Open Marsh Water Management

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Open marsh water management

Open Marsh Water Management (OMWM) refers to the practice of controlling the mosquito population in salt marshes by creating an appropriate habitat for the natural enemies of the mosquitoes; and by reducing flooding in areas that are not wet on an ordinary basisthus reducing an environment that would support mosquitoes but not their predators. Without use of chemicals that might be harmful to the natural resources surrounding the water body, as well as harmful to wildlife and humans, OMWM calls on nature's own ecological balance in order to successfully alter the pestmosquito. The techniques were eventually employed by individuals and municipalities in order to control mosquitoes even in their own backyards, no matter how far that might be from tidal wetlands .

Before this plan was developed, and first used in New Jersey to control the mosquito problem in their tidal wetlands and marshes, the normal practice utilized a network of ditches. Under this method, which started around the time of the Civil War, the shallow ponds and water pools, the natural habitats of the birds and fish that fed on mosquitoes were changed or destroyed through ditch digging to drain the marshes and remove standing water. According to the Conservationist in an article published in June 1997, the digging also created a problem because, "Spoil generated by the ditch digging was often disposed of in marsh areas, creating high areas. The high areas were soon invaded by reeds and shrubsvegetation of lesser value for marsh wildlife. In addition spoil mounds restricted the ebb and flow of tides, reducing flushing and creating habitat that actually favored mosquitoes. Many marshes became cut off from tidal flow, causing them to gradually become less saline [salty]. This resulted in changes in the composition of flora and fauna , enabling such non-native plants as the common reed (Phragmites) to invade the marsh. While somewhat attractive and often collected for dry plant arrangments, this plant has little wildlife value and displaces other more valuable species."

The key to OMWM is creating a habitat for one particular mosquito predator, the mummichogs, or marsh killfish. By restoring the body of water to its natural state, marsh killfish are provided with a habitat for survival, as are other fish and wildlife. The killfish are small but have appetites for mosquito larvae that never subside. Natural habitats are created by the tidal flow that go between being pools of deep water and shallow ponds. Killfish thrive under these conditionslaying their eggs when it is dry, and hatching them when it floods. The mosquito produces a vast number of larvae (all waiting to emerge as mosquitoes) as soon as the marshes are once again full of water. Yet simply producing more killfish does not ensure that they will reach the mosquitoes, and thus control the population. Again, the Conservationist has pointed out that, "For a project to be effective over a wide tidal range, the design of the tidal channels must be carefully tailored to the specific conditions of the marsh. Shallow access channels are constructed to allow the fish access to all parts of the marsh surface. However, pool and ditch sections must provide enough water and adequate depth to prevent excessive predation of the fish by wading birds."

Mosquito control in history

The state of New Jersey was the first place where OMWM was usedand the reasons for that go as far back as the early settlers. According to the New Jersey Mosquito Homepage (NJMH) Yellow Fever, contracted from mosquitoes, was the known as the "American plague," infecting the Massachusetts colony as early as 1647. In 1793, Philadelphia was hit and the city's population was devastated. When an army surgeon named Walter Reed finally traced the virus to the mosquito, Aedes aegypti, in 1900, the disease was dealt a death blow like the one it once had wagered on its victims. But the mosquito remained more than a simple annoyance that even the window screen introduced in the 1880s could not totally erase. Humans could contract Eastern Equine Encephalitis through mosquitoes, a disease of epidemic proportions even into the middle half of the twentieth century. Horses continue to be infected if they are not inoculated against the disease. Mosquitoes can also transmit heartworm disease to dogs. Even in modern-day America, a variety of the species has been found to infect humans with the deadly West Nile Virus causing illness and death to wildlife and people from New York to Ohio, and down the east coast, by 2002.

The NJMH elaborates further on the history of mosquito control as an early priority in that state, and others. "Considerable public debate was given to the question whether mosquitoes could ever be controlled. Mosquito control operations grew in some towns but not in all towns. Newspaper battles raged when it was painfully noted that mosquitoes ignored municipal and even state borders. Local boards of health funded most of the extermination work. Laws in 1906 required support for local efforts from the state experiment station. Another law in 1912 directed the creation of county mosquito extermination commissions to assure full-time mosquito work With an increase in mosquito control workers and their rapid progress, it became clear than an organization was needed within which these workers could discuss their problems and share their experiences." Following the first convention of county commission in February 1914 in Atlantic City, the permanent organization known as the New Jersey Mosquito Extermination Association was formed. In 1935, a regional organization was formed with ten other states, the Eastern Association of Mosquito Control Workers , from a meeting at Trenton; it was re-named in 1944. The American Mosquito Control Association remains the premier organization concerned with mosquito control.

The common salt marsh mosquitoes include those known as Aedes sollicitans ; Aedes contator, ; and, Aedes taeniorhynchus. Adult females deposit their eggs on the marsh surface, which dry for 24 hours. The egg-filled depressions fill with water during the monthly high tides when the marsh is flooded, causing the larvae to hatch quickly. In addition to killfish acting as predators for the larvae, they can be controlled through the OMWM technique of utilizing a series of ditches to connect mosquito breeding depressions to more permanent bodies of water. It eliminates standing water, and also provides for predator fish to reach any mosquito larvae that is still there.

One example of a project to turn around the results of the former ditch digging practice that had been used and proven harmful(see History of to the natural environment is a research project operating in 2000, and supervised by the U.S. Fish and Wildlife Service (USFWS), Region 5, at the Rachel Carson Wildlife Refuge in Maine. The U.S. Department of the Interior , U.S. Geological Survey information system project of the Patuxent Wildlife Research Center in Laurel, MD provided a history and description of the project. "By the 1930s, most salt marshes throughout the northeastern United States had been ditched for mosquito control purposes. Documented impacts of this extensive network of ditching include drainage of marsh pools, lowered water table levels, vegetation changes, an associated trophic [feeding ] responses. To restore the ecological functions of ditched salt marshes, while maintaining effective mosquito control, the USFWS (Region 5) is in the process of plugging the salt marsh ditches and establishing marsh pools. Ditch plugging is an adaptation of the mosquito control practice known as Open Marsh Water Management. The purpose of this proposed study is to compare and evaluate marshes that have been ditch plugged with unditched and parallet ditched marshes. Physical (tidal hydrology , water table level), chemical (soil salinity ), and ecological (vegetation, nekton , marsh invertebrates, waterbirds, mosquito production) factors will be evaluated." Before data could be collected, whatever the results, this study was expected to create an established pattern for long-term salt marsh monitoring.

East coast states, as well as areas around the country, have engaged in OMWM not merely for simple mosquito control; but as a part of the restoration of estuaries and other wildlife management and restoration projects. OMWM techniques were still being refined as of 2002 as part of a return to an ecologically-balanced system that provides for the survival of natural resources, and for the survival of humans and wildlife.

[Jane E. Spear ]



"Tidal Wetlands in New York State Diversity." Conservationist 51, no. 6 (June 1997): 4.


"Cape May County Mosquito Extermination Commission." Cape May County Web Page. 2001 [June 2002]. <>. "Coastal Mosquito Control in Your Neighborhood." Norfolk County Mosquito Control Project. May 3, 2001 [June 2002]. <>.

"Connecticut River Estuary and Tidal River Wetlands Complex." State of Connecticut Department of Environmental Protection. 2002 [July 2002]. <>.

James-Pirri, Mary Jane. "Mosquito Beach Salt Marsh OMWM Restoration." Northeastern Mosquito Control Association. 2002 [July 2002]. <>.

Johnson, David. "Africana." Patuxent Wildlife Research Center. July 25, 2000 [cited June 2002]. <>.

"Mosquito Breeding Habitats." Monmouth County, NJ Web Page. 1999 [June 2002]. <>.

Patuxent Wildlife Research Center. Research Project; Ecosystem response of salt marshes to Open Marsh Water management (OMWM): Rachel Carson National Wildlife Refuge (Maine). "Angolan deepwater fields emerge asworld's most exciting oil frontier." 2001. (June 2002). <>

Purdue University/Forestry and Natural Resources. Did You Know? Healthy Wetlands Devour Mosquitoes? [cited June 2002]. <>.

Rutgers University/Entomology. "History of Mosquito Control in New Jersey (Where it all Began)." New Jersey Mosquito Control Homepage. [cited June 2002]. <>.


U.S. Environmental Protection Agency, 1200 Pennsylvania Avenue, N.W., Washington, DC USA 20460 202-260-2090, <>

U.S. Fish and Wildlife Service, Washington, D.C. USA , <>