August 26, 2011 – Fisheries biologists with the Louisiana Department of Wildlife and Fisheries (LDWF) are investigating reports of crab mortalities by fishermen in Lake Borgne. The reports, which began on Sunday, August 21, were made by commercial fishermen pulling dead fish and crabs from traps through much of the lake. Initial reports indicate that areas of hypoxia, as a result of limited exchange of oxygen from the surface to the bottom layers of the water column, are the cause of these mortalities.
Hypoxic areas in the Gulf of Mexico, sometimes called “dead zones” can cause mortalities for those species unable to quickly relocate to areas where there is oxygen in the water for them to “breathe.” Hypoxic zones can be common along Louisiana’s coast in the summer months. Current hypoxic zones have been occurring in the northern Lake Borgne, the Mississippi Sound, Chandeleur Sound, Breton Sound, Black Bay and the Mississippi River Gulf Outlet (MRGO) for approximately the last month, as well as the more well-known areas offshore west of the Mississippi River. Some of these areas have been experiencing some levels of hypoxia for the last two to three months.
While the current series of crab mortalities from hypoxia are in the general region at the mouth of the Pearl River where a dramatic fish kill was reported on August 13, fisheries experts do not believe the two events to be linked. However, out of an abundance of caution, LDWF fisheries biologists collected crab samples and sent them to the independent laboratory, Eurofins Central Analytical Laboratory, contracted by the Louisiana Department of Health and Hospitals for both volatile and semi-volatile testing.
LDWF fisheries biologists report that high levels of freshwater that have inundated Lake Borgne and other waterbodies near the mouth of the Mississippi River likely created hypoxic areas that crab fishermen have encountered.
Unusually large amounts of freshwater has reduced the ability of the water to mix from top to bottom in many areas, affecting dissolved oxygen levels in the lower part of the water column. In addition,high water temperatures further reduce the amount of oxygen the water can hold. Also, the large input of nutrients has led to increased organic production whose decomposition in the bottom layers further reduces oxygen concentrations.
Fisheries biologists with the state will continue to investigate crab and fish mortalities as they are reported. Once results of the tests are made available with DHH, they will be made available for the public. Fish kills are common occurrences in coastal Louisiana during the summer months. LDWF cooperates with other state agencies in monitoring these occurrences on an ongoing basis.
The Louisiana Department of Wildlife and Fisheries is charged with managing and protecting Louisiana's abundant natural resources. For more information, visit us at www.wlf.louisiana.gov on Facebook at www.facebook.com/ldwffb or follow us on Twitter @LDWF.
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