Carter and VanBlaricom: Effects of experimental han/ersl on Strongylocentrotus franascanus in northern Washington 



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Effect of harvest on density 



Sea urchin densities in the SJC sites initially averaged 

 1.35/m2 (±0.55, Table 1). Differences in initial sea urchin 

 density between sites were marginally significant 

 (P=0.05) because of the low density of sea urchins at 

 North McConnell. Because of the difference in initial den- 

 sity, we expressed changes from preharvest to posthar- 

 vest densities in SJC sites as percentages of the original 

 population. 



Initial harvest in March 1997 decreased the number 

 of sea urchins in complete harvest sites by 94.7% (range; 

 93.7-96.3%) and in selective harvest sites by 66.6% 

 (range: 61.7-74.9%, Table 3). Reharvest of selective har- 

 vest sites in March 1998 reduced sea urchin numbers 



46.9% (range: 37.6-55.6%). All decreases were significant 

 (P<0.04). Yields from selective harvest sites averaged 0.95 

 sea urchins/m' in 1997, and 0.31 sea urchins/m^ in 1998. 

 The number of sea urchins in control sites did not differ 

 between 1997 and 1998 (P=0.77, average 1.41/m2). 



Small-scale sampling also demonstrated significant 

 changes in sea urchin density as a result of the experimen- 

 tal harvest treatments (Table 4). Initial harvest reduced 

 sea urchin densities by 97.6% and 69.5% in complete and 

 selective harvest sites, respectively. Mid-March (5 days 

 after harvest) densities averaged 0.03/m- (±0.07/m-) and 

 0.37/m- (±0.45/m^) in complete and selective harvest sites, 

 respectively. Sea urchin densities in complete harvest sites 

 remained low for the duration of the study (<0.04/m2). Sea 

 urchin densities in selective harvest sites increased slight- 



