Tuesday, May 4, 2010

Hot Tuna and Oil: Bluefin in the Gulf off Mexico

This species, Thunnus thynnus, never gets a break. Imagine a fish with a reproductive strategy where it has to grow from a tiny egg to a 225 lb fish, and than once mature, it goes to spawn in the Gulf of Mexico for 4-6 weeks. If that’s your evolutionary strategy for reproduction, you’d better hope that your spawning ground is just like you left it the year before- warm and clean. The rest of the year the giant bluefin swims the entire North Atlantic ranging from frigid seas off the Maritimes of Canada to Iceland, from the Azores to the shores of North Carolina. Once a year this remarkable fish - ranging in size as a breeder from 200-1400 lbs - comes to the warm waters of the Gulf of Mexico and spawns. I’ve only recently seen bluefin spawn in captivity and they have a remarkable courtship- they swim in a tight school and twirl up a tornado- female just ahead of male, as they move at excessive speeds and then broadcast their gametes into the water.

At a time when our Gulf of Mexico bluefin stock is vitally important to protect, where every last fish that makes it out of the Gulf is vital to the future of this fishery, when it is imperative they spawn without disturbance, an oil spill happens.

Atlantic bluefin tuna are comprised of several populations and the bluefin that gets the largest are from the Gulf of Mexico spawning stock. Over the past three years we’ve been working in Canada to tag the largest fish- to see where the bluefin spawn. From analyzing fish carrying electronic tags we have learned these “Canadian” bluefin go often to the region just north of the Loop current right where the shelf break occurs (see track below). Here on the frontal edge of the Loop Current, the bluefin spawn- leaving their eggs to hatch into larvae. They require warm temperatures and spectacularly clean water. Unfortunately where the oil spill is occurring is essential bluefin spawning habitat. These are places that are the most important in our waters to protect.

Map shows the track from an 8-1/2 foot long Atlantic bluefin tuna tagged off Nova Scotia that swam into the Gulf of Mexico, spent several weeks in the spawning area, then departed (yellow track). The dark blob off Louisiana is the estimated coverage area of the oil spill as of May 3.

Our TAG team spent five years working in the Gulf of Mexico, trying to establish the critical spawning habitat for this species - one of the largest fish in the sea. We saw the seismic boats from several companies out on the shelf break searching with sound for oil. Blasting right where the tuna were spawning. It bothered me back then and I told the head of NOAA about the problem (Dr. Rollie Schmitten). I often wondered if they (the oil companies) had an environmental impact statement that included the bluefin tuna. In these waters not only are bluefin spawning, but many other large pelagics as well- blue marlin, swordfish, and later in the spring, yellowfin tuna. Springtime is a time of renewal in the Gulf of Mexico. This place is the fertile nursery grounds to America’s North American fisheries- so the pain may reach far beyond the shores of Louisiana. For the future of the Atlantic bluefin in North American waters, let’s hope that BP and the Federal government act quickly to shut down the leaks.

-Dr. Barbara Block

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