A dead zone off the coast of West Africa is reducing the amount of available habitat for Atlantic tuna and billfish species, reports the National Oceanic and Atmospheric Administration in a study published in Fisheries Oceanography. The zone is growing due to rising water temperatures and is expected to cause over-harvest of tuna and billfish as the fish seek higher levels of oxygen in areas with greater fisheries activity.
Dead zones are areas of the ocean which are too low in oxygen to support many marine species. There are about 400 of these “hypoxic” regions throughout the world, many caused by human activities. Perhaps the most notorious, the New Jersey-size dead zone in the Gulf of Mexico is caused by fertilizer runoff released by the Mississippi which encourages oxygen-depleting algae to proliferate out of control. Another dead zone was discovered in 2007. It occurs off the coast of Texas, where the Brazos river empties into the Gulf.
Dead zones can also be caused by climate change. Increases in ocean temperature can change the course of currents, isolating certain areas from influxes of deeper, colder water. As the water in the area sits, it warms and releases its oxygen, making it inhospitable to many aquatic species. Three major dead zones are known to have been caused by climate change: one off the coast of Chile and Peru, one off the east coast of Africa, and another off of Africa’s west coast. A new dead zone was reported off the US west coast in 2002. It occurs seasonally and is believed to be part of a continuum of South America’s dead zone.
Freshly-landed striped marlin on a Pacific longline fishing vessel. Photo by Mongabay.com
NOAA scientists teamed up with researchers from the University of Miami and The Billfish Foundation to study the West African dead zone and its effect on fish species.
“The hypoxic zone off West Africa, which covers virtually all the equatorial waters in the Atlantic Ocean, is roughly the size of the continental United States, and it’s growing,” said Dr. Eric D. Prince, a NOAA Fisheries Service research fishery biologist who led the study. “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 fish.”
The team of scientists tagged sailfish and blue marlin with satellite tracking devices which monitored horizontal and vertical movement patterns. The tags confirmed that billfish prefer oxygen rich waters closer to the surface and will actively avoid waters low in oxygen.
Tuna and billfish – a catch-all category which includes marlin, swordfish, and sailfish species – are already experiencing global declines due to pressure from fisheries. The habitat loss caused by the dead zone is expected to push Atlantic stock down even further as fish escape to near-shore areas with more fishing activity. The increase in catch rates will make it appear that stocks are up when really they’re just concentrated in a smaller area. This may lead to over-harvest.
Scientists are calling for stringent monitoring and revamped population assessments which will take into consideration the fishes’ shrinking habitat.