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Fishery Bulletin 102(2) 



Table 2 



Results of one-way ANOVAs comparing differences in the proportion of oysters found dead ("mortality" i on reefs before and after 

 harvesting by different methods (dredging, tonging, diver-harvesting, and controls), and the absolute difference (\after-before]) 

 in the proportion of dead oyster found before versus after harvesting, df = degrees of freedom; ms = mean square; F = F-value; 

 P = P-value; ss = sum of squares. Partial r 2 = treatment ss/total ss. 



Before mortality 



After mortality 



partial 



Source 



df ms 



partial 

 r 2 



Difference in mortality 



partial 



7.90 0.004 



0.58 



II us 



0.01 

 0.08 



7.56 0.004 0.57 



0.04 



increased reef diameter compared to the other treat- 

 ments (SNK; P<0.05). The increase in diameter of 

 diver-harvested reefs was also greater than that for 

 controls (SNK; P<0.05). The substantial increase in 

 shell material (with oysters of all sizes) spread out on 

 the seafloor on dredged reefs indicates that the collec- 

 tion efficiency of dredges is less than 100%. 



Catch per unit of effort 



Catch per unit of effort of oysters included the time 

 required to collect oysters from the reef and the time 

 needed to separate (i.e., cull) them from undersized 

 oysters and shell material. Two of the harvesting 

 methods, hand-tonging and oyster dredging, are one- 

 man operations in which one fisherman can operate 

 the harvesting gear, cull oysters, and drive the boat. 

 Therefore, measurements of catch per unit of effort for 

 dredging and tonging represent the numbers of bush- 

 els of oysters one fisherman can collect per hour. In 

 contrast, scuba diving is rarely attempted alone and it 

 is usually necessary for someone else to tend the diver 

 (e.g., helping him or her in and out of the water) and 

 to haul oysters up to the boat when given a signal by 

 the diver on the reef. Divers should preferably work as a 

 team using the "buddy" system for safety reasons. Data for 

 diver-collections are given in bushels per hour collected by 

 one diver but hauled up to the boat and culled by a second 

 person. 



There was a significant difference in the numbers of 

 bushels collected per hour by the different harvesting 

 techniques (Table 1). Diver-harvesting had a higher catch 

 efficiency than all other treatments ( SNK; P<0.05; Fig. 2). 

 Diver-harvesting was about 25% more time efficient than 

 dredge harvesting and 32% more efficient than tonging. 

 There was no statistically significant difference in effi- 

 ciency between dredging and tonging (SNK; P>0.05). 



Incidental oyster mortality 



The proportion of oysters found dead on experimental 

 reefs in February 1996 (-20%), prior to experimental 



harvesting, was similar to that found on other nearby ex- 

 perimental and natural reefs in the Neuse River estuary 

 in preceding years (e.g., Lenihan and Peterson. 1998: 

 Lenihan 1999). In February, the proportions of dead oys- 

 ters did not differ among the four sets of reefs destined 

 to be experimentally harvested (Table 2. Fig. 3A). In 

 contrast, there was a large and statistically significant 

 difference in the proportions of dead oysters on the reefs 

 after harvesting (Table 2, Fig. 3A). The proportions of 

 dead oysters on reefs that had been tonged and dredged 

 were significantly greater than on diver-harvested and 

 control reefs (SNK; P<0.05). 



Before-after-control-impact (BACI) comparison of the 

 change in proportions of dead oysters from before to after 

 harvesting ( [after— before] ), a direct estimate of incidental 

 mortality caused by harvesting gear, showed a similar 

 pattern to mortality inferred from in situ proportions of 

 dead oysters in March after harvesting (Table 2, Fig. 3B). 



