Oxygen Levels Affect Supply
By Susan Cocking, MiamiHerald.com

Avid billfish anglers become starry-eyed and poetic when they talk about the abundance and size of sailfish and marlin they catch off Central America's Pacific coast.

When asked to explain why the fishing off Guatemala and Costa Rica is superior to South Florida, they often assume the pose of sage billfish scientists and reply with something such as, ``Cold-water upwellings in the Pacific concentrate more bait in those areas than in the Atlantic.''

However, that's not the real reason for the Pacific's more-and-better billfishing, according to new research by scientists with the Billfish Foundation and NOAA Fisheries Service.

In a four-year study, Phil Goodyear and Eric Prince found that lower oxygen levels in waters as shallow as 85 feet in the eastern Pacific force sailfish and marlin to stick closer to the surface where they can be easily picked off by fishermen trolling bally hoo -- or deploying purse seines or longlines.

In the western North Atlantic -- including South Florida, the Virgin Islands and the Bahamas -- the waters contain enough oxygen at greater depths to enable billfish to go pretty much wherever they want.

''It's akin to filling your bathtub with water and putting in a half-dozen big carp and trying to catch them with your hands,'' Prince said. ``Then you pull the plug, and they're a lot easier to catch.''

This ''habitat compression,'' as Prince calls it, also explains why Pacific sailfish, on average, seem to be half-again as large as their skinny Atlantic counterparts. Predators (billfish) and prey (goggle eyes) are crowded into the same small area, enabling the larger fish to chow down to their hearts' content and grow ever larger.

Goodyear and Prince base their findings on data from pop-up satellite tags placed in a total of 32 sailfish, blue marlin and black marlin in the Atlantic and Pacific. The tags record depth, water temperature, light conditions, and other data before popping to the surface at a pre-programmed interval. The data is beamed to a satellite, which transfers it to the scientists' computers. The billfish in this study were monitored for between 400 and 800 days -- plenty of time to draw solid conclusions.

When the researchers started examining the data, they noticed something odd -- sailfish would never go below 330 feet in the eastern Pacific, yet they dived more than 1,100 feet deep off Miami.

''There's something going on here. What the heck is it?'' Prince wondered at the time.

They started looking at marlin and found the same phenomenon: fish in the Pacific spent far more time at shallower depths than their Atlantic counterparts.

The scientists ruled out temperature because the waters favored by Atlantic billfish were colder than those frequented by the Pacific fish.

Earlier research studies had shown that sailfish and marlin require plenty of oxygen to sustain life. Below dissolved oxygen concentrations of 3.5 milliliters per liter, they weaken and die. That's because they are ''ram ventilators'' -- constantly moving forward to force water across their gills.

When Prince and Goodyear compared dissolved oxygen levels at various depths in the Atlantic and Pacific, they found the ''hypoxic plume'' of water low in oxygen was a lot closer to the surface in the Pacific than the Atlantic.

''In the Pacific, they're squeezed into a narrow layer of habitat at the surface of the ocean,'' Prince said.

These findings are important for two reasons: first, billfish in the eastern Pacific are much more vulnerable to surface fishing gear, such as purse seines and longlines, than their Atlantic counterparts. And billfish released by recreational anglers in that region must be resuscitated by moving them slowly through the water so that they don't sink down into waters with insufficient oxygen.