American oyster, continued 



Sellers 1986). Stone crabs are also major oyster 

 predators in the Gulf of Mexico (Menzel and Hopkins 

 1956, Berrigan et al. 1991). In Louisiana, it was 

 estimated that one stone crab could kill up to 219 

 oysters per year. In addition, the blue crab and smaller 

 mud crabs (Xanthidae), prey on oyster spat and young 

 thin-shelled oysters. The black drum is an important 

 predator of oysters as well (Pearson 1 929, Cave 1 978, 

 Cave and Cake 1980, Berrigan et al. 1991). Black 

 drum will attempt to crush and consume any oyster that 

 will fit in their pharyngeal apparatus. Large black drum 

 (>900 mm TL) can consume oysters up to 1 12 mm in 

 length, while smaller drum (<900 mm TL) consume 

 oysters less than 75 mm. It has been estimated that 

 black drum consume up to two oysters per day for 

 every kilogram of body weight, and a single large drum 

 can consume an average of up to 48 oysters per day. 

 Other predators include the oyster leech (Stylochus 

 frontalis), the lightning whelk (Busycon contrarium), 

 the crown conch (Melongena corona), echinoderms, 

 flat worms, cownose ray (Rhinoptera bonasus), south- 

 ern eagle ray (Mylibatisgoodei), Atlantic croaker, spot, 

 toad fish (Opsanus sp.), sheepshead, pinfish, and 

 striped burrfish (Chilomycterus schoepfl) (Hopkins 

 1 955, Menzel et al. 1 966, Hofstetter 1 977, Cake 1 983, 

 Stanley and Sellers 1986, Berrigan et al. 1991). 



Factors Influencing Populations : Salinity is probably 

 the single most important factor that influences the 

 distribution and abundance of estuarine organisms 

 (Copeland and Hoese 1 966, Berrigan et al. 1 991 ), and 

 this is particularly important with respect to oysters. 

 Droughts can increase salinities over oyster reefs and 

 contribute to higher mortality due to increased num- 

 bers of high salinity, stenohaline oyster predators 

 '(Gunter 1 955, Cake 1 983, Lowery 1 992). High mortal- 

 ity due to prolonged exposure to lowered salinities can 

 occur during episodes of heavy flooding from storm 

 events (Gunter 1 953, May 1 972, Burrell 1 977, Hofstetter 

 1 977, Soniat et al. 1 989, Berrigan et al. 1 991 ). Some 

 flooding is beneficial because it maintains low levels of 

 Perkinsus marinus infection (Soniat et al. 1989), and 

 excludes marine predators and parasites (Hofstetter 

 1 977) by keeping salinities low. Increased salinities in 

 estuaries due to a reduction of freshwater inflow have 

 caused oysters beds to relocate toward the headwa- 

 ters of estuarine basins to more favorable salinities 

 (Berrigan et al. 1991). Since this shift in location has 

 occurred over a relatively short period of time, these 

 areas lack extensive reefs for larval settlement. Oys- 

 ters are also more prone to mortalities from freshwater 

 flooding events in these areas. Another problem is that 

 these locations are closer to areas of human habitation 

 where sanitary conditions can become compromised, 

 and other pollutant-related diseases and mortality will 

 occur. 



Hurricanes, tropical storms, and flooding can have 

 both positive and negative effects on oyster popula- 

 tions in Gulf of Mexico estuaries (Berrigan et al. 1991, 

 Lowery 1992). Hurricanes impact oyster production 

 through several mechanisms. They can destroy reef 

 integrity, remove live oysters and shell cultch, cause 

 sedimentation that buries reefs, increase current ve- 

 locity causing scouring and abrasion, and bring fresh- 

 ets into the estuary that drop salinities to lethal levels. 

 The severity of the damage may be affected by local 

 tidal conditions, proximity to the storm, wave surge, 

 rainfall and other climatological factors. Runoff from 

 storm events, along with dredge and fill activities and 

 effluent discharges, can also increase turbidity and 

 sedimentation in the aquatic environment (Killam et al. 

 1 992). This can lead to silt settling out over oyster spat 

 and inhibiting normal growth. This sedimentation also 

 results in a soft muddy habitat that is undesirable for 

 spat settlement. Currents are necessary for removal of 

 feces and pseudofeces to prevent burial of the oyster 

 reef. However, turbulent currents that carry sand or 

 pebbles can damage oysters by eroding shell sur- 

 faces. Suspended solids may clog gills and interfere 

 with filter feeding and respiration. If covered with 

 sediment, oysters can die within a week (Stanley and 

 Sellers 1986). Despite initial mortality resulting from 

 hurricanes, long-term oyster production may be en- 

 hanced by the subsequent destruction of high-salinity 

 predators and diseases, and the scouring of extant 

 reefs making more clean shell available for spat settle- 

 ment. 



The loss of suitable habitat is probably the most impor- 

 tant factor in the decline of oyster populations in the 

 Gulf of Mexico (Berrigan et al. 1991). Reef substrate 

 which is necessary for spat settlement is removed 

 during harvest, and fossil reefs are mined for shell 

 material. The continuing development of Gulf coastal 

 areas is resulting in habitat areas being filled or dredged 

 to accommodate human needs. Spoil banks from 

 dredging projects modify the bottom morphology of 

 bay bottoms and alter current patterns causing condi- 

 tions that can result in mortality (Hoese and Ancelet 

 1987). Freshwater inflow into estuaries has been 

 reduced due to the damming of rivers, leveeing of 

 rivers preventing overflow into surrounding marshes, 

 channelization, pumping for redistribution, and other 

 construction projects that alter salinity regimes, reduce 

 available nutrients, and allow the influx of predators. 

 Development of coastal areas has also led to in- 

 creased pollution and pollution-related mortality (Menzel 

 et al. 1 966, Berrigan et al. 1 991 ). The development of 

 power equipment for commercial oyster harvest has 

 increased the potential for depleting and damaging 

 oyster beds (Stanley and Sellers 1986). 



26 



