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October 25, 2001

Warming Up

Why do swordfish sun themselves on the surface more in New England than in other areas?

Q: Why do swordfish sun themselves on the surface more in New England than in other areas? And, if they are tailing on top of the water for sun, why do I see them at the surface more often on hazy or foggy days?

- Marty Weiss, Lawrence, New York
A: Billfish tail just about as much in California as they do in New England, or anywhere else that the ocean's surface temperatures are chilly. When fish dive into deep waters to feed, their body temperature drops, slowing them down. Many large predators, including tunas and large sharks, conserve heat to maintain their bodies a few degrees warmer than the surrounding water. Billfishes, in addition, have a specialized organ which actually produces heat to warm the essential eye and brain functions, enabling billfish to dive deeper and longer than tunas.
When swordfish dive into cold water, the blood flowing across their gills is quickly chilled. To prevent rapid heat loss, they close a main artery, shunting their blood through a countercurrent heat exchanger and slowing the loss of body heat through the gills. Swordfish can swim in cold water for eight hours or more using their brain/eye warmer and heat-exchange system.
After a long, body-chilling dive into cold water, however, swordfish must tail on the surface to warm themselves - but that doesn't mean they are sunning themselves like a lizard on a rock does. In fact, research shows that swordfish returning to the surface warm themselves 10 times faster than they cooled down. To do this, swordfish open the artery that was closed while diving and shunt blood around the heat exchanger to flow directly across the gills, quickly warming the blood to the temperature of the surface water. Technically, tailing swordfish are warmed by the waters that are warmed by the sun more than by the direct rays of the sun.
When the ocean is calm and the sun is weak, as on hazy, foggy days, less mixing of surface waters and a thinner, sun-heated top layer bring fish closer to the surface to warm themselves. At such times, they become visible to anglers.