Ocean dead zones called growing threat to game fish

A new study says that fish vital to the sports fishing industry are becoming more vulnerable because they are being concentrated as large patches of the sea lack sufficient oxygen.

Billfish and other marine species become more susceptible to overfishing because they are compressed into oxygen rich waters at the ocean’s surface where they are easier to catch, while avoiding deeper waters low in oxygen, said The Billfish Foundation, which participated in the study.

The areas of depleted oxygen are called hypoxic zones, but more recreational anglers refer to them as dead zones, the foundation said.

The study involved scientists from the U.S. National Oceanic and Atmospheric Administration, the University of Miami Rosenstiel School of Marine and Atmospheric Science, and the foundation. The findings were published in the Journal of Fisheries Oceanography, the foundation noted.

Ellen Peel, president of The Billfish Foundation, said in a release that scientists outfitted 79 sailfish and blue marlin in two strategic areas of the Atlantic with pop-off archival satellite tags which monitored their horizontal and vertical movement patterns.

“Billfish favor abundant habitats of oxygen rich waters closer to the surface while avoiding waters low in oxygen,” Ms. Peel was quoted as saying.

The foundation said that these hypoxic zones occur naturally in areas of the world’s tropical and equatorial seas because of weather patterns and oceanographic and biological processes. In the current cycle of climate change and accelerated global warming, hypoxic areas are expanding and may continue to expand as sea temperatures rise, it added.

The research waters included areas off south Florida and the Caribbean and off the coast of West Africa, said the foundation.

A companion graphic from the

Colors show dissolved oxygen levels at 100 meters. Red and black show depleted or so-called dead zones. Journal of Fisheries Oceanography graphic

al of Fisheries Oceanography used data from the World

Ocean Atlas to show depleted levels of dissolved oxygen 100 meters deep off Africa and in a broad swath in the Pacific, said the foundation.

Because the fish congregate nearer the surface, a census of species and their numbers may give a false result and cause gross overestimations, noted the study. The foundation provided these assessments:

“The zone off West Africa encompasses virtually all the equatorial waters in the Atlantic Ocean, is roughly the size of the continental United States and is growing,” said Eric D. Prince, a fisheries research biologist. “With the current cycle of climate change and accelerated global warming we expect the size of this zone to increase, further reducing the available habitat for these fishes.”

He is with the U.S. National Oceanic and Atmospheric Administration.

“As water temperatures increase, the amount of oxygen dissolved in water decreases, squeezing billfish into less available habitat and exposing them to even higher levels of overfishing,” said the foundation’s Phillip Goodyear.

“While most recreational anglers are practicing catch and release, sailfish and marlin will become more vulnerable to commercial netters, purse seiners, and longliners that fish the oxygen-rich zones,” said Ms. Peel. Reduced habitats can lead to higher catch rates of fish not because there are more fish in an area, which is the usual indication, but because the billfish are more densely concentrated near the surface where fishing gear is more likely to catch them, she added.

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