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Giant salvinia (Salvinia molesta) is among the worst invasive aquatic weeds in the world (Holm et al. 1977) and has invaded aquatic habitats in numerous countries (Julien et al. 2002). Under ideal conditions giant salvinia can double its biomass within 2 (Cary and Weerts 1983) to 4 d (Gaudet 1973, Mitchell and Tur 1975, Sale et al. 1985), clogging waterways with dense mats of vegetation. A single plant is capable of starting a population that may cover 103.6 km (40 mi) in 3 mo (Creagh 1991/1992 in Jacono and Pitman 2001). Giant salvinia can out-compete native vegetation and if control measures are not implemented, will completely cover the water surface and form mats up to 1 m thick (Creagh 1991/1992 in Oliver 1993). These mats deplete dissolved oxygen, make waterways uninhabitable by most fish species, impede boat traffic, reduce habitat for waterfowl, limit access for fishing and swimming and can interfere with water use for electrical generation, irrigation and municipal water supplies (Julien et al. 2002). Giant salvinia has been listed as a Federal Noxious Weed since 1984 (McFarland et al. 2004), and as such, it is illegal to import into, or transport this plant within the United States. Giant salvinia also appears on many state noxious weed lists, including Mississippi’s (U.S. Department of Agriculture 2009). However, if giant salvinia is not declared by a state as a noxious weed, it can still be cultivated and sold within that state. Despite regulations, this species continues to be moved through the water garden trade as an ornamental plant for backyard ponds. Escapes or releases from such areas may have resulted in its establishment in numerous southern states such as Florida, Louisiana, and Texas and in major waterways like the Colorado River (U.S. Geological Survey 2009). A population of giant salvinia was discovered in June 2005 in the lower west Pascagoula River distributary and a major tributary, Bluff Creek, in Jackson County, Mississippi. At the first report of infestation, the Mississippi Department of Marine Resources (MDMR) conducted an initial survey of the watershed and determined that giant salvinia was restricted to areas on the western side of the river both north and south of Interstate 10 and was densest in backwater sloughs (Figure 1). Biologists from MDMR also concluded that the infestation had become too extensive (about 789 ha) to eradicate with herbicides or mechanical control methods, so Australian salvinia weevils (Cyrtobagous salviniae) were chosen as a bio-control agent. The weevils were seeded in 3 locations on 18 August 2005 in an effort to establish a population that would ultimately hinder the spread. An outreach campaign, in the form of signs at boat ramps and other high-visibility areas, was also initiated to increase public awareness of giant salvinia and on measures to prevent its spread. Interviews of local residents by MDMR indicate that the source of the infestation was likely a direct release of the plant during a clean-out of a water garden pond. The densest area of infestation was in a canal that drains a residential area just south of I-10. Hurricane Katrina struck the Mississippi coast 29 August 2005 causing a storm surge in the study area ranging from 4.6-5.2 m (FEMA 2005). At the time, there was concern and speculation over the fate of the giant salvinia. Several hypotheses were considered to have merit: 1) The storm surge may have beached much of the plant material on land, rendering the population small enough for control efforts; 2) the salinity of the surge may have killed at least the downstream portion of the infestation; 3) the plant may have been spread to new areas during the flooding; and 4) the growth and spread of remaining giant salvinia may be augmented by increased nutrient levels resulting from the flooding. The objectives of this study were to: 1) survey the lower Pascagoula River Basin and determine the post-storm distribution and abundance of giant salvinia; 2) control any remaining giant salvinia through physical and/or chemical means; 3) determine the fate of the bio-control agents; and 4) determine if re-introduction of salvinia weevils is needed and if so, to decide where best to release them.

Effects of Hurricane Katrina on an Incipient Population of Giant Salvinia Salvinia molesta in the Lower Pascagoula River, Mississippi