Q: I know that Pacific sailfish and Atlantic sailfish are different species, and Pacific sails average a good bit larger than Atlantic sails. What's the reason for the size difference? Other than the fact that the Pacific is cooler than the Atlantic, I can't really think of another reason.

Jamie Bartono
Lexington, South Carolina

A: In fact, sailfish in the Pacific and Atlantic oceans are not considered to be different species. The answer to your question can be found in a paper I co-authored with C. Phillip Goodyear and published in Fisheries Oceanography a couple of years ago: "hypoxia-based habitat compression of tropical pelagic fishes." As you say, sailfish in the western north Atlantic average a considerably smaller size than those in the eastern tropical Pacific or the eastern tropical Atlantic. This is due to the development of hypoxic plumes (also called oxygen-minimum zones) that develop off the Pacific side of Central America and west Africa that compress the available habitat for this species into a very narrow layer extending down often only 80 feet or so from the surface of the ocean. The low oxygen levels below this shallow thermocline create a lower habitat boundary (a compressed area beneath the surface) for tropical pelagic fishes such as  marlin, sailfish and yellowfin tuna. All are ram ventilators (relying on constant high-speed swimming to force large amounts of water through their gills) and thus are high-oxygen-demand fishes. In the plume areas described in our paper, prey (herring, small jacks and small tunas, all of which are also ram ventilators) and predators are all concentrated in the same narrow surface layer, thus enhancing the foraging opportunities for these predators. — Eric Prince