86 



Abstract.— We conducted a large-scale 

 field experiment to test whether clam 

 and oyster harvesting applied alone and 

 in combination on intertidal oyster reefs 

 have impacts on resident shellfish pop- 

 ulations. This experiment was conducted 

 to resolve a long-standing conflict be- 

 tween oyster iCrassostrea virginica 

 (Gmelin, 1791)) and clam (Mercenaria 

 mercenaria (Linneaus, 1758)) fishermen 

 who contend that the other fishery causes 

 high rates of mortality to their respective 

 species. Intertidal oyster reefs located in 

 two estuarine creeks near Wilmington, 

 North Carolina, were harvested for clams 

 only, oysters only, both clams and oys- 

 ters, or were left undisturbed as controls. 

 Experimental harvesting was conducted 

 over a one-year period by a professional 

 shellfisherman who used realistic fish- 

 ing techniques (clam rakes and oyster 

 tongs), intensity, and frequency. Har- 

 vesting impact on hard clam and oyster 

 populations was assessed by sampling 

 naturally occurring oysters before and 

 after harvesting, and sampling both nat- 

 urally occurring clams (all size classes) 

 and transplanted, hatchery-raised clams 

 (20-37 mm in length) after harvesting. 

 Clam and oyster harvesting had obvious 

 negative effects on populations of oys- 

 ters. There was a substantial decrease in 

 the number of live oysters on clam-har- 

 vested and oyster-harvested reefs com- 

 pared with unharvested, control reefs. 

 Clam and oyster harvesting, applied 

 together, reduced oyster densities and 

 killed unharvested oysters at a level sim- 

 ilar to that caused by each type of har- 

 vesting applied separately. The effects of 

 the shellfish harvesting on populations of 

 hard clams varied between the two sites 

 (i.e. creeks). In both creeks, clam har- 

 vesting, alone and combined with oyster 

 harvesting, significantly decreased the 

 number of live, naturally occurring 

 clams. Oyster harvesting alone decreased 

 the number of live, naturally occurring 

 clams only at one site. Clam harvesting 

 also decreased the number of live, trans- 

 planted clams on reefs, but there was 

 no effect of oyster harvesting, because 

 the transplanted clams were juveniles 

 too small to be harvested with oyster 

 tongs. Overall, the combined effect of 

 both types of harvesting applied together 

 did not have a negative synergistic effect 

 on clam and oyster populations. Conse- 

 quently, both clamming and oyster har- 

 vesting should be permitted on some 

 reefs, but maintaining large populations 

 of oysters and clams on intertidal oyster 

 reefs will require protection of some reefs 

 from both types of harvesting. 



Manuscript accepted 24 March 1999. 

 Fish. Bull. 98:86-9.-) (2000). 



Biological effects of shellfish harvesting on 

 oyster reefs: resolving a fishery conflict by 

 ecological experimentation 



Hunter S. Lenihan 



Beaufort Laboratory 



National Marine Fishenes Sen/ice, NCAA 



101 Piver's Island Road. Beaufort. North Carolina 28516 



Present address Institute of Manne Sciences 



University of North Carolina at Chapel Hill, 



3431 Arendell St., Morehead City, North Carolina 

 E-mail address fisleniha(S)email uncedu 



Fiorenza Micheli 



National Center for Ecological Analysis and Synthesis 

 Santa Barbara, California 93101 



Marine fisheries are an important 

 source of employment and protein 

 for humans but can negatively affect 

 marine organisms and ecosystems 

 (Dayton et al., 1995; Engel and 

 Kvitek, 1995; Botsford et al., 1997). 

 The most obvious negative ecolog- 

 ical effects of fishing result from 

 over-harvesting of target species, 

 incidental mortality of non target spe- 

 cies ("bycatch"), and fishery-related 

 disturbances to marine habitat ( FAO, 

 1993; Dayton et al., 1995). Of course, 

 fisheries over-exploitation and hab- 

 itat destruction also threaten the 

 sustainability of the fishing indus- 

 try. At present, 44'^ of the worlds 

 fish stocks are fully to heavily 

 exploited, and 22'7( are overexploited 

 or depleted, indicating most fisher- 

 ies are not managed for long-term 

 sustainability (Botsford et al., 1997). 

 The degradation and destruction of 

 marine biogenic habitat (e.g. coral 

 reefs, seagrass beds, mangrove for- 

 ests, and oyster reefs) by dredging, 

 trawling, use of explosives, and poi- 

 soning reduces fishery production by 

 removing habitat essential for the 

 recruitment, growth, and survival of 

 fishery and prey organisms ( Winslow 

 1881, a and b; Peterson et al, 1987; 

 Norse, 1993; Rothschild et al., 1994; 

 NRC, 1995; Lenihan and Peterson, 



1998). The sustainability of a fishery 

 is often threatened when competing 

 fisheries exploit a common resource 

 or negatively impact a commonly 

 used habitat. For example, when the 

 bycatch of one fishery is within a 

 food web supporting another fishery 

 (West and Gordon, 1994), or when 

 a fishery destroys habitat impor- 

 tant to the life history of other fish- 

 ery species (Russ and Alcala, 1996), 

 heated political battles arise and the 

 livelihood of many people may be 

 lost. Resolving such fishery conflicts 

 has important ecological and eco- 

 nomic consequences and is of major 

 concern to fisheries managers and 

 ecologists worldwide (McAllister and 

 Peterman, 1992). This paper pres- 

 ents the results of an experimental 

 analysis of whether two economi- 

 cally valuable fisheries conflict and 

 provides management recommenda- 

 tions to resolve the conflict. 



High productivity of fishery stocks 

 in estuaries and shallow water coastal 

 habitats often induces intense exploi- 

 tation of a common species or habitat 

 by multiple, potentially conflicting 

 fisheries (Peterson et al., 1987). In 

 many estuaries along the Atlantic 

 coast of the USA, intertidal oyster 

 reefs are harvested for hard clams 

 {Mercenaria mercenaria ) year round, 



