Lenihan and Micheli: Biological effects of shellfish harvesting on oyster reefs 



93 



tions on intertidal oyster reefs. Both types of shellfish 

 harvesting, applied separately or together, reduced 

 the densities of live oysters by 50-80% compared with 

 densities at unharvested reefs. Surprisingly, there 

 was no difference among the effects of clam harvest- 

 ing only, oyster hai'vesting only, and clam and oyster 

 harvesting combined on the density of live oysters. 

 We expected oyster harvesting to reduce oyster pop- 

 ulations more than clam harvesting because oyster 

 harvesting removes oysters whereas clam harvesting 

 targets clams only (see Table 2). We do not know the 

 effect of clam and oyster harvesting on oysters <1 

 mm in length; therefore further experiments should 

 be conducted to determine their fate. Results of our 

 experiment show conclusively that the density of live, 

 adult oysters was significantly reduced on reefs that 

 were harvested for clams only (Fig. 1 ). Therefore, clam 

 harvesting has important negative effects on oysters, 

 most likely through increased oyster mortality. 



We did not investigate the specific mechanisms 

 underlying the negative effect of clam harvesting 

 on oyster populations, but obsei-vations made during 

 experimental harvesting indicated that clamming with 

 rakes killed oysters in two ways. First, during the pro- 

 cess of clamming oyster shells were cracked or punc- 

 tured (senior author, personal obs.) Severely wounded 

 oysters probably died. Oysters were also indirectly 

 killed during clamming when they were buried or 

 smothered beneath sediments that were removed in 

 the process of digging for buried clams (senior author, 

 personal obs.). Another potential, but unobserved, 

 mechanism potentially leading to enhanced oyster 

 mortality during clamming was that predators (e.g. 

 blue crab and the sheepshead fish, Archosargus pro- 

 batochephal us) were attracted to the reefs by wounded 

 oysters and by sediment disruption, thereby enhanc- 

 ing predation intensity on oysters (e.g. Dayton et al., 

 1995). It did not appear that oysters were spread 

 around on the experimental reefs by clam harvesting, 

 thus reducing their densities in sampling plots. 



Effects of clam and oyster harvesting on naturally 

 occurring populations of hard clams were less clear 

 than effects of clam and oyster harvesting on oysters. 

 Clam harvesting, both alone and in combination with 

 oyster harvesting, decreased densities of live clams 

 by 50—90'%^ compared with unharvested reefs. This 

 result was expected because clam harvesting removes 

 large numbers of clams (see Table 2). Because clams 

 are motile, it is possible that some clams emigrated 

 from sampling plots following the harvest disturbance, 

 thereby accounting for some reduction in clam density. 

 However, this movement is unlikely to have accounted 

 for a large proportion of the reduction in clam densi- 

 ties because the sampling plots covered much of the 

 area on reefs inhabited by clams. Oyster harvesting 



E 

 o 



E 

 O 



Clams 



+ 

 Oysters 



Harvest type 



Figure 5 



Mean density of live, dead, and missing hatchery-raised 

 hard clams after (10-23 July 1997) application of experi- 

 mental harvest treatments in Pages and Whiskey Creeks, 

 NC. Sixteen clams were placed in each 1.0-m-^ quadrat 

 between 5-10 July 1996. Data are means and standard 

 errors («=4) of counts taken within 1.0-m'-' quadrats. 

 Results of SNK post hoc comparisons are illustrated with 

 letters above bars (a>b at P<0.05). Separate ANOVAs and 

 SNK tests were used to compare numbers of live, dead, and 

 missing clams. 



alone also reduced the density of live clams but only at 

 one site. Pages Creek. At Whiskey Creek, the density 

 of live clams after harvesting was similar between oys- 

 ter-harvested and control plots, indicating that oyster 

 harvesting had little effect on clam survival (Fig. 3). 

 A negative effect of oyster harvesting on clams may 

 be caused both by direct removal of clams as bycatch 

 (Table 2) and enhanced clam mortality through mecha- 

 nisms analogous to those hypothesized for oysters (see 

 above). Some clams may also have emigrated from the 

 oyster harvesting treatments following harvesting. 



Patterns of survival and mortality of hatchery- 

 raised clams transplanted to experimental reefs varied 

 with site and harvest type (Table 5). Fewer live and 

 dead transplanted clams were recovered from reefs at 

 Whiskey Creek than at Pages Creek (Fig. 5). In con- 



