Lenihan et al.: Conserving oyster reef habitat 



301 



Table 1 



Results of one-way ANOVAs comparing differences in reef height (cm), reef diameter (cm), and catch per unit of effort (number 

 of oysters collected per hour) among experimental reefs harvested by different methods (dredging, tonging, diver-harvesting, 

 and controls), df = degrees of freedom; ms = mean square; F = F-value; P = P-value; ss = sum of squares. Partial r 2 = treatment 

 ss/total ss. 



Reef height 



Reef diameter 



partial 



Source 



df ms 



partial 



7' 2 



Catch per unit of effort 



partial 



0.0002 0.80 



0.27 



3.21 

 0.08 



17.84 0.0001 0.11 



9.64 



the ANOVA. Before ANOVA, homogeneity of variances 

 was tested by using Cochran's method (a=0.05). All data 

 passed this test. After ANOVA, post hoc differences among 

 means were compared by using Student-Newman-Keuls 

 (SNK) tests (a=0.05). 



Results 



Reef height and diameter 



Dredge harvesting on experimental reefs removed the 

 largest amount of shell material from the reefs, based on 

 the reduction of reef height (Fig. 1A) and on the qualitative 

 assessment of increases in numbers of oyster shells found 

 on the seafloor around the reefs. Hand-tonging removed 

 an intermediate amount of reef materials, and diver-har- 

 vesting removed far less shell matrix than either dredging 

 or tonging. All harvesting methods reduced the height of 

 restored oyster reefs (Fig. 1A), but dredging (34% of reef 

 height) and tonging (23%) had greater impacts than did 

 diver-harvesting (6%). ANOVA demonstrated significant 

 differences among harvest treatments in mean change in 

 reef height (Table 1); all harvest treatments induced a loss 

 in reef height as compared with unharvested control reefs 

 (SNK; P<0.05). Dredging reduced reef height more than 

 any other treatment (SNK, P<0.05), and tonging reduced 

 reef height more than diver-harvesting (SNK, P<0.05). 

 The reduction in reef height caused by diver-harvesting 

 was small (mean ±SD: 6 ±3 cm). However, diver-har- 

 vesting nearly eliminated the veneer of live market-size 

 oysters on reefs, which provides substantial structure on 

 reef surfaces. 



Oyster harvesting either slightly increased or slightly 

 decreased reef diameter, depending upon method ( Fig. 

 IB). Reef material was apparently removed from edges 

 of reefs by tonging. thereby reducing reef diameter. Shell 

 was spread around the reefs by dredging, thereby increas- 

 ing reef diameter after application of that harvesting 

 method. The effects of oyster harvesting on reef diameter 

 proved significant (Table 1). Tonging significantly re- 

 duced reef size compared with controls and the other two 

 harvesting treatments (SNK; P<0.05), whereas dredging 



DC 



control diver-harvested tonged dredged 



Figure 1 



Modification of reef size and structure caused by various 

 harvesting techniques. (A) Mean (+SE) reduction in the 

 height of experimentally restored oyster reefs caused 

 by three types of oyster harvesting: hand-harvesting by 

 divers, hand tonging. and dredging. Dredges are pulled 

 behind power boats. Reefs were located in the Neuse River 

 estuary, North Carolina. Letters represent results of SNK 

 post hoc tests: dredged>tonged>diver-harvested>control 

 at P<0.05. (B) Mean ( + SE) change in the diameter of 

 experimental oyster reefs caused by different oyster- 

 harvesting techniques. Letters represent results of SNK 

 post hoc rests: dredged>diver-harvested>control>tonged 

 atP<0.05. 



