Biological effects of shellfish harvesting on oyster reefs: resolving a fishery conflict by ecological experimentation

We conducted a large-scale held experiment to test whether clam and oyster harvesting applied alone and in combination on intertidal oyster reefs have impacts on resident shellfish populations. This experiment was conducted t o resolve a long-standing conflict between oyster (Crassostrea virginica (G melin, 1791)) and clam (Mercenaria mercenaria (Linneaus, 1758)) fishermen w ho contend that the other fishery causes high rates of mortality to their r espective species. Intertidal oyster reefs located in two estuarine creeks near Wilmington, North Carolina, were harvested for clams only, oysters onl y, both clams and oysters, or were left undisturbed as controls. Experiment al harvesting was conducted over a one-year period by a professional shellf isherman who used realistic fishing techniques (clam rakes and oyster tongs ), intensity, and frequency. Harvesting impact on hard clam and oyster popu lations was assessed by sampling naturally occurring oysters before and aft er harvesting, and sampling both naturally occurring clams (all size classe s) and transplanted, hatchery-raised clams (20-37 mm in length) alter harve sting. Clam and oyster harvesting had obvious negative effects on populatio ns of oysters. There was a substantial decrease in the number of live oyste rs on clam-harvested and oyster-harvested reefs compared with unharvested, control reefs. Clam and oyster harvesting, applied together, reduced oyster densities and killed unharvested oysters ata level similar to that caused by each type of harvesting 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 harvesting, alone and combined with oyster h arvesting, significantly decreased the number of live, naturally occurring clams. Oyster harvesting alone decreased the number of live, naturally occu rring clams only at one site. Clam harvesting also decreased the number of live, transplanted clams on reefs, but there was no effect of oyster harves ting, because the transplanted clams were juveniles too small to be harvest ed with oyster tongs. Overall, the combined effect of both types of harvest ing applied together did not have a negative synergistic effect an clam and oyster populations. Consequently, both clamming and oyster harvesting shou ld be permitted on some reefs, but maintaining large populations of oysters and clams on intertidal oyster reefs will require protection of some reefs from bath types of harvesting.