Image by Dylan Baist-Bliss/<a href="http://www.flickr.com/photos/dbaist/7678962530/">Flickr</a>
Anyone who saw “Finding Nemo” remembers the cool sea turtles – including Crush and his son— dude! – Squirt riding the East Australian Current. What few realize is that currents like the “EAC” stitch together all of the oceans of the world into a single global fabric.
Differences in temperature, salinity and density drive currents that move water, creating a giant system of currents called the “global ocean conveyor” or thermohaline circulation – THC for short. These currents also move tropical energy from equatorial zones into the northern ocean, one of the factors that keep Northern Europe warm enough to live in.
“Finding Nemo” got it right in showing how this current system conveys animals from one place to another. In the Atlantic, baby sea turtles ride ocean currents away from the beaches where they were born, into the vast aquatic jungle of the Sargasso Sea, and then back to those same beaches again when they are old enough to reproduce.
American eels do the same thing, but in reverse; transparent baby eels, spawned in the Sargasso Sea, ride the great wheel of the North Atlantic to coastal estuaries and freshwater streams. There they grow to adulthood, before making the return pilgrimage to the Sargasso.
Salmon in the North Pacific have their own wrinkle on this pattern. Stream-spawned young ride massive current-driven gyres, around and around, until they are old enough to return to spawn and die in the streams where they began life.
Myriad other species have adapted their life histories to this fundamental ocean architecture. Many reef fishes and invertebrates like lobsters spawn at certain places and certain times, producing buoyant larvae that float down-current for weeks or months (sometimes more than a year for spiny lobsters), until they find the right conditions or habitats for them to settle on the bottom and live their lives.
Sometimes larvae circle in place, in localized gyres, repopulating the habitats where they were spawned; sometimes they are delivered far down-stream to habitats hundreds of miles away. In that way, a grouper or snapper spawned in Banco Chinchoro, off Mexico’s Yucatan Peninsula, may populate coral reefs off Florida or rocky reefs off North Carolina. Adult groupers often move back up current to spawn, giving their offspring a fair chance at the same journey to good quality habitats they made. Even offshore fishes are adapted to this pattern.
The best remaining spawning ground for North Atlantic Bluefin tuna is actually in the northern Gulf of Mexico. Adults swim against the currents south to Florida, and then through the Straits of Florida and into the Gulf, where they spawn, allowing their young to drift back with the currents, feeding all the while, to the cold waters of the North Atlantic. Many sharks do approximately the same thing, timing their migrations and reproduction to assure that their babies will find the best nurseries.
Again and again, this pattern of life in the ocean repeats.
Protecting all of these creatures requires that the nations whose waters hold these long distance swimmers work together to protect spawning grounds, nursery grounds, shared adult populations, and – perhaps most importantly – the integrity of the THC itself.
Disturbing recent science suggests that global warming could, at some point, alter this great circulating system. There are many factors involved, and the computer models are ambiguous, but the possibility is very real.
This threat to the great THC is one more good reason for the world to get serious about lowering global warming emissions, while we do everything we can – and there is a lot we can do – to protect the ocean’s reefs, spawning grounds and nurseries, and nourish abundant fish populations.