Grabowski et al.: Estimating stock biomass of Strongylocentrotus droebachiensis 



325 



50,000 100.000 150.000 



h(m) 



200.000 



Figure 3 



Sample variogram of mean green sea urchin (Stron- 

 gylocentrotus droebachiensis) density by site, showing 

 small-scale variability, gamma (y), with respect to 

 the distance between sample point pairs, /;. 



spatial distributions. Exploitable biomass estimates for 

 method 2 were more than 2 times greater than those 

 for method 1 (Table 3). With method 1, legal-size sea 

 urchins were concentrated in the northeastern corner 

 of management area 2, but with method 2, they were 

 concentrated in the northeastern portion of area 1 and 

 the central portion of area 2 (Fig. 6). Exploitable sea 

 urchin biomass showed different patterns by manage- 

 ment area and depth than did total biomass and fish- 

 able biomass (Fig. 5). For example, management area 

 1 had a larger share of the total exploitable biomass, 

 39% or 47%, for methods 1 and 2, respectively, and 



U\ 



 Area 1 

 □ Area 2 



u. 



0-29 



30^19 50-64 65-80 

 Diameter (mm) 



81 + 



Figure 4 



Total biomass by green sea urchin (Strongylocentrotus droe- 

 bachiensis) test diameter according to management area. Sea 

 urchins between 50 and 80 mm were considered legal size 

 for this study, and the biomass within these limits, indicated 

 by the dashed lines, constitutes the fishable biomass. 



this biomass was almost exclusively found in the 0-5 m 

 depth zone, accounting for 98% or 93%, respectively, of 

 the area's biomass. 



TIN biomass estimates were similar to ones produced 

 with the arithmetic mean but were higher for total 

 biomass and lower for fishable biomass. Exploitation 

 rates for method 1 were estimated at 0.59 and 0.55 

 for management areas 1 and 2, respectively, and 0.20 

 and 0.27 for method 2, respectively. Exploitation rates 



