Conserving oyster reef habitat by switching from dredging and tonging to diver-harvesting
Abstract–A major cause of the steep declines of American oyster (Crassostrea virginica) fisheries is the loss of oyster habitat through the use of dredges that have mined the reef substrata during a century of intense harvest. Experiments comparing the efficiency and habitat impacts of three alternative gears for harvesting oysters revealed differences among gear types that might be used to help improve the sustainability of commercial oyster fisheries. Hand harvesting by divers produced 25-32% more oysters per unit of time of fishing than traditional dredging and tonging, although the dive operation required two fishermen, rather than one. Per capita returns for dive operations may nonetheless be competitive with returns for other gears even in the short term if one person culling on deck can serve two or three divers. Dredging reduced the height of reef habitat by 34%, significantly more than the 23% reduction caused by tonging, both of which were greater than the 6% reduction induced by diver hand-harvesting. Thus, conservation of the essential habitat and sustainability of the subtidal oyster fishery can be enhanced by switching to diver hand-harvesting. Management schemes must intervene to drive the change in harvest methods because fishermen will face relatively high costs in making the switch and will not necessarily realize the long-term ecological benefits.
Commercial fishing for demersal fishes and benthic invertebrates, such as mollusks and crabs, is commonly undertaken with bottom-disturbing gear that can inflict damage to seafloor habitats (Dayton et al., 1995; Engel and Kvitek, 1995; Jennings and Kaiser, 1998; Watling and Norse, 1998). Habitat damage from dredges and analogous gear, designed to excavate invertebrates that are partially or completely buried beneath the surface of the seafloor, is generally much more severe than the damage caused by bottom trawls (Collie et al., 2000). Furthermore, impacts on and recovery from bottom-disturbing fishing gear vary with habitat type; generally smaller effects and more rapid rates of recovery are found for infauna in sedimentary habitats and the most severe and long-lasting damage in biogenic habitats that emerge from the seafloor (Peterson et al., 1987; Collie et al., 2000). Such biogenic habitats include seagrass beds, fields of sponges and bryozoans, and invertebrate reefs. Biogenic reefs that provide important ecosystem services such as habitat for other organisms include not only tropical coral reefs but also temperate reefs constructed by oysters (Bahr and Lanier, 1981; Lenihan et al., 2001), polychaetes like Petaloproctus (Wilson, 1979; Reise, 1982), and vermetid gastropods (Safriel, 1975). The recovery of such emergent invertebrate reefs is a slow process because of the relative longevity of the organisms that provide structure for the reef after they die and because of the nature of reefs as accumulations of multiple generations of reef builders.
One widespread temperate reef builder, the American oyster (Crassostrea virginica, also known as the “eastern oyster,” Am. Fish. Soc.), has been especially affected by bottom-disturbing fishing gear as the target of fisheries. More than one hundred years of dredging and tonging oysters in the Chesapeake Bay and Pamlico Sound have caused severe degradation of the oyster reef matrix (deAlteris, 1988; Hargis and Haven, 1988), such that reef area and elevation have been dramatically reduced (Rothschild et al., 1994; Lenihan and Peterson, 1998). Reduction in reef height has a serious consequence for the oyster population because one function of naturally tall subtidal oyster reefs is to elevate the oysters up into the mixed surface layer of the estuary; this layer of mixed surface water allows them to avoid mass mortality from persistent exposure to seasonally anoxic and hypoxic bottom water (Lenihan and Peterson, 1998). Reef height and structure also control reef hydrodynamics (e.g., flow speed, turbulent mixing, and particle delivery and deposition), which influence oyster population dynamics and production (Lenihan, 1999). Consequently, harvest-related reef destruction and degradation are considered major factors that have led to declines of American oysters in many estuaries located along the coasts of the Atlantic Ocean and Gulf of Mexico (Lukenbach et al., 1999).
Loss of oysters and the biogenic habitat that they provide appears from archaeological and paleontological evidence to be a worldwide phenomenon in temperate estuaries (Jackson et al., 2001). Oyster loss hurts not only the oyster fishery but, more importantly, imperils the ecosystem services provided by the oysters. These include, especially, the provision of emergent habitat and reef-dependent prey resources for many fish and crustacean populations of commercial and recreational importance (Peterson et al., 2000; Lenihan et al., 2001; Peterson et al., 2003), the filtration of estuarine waters (Newell, 1988), and the promotion of estuarine biodiversity by provision of hard-bottom habitat in fields of mobile sediments (Wells, 1961).