Lenihan and Micheli: Biological effects of shiellfisfi fiarvesting on oyster reefs 



87 



and for oysters iCrassostrea virginica ) in the fall and 

 winter (i.e. October-March). In recent years, clam and 

 oyster (i.e. "shellfish") harvesting on oyster reefs has 

 led to conflict between the two fisheries, and between 

 fishermen and habitat managers over the issue of hab- 

 itat degradation, especially in the southeastern United 

 States (e.g. Frankenberg^; Noble-^). Oyster fishermen 

 claim that the harvesting of clams from intertidal 

 oyster reefs decreases resident oyster populations, 

 and vice-versa, because each type of fishing kills or 

 removes the other species. Rakes and hand tongs used 

 for the two types of shellfishing appear to increase the 

 mortality of the sessile reef animals by burying them 

 beneath sediments, fracturing their shells, or causing 

 other physical damage (Noble^). In addition, bodies of 

 dead and wounded animals left behind may attract 

 scavengers and predators, thereby increasing preda- 

 tion intensity on healthy live animals (Dayton et al., 

 1995). Habitat and fishery managers are concerned 

 that the physical destruction of oyster reefs caused by 

 shellfishing will negatively affect many other fishery 

 organisms that recruit to and utilize oyster reef habi- 

 tat, including many species of fishes (Breitburg et al. 

 1995, Lenihan et al., 1998, Luckenbach et al., 1998) 

 and the blue crab (Callinectes sapidus (Rathbun)) 

 (Bahr and Lanier, 1981; Zimmerman etal., 1989; Leni- 

 han et al., 1998; MicheU and Peterson, 1999). Shell- 

 fishing also reduces the overall size of reefs because 

 shell material is broken or removed along with the 

 target species (Lenihan and Peterson, 1998; Coen^). 

 Reducing the size of reefs is thought to decrease the 

 abundance of clams because less habitat is available 

 for adults and recruits (Arnold, 1984; Sponaugle and 

 Lawton, 1990; Peterson et al., 1995). Decreasing the 

 size (i.e. height) of oyster reefs also reduces oyster 

 production because flow speed over reefs diminishes, 

 causing sediment to accumulate and oyster growth 

 and survival to decrease (Lenihan, 1999; Lenihan et 

 al., 1999). In contrast to the negative effects of shell- 

 fish harvesting, many fishermen claim that "turning 

 over" the shell matrix of oyster reefs during harvest- 

 ing improves clam and oyster production because it 

 removes accumulated sediment. In North Carolina 

 and many other Atlantic coast states, both types of 

 shellfishing are allowed on reefs and conflicts between 

 the fisheries continue (e.g. Frankenberg'; Marshall''). 



' Frankenberg, D. 1995. Report of North Carolina Blue Ribbon 

 Advisory Council on oysters. North Carolina Department of Envi- 

 ronmental Health, and Natural Resources, Raleigh. NC, 101 p. 



^ Noble, E. B. 1995. Destruction of oyster rocks by individuals 

 taking clams by legal hand harvest methods. Report of the North 

 Carolina Division of Marine Fisheries, Morehead City, NC, 11 p. 



■^ Coen. L. D. 1995. Areview of the potential impacts of mechani- 

 cal harvesting on subtidal and intertidal shellfish resources. A 

 report prepared by the South Carolina Department of Natural 

 Resources, Marine Resources Research Institute, SC, 111 p. 



Whether oyster harvesting, clam harvesting, or both 

 types of fishing activities together have negative 

 impacts on shellfish populations of intertidal oyster 

 reefs is a matter of opinion and has not been tested 

 experimentally. 



A comparison of the biological impact of various fish- 

 ing practices by using large-scale field experiments is 

 an efficient method of resolving many fishery-related 

 conflicts (McAllister and Peterman, 1992) and is an 

 important component of adaptive management strat- 

 egies (Walters, 1986). Such experiments are usually 

 designed so that replicate areas (i.e. treatments) are 

 fished, by using each technique separately and by using 

 a combination of techniques, while other areas (i.e. con- 

 trols) are closed to fishing. For these experiments to be 

 meaningfial, they must be conducted on realistic tem- 

 poral and spatial scales, and the fishing treatments 

 must be applied through the actual fishery (McAllister 

 and Peterman, 1992). The success of such experiments 

 also depends heavily on close working relationships 

 among fishermen, fisheiy ecologists, and fishery man- 

 agers (Grumbine, 1997). For adaptive management, 

 the results of initial (i.e. "prototype") experiments are 

 used to design new management strategies that are 

 subsequently tested on even larger temporal and spa- 

 tial scales. Such adaptive management strategies and 

 the use of experimental approaches are often discussed 

 in fisheries management, but in reality are rarely 

 attempted (e.g. Walters, 1986; Botsford et al., 1997). 



We conducted a large-scale field experiment to test 

 the effects of hard clam and oyster harvesting, sepa- 

 rately and in combination, on oyster and hard clam 

 populations living on intertidal oyster reefs in North 

 Carolina. Specifically, we tested whether 1) the den- 

 sity of live and dead oysters varied among oyster 

 reefs that were harvested for clams, harvested for oys- 

 ters, harvested for clams and oysters, or were unhar- 

 vested; 2) the density of live and dead clams varied 

 among oyster reefs subjected to the same four harvest- 

 ing treatments; and 3) the joint application of both 

 shellfish harvesting practices has a synergistic (i.e. a 

 multiplicative) effect on each species. If applying both 

 types of harvesting to the same reefs enhances poten- 

 tial negative effects of each harvesting type, a possible 

 management option would be to allow clam and oyster 

 harvesting only on separate reefs. This experiment was 

 designed and conducted with the combined effort of a 

 clam and oyster fisherman, "> ecologists,^ and habitat 



^Marshall, M. 1996. North Carolina Division of Marine Fisheries, 



3431 Arendell St.. Morehead City, NC, 28557. Personal commun. 

 ■'' Cummings, R. A. 1996. For address contact H. S. Lenihan. 



Institute of Marine Sciences, 3431 Arendell, Morehead City, NC 



28557. Personal commun. 

 " Peterson, C. H., and H. C. Summerson. 1997. Institute of 



Marine Sciences, 3431 Morehead City, NC 28557. 



