Lenihan et al.: Conserving oyster reef habitat 



299 



ters in many estuaries located along the coasts of the At- 

 lantic 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 oys- 

 ter 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 com- 

 mercial 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). 



Because of the importance of restoring and sustaining 

 oysters and their reefs to serve both the oyster fishery and 

 the ecosystem, we designed a field test of the habitat im- 

 pacts of three oyster harvesting methods: dredging, tong- 

 ing, and hand extraction by divers (diver-harvesting). Our 

 study is a gear comparison, in which we assess not only 

 the traditional response variable of quantitative harvest 

 per unit of effort with each gear but also the degree of reef 

 habitat damage induced by the extraction of the oysters 

 (analogous to Peterson et al., 1983). We additionally ex- 

 amine the quality of the oysters harvested as a function 

 of gear type. The results indicate that diver-harvesting is 

 a more environmentally sound way of harvesting oysters 

 than traditional methods with dredges and tongs and may 

 be more compatible with conserving oyster reef habitat. 



Methods 



Study site 



Gear comparisons were conducted on subtidal oyster reefs 

 in the Neuse River estuary. North Carolina (35°00'20"N, 

 76°33'50"W). Environmental conditions of this estuary 

 are well described elsewhere (Paerl et al., 1998; Lenihan, 

 1999). Briefly, the estuary is mesohaline, an optimal 

 habitat for the American oyster, and was once an impor- 

 tant oyster fishery ground (Lenihan and Peterson, 1998). 

 The estuary contains remnants of many large, natural 

 subtidal oyster reefs that have been intensely mined by 

 oyster harvesting gear for over a century. Dredging is the 

 most common fishing practice. Mining of the reef matrix 

 has combined with sediment loading and eutrophication- 

 associated hypoxia (Paerl et al., 1998) to degrade the 

 oyster reef habitats and greatly reduce oyster populations 

 (Lenihan and Peterson, 1998). In harvested areas, reefs 

 that were 2-3 m tall in quantitative surveys in the late 

 1800s (n = 8 reefs) were all <1 m tall in our survey con- 

 ducted in 1994 — a modification of habitat caused entirely 

 by the removal of oysters and shells during harvesting 

 with dredges and tongs (Lenihan and Peterson, 1998). 

 To help maintain oyster harvests, the North Carolina 

 Division of Marine Fisheries (NCDMF) restores oyster 

 reefs throughout many locations in the estuary by creat- 

 ing piles of oyster shell, or marl, on the seafloor. These 



restored oyster reefs are also targeted by oyster fishermen 

 using dredges and, less often, using manual oyster tongs 

 (Marshall 1 ). 



Experimental oyster reefs 



Gear comparisons were conducted in March 1996 on 16 

 subtidal oyster reefs that had previously been created in 

 July 1993 as part of a reef restoration experiment (Leni- 

 han and Peterson, 1998; Lenihan, 1999) in collaboration 

 with NCDMF. The experimentally restored reefs (referred 

 to as "experimental reefs" in this gear-comparison study) 

 were piles of oyster shells 1 m tall, 6-7 m in diameter 

 (28.3-38.5 m 2 in area), and generally hemispherical in 

 shape. Natural subtidal reefs located elsewhere in the 

 estuary are typically larger, rectangular biogenic struc- 

 tures, ranging from 8-13 m wide and 20-30 m long. 

 Experimental reefs were constructed in 3-4 m of water on 

 a firm and sandy bottom, and were separated by at least 

 50 m. From the time of their construction until use in our 

 experiments, the restored oyster reefs remained research 

 sanctuaries, protected from commercial and recreational 

 shellfishing. 



As oysters settle and undergo metamorphosis on the 

 shells of other (live and dead) oysters, to which they are 

 attracted by chemical cues (Tamburri et al., 1992), they 

 help cement together and add to the shell matrix of the 

 reef over years. Prior to our harvest treatments, the ex- 

 perimentally restored reefs were colonized by at least three 

 generations of oysters, many of which grew to adult size 

 (range of oyster sizes on experimental reefs at the start 

 of our experiment: 2-11 cm in shell height). Consequently, 

 the shell matrices of the reefs had become somewhat cohe- 

 sive, although probably less so than natural oyster reefs. In 

 February 1996, before initiation of experimental harvests, 

 there was no significant difference in the mean density 

 of adult (>1 cm in shell height) oysters (mean ±SD 179.1 

 ±18.4/m 2 ) among the four sets of four experimental reefs 

 randomly selected to receive the four harvesting treatments 

 (one-way ANOVA; F 3 12 =0.29; mean square error=285.06; 

 P=0.83). Experimental reefs in the Neuse River usually 

 had slightly higher oyster densities nearer their base and 

 larger oysters near the crest (see Lenihan, 1999). 



Experimental harvests 



We compared three types of oyster-harvesting techniques: 

 dredging, hand-tonging, and diver-harvesting. In March 

 1996, each of 16 reefs was either dredged, tonged, diver- 

 harvested, or left unharvested as a control (four replicates 

 of each treatment). Experimental dredging and hand-tong- 

 ing were conducted in the manner applied by commercial 

 oyster fishermen. The dredge, 25 kg in weight and 1 m in 

 width, was pulled behind a powerboat operated by NCDMF 

 personnel with commercial oyster-dredging experience. 

 Hand-tonging was also conducted by a professional oyster 



Marshall, M. 1999. Personal commun. North Carolina 

 Division of Marine Fisheries, 3431 Arendell St., Morehead 

 City, NC 28557. 



