Oceanic  Dead Zones  Exposing Tuna And Billfish To More Overfishing With the New Year comes new challenges to fish in our world s oceans and one of the major concerns is the expansion of hypoxic zones. That s the scientific name but more recreational anglers are becoming aware of them as  dead zones.

 They are areas in the oceans with low or non-existent oxygen levels which, according to a recently released research study by scientists and fish management experts, are increasing in size while decreasing the habitats of billfish and tuna.  In scientific circles this phenomena is called “habitat compression.”
Ellen Peel, President of The Billfish Foundation (TBF) said 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 favour abundant habitats of oxygen rich waters closer to the surface while avoiding waters low in oxygen,  Peel said.

The study, composed of scientists from the National Oceanic and Atmospheric Administration (NOAA), the University of Miami Rosenstiel School of Marine and Atmospheric Science, and TBF, found a massive expanding low oxygen zone in the Atlantic Ocean is encroaching upon the fish forcing them into shallower waters where they are more likely to be caught.

The research waters included areas off south Florida and the Caribbean (western North Atlantic); and off the coast of West Africa (the eastern tropical Atlantic).
Hypoxic zones occur naturally in areas of the world s tropical and equatorial seas because of ongoing weather patterns, oceanographic and biological processes. In the current cycle of climate change and accelerated global warming, hypoxic areas are expanding and shoaling closer to the sea surface, and may continue to expand as sea temperatures rise.
 The zone off West Africa,  said Dr Eric D. Prince, NOAA Fisheries Service research biologist,  encompasses virtually all the equatorial waters in the Atlantic Ocean, is roughly the size of the continental United States and is growing. 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.  Dr Phillip Goodyear of TBF explained that fishery managers should start incorporating oxygen depleted zones into assessing population abundance and making management decisions.

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.  Peel added,  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. 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.
These higher catch rates from compacted habitat can skew estimates of population abundance, producing a false signal of stock size. This important issue is whether the change in habitat will cause a change in CPUE (catch per unit of effort) with no corresponding change in species abundance. This issue will be important for future stock assessments.  The findings were published in the November edition of Fisheries Oceanography, where a full discussion of this challenging phenomenon in both the Pacific and Atlantic Oceans is reviewed.

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