuna tonight for dinner.
Log by Genevieve Johnson