Grabowski et al.: Estimating stock biomass of Strongylocentrotus droebachiensis 



323 



2H Kilometers 



^ 



Urchin density 



0-5 



 5 - 10 



| Id- 15 

 H 15 



~\ No data 



20 kiloaielere 



Urchin density 



I 10-5 

 1 5 - 111 



 I"- 15 

 ^B 15 - 

 HNo data 



5 10 15 20 Kilometer; 



Urchin density 



(5-10 



| HI- 15 

 ^B 15 



J No data 



4- 



Figure 2 



Representations of the triangulated irregular networks (TINs), used to characterize the large-scale patterns in green sea 

 urchin [Strongylocentrotus droebachiensis) density (number of sea urchins/m 2 ), for the 50-64 mm sea urchin size category in 

 the central portion of management area 2. Top left, 0-5 m depth zone; top right, 5-10 m depth zone; bottom left, 10-15 m depth 

 zone; bottom right, 15-40 m depth zone. 



To determine total sea urchin biomass <6) for each sce- 

 nario, the volume beneath the modified TIN surface was 

 calculated, from Riemann sums, and multiplied by the 

 mean weight (w) according to the following equation: 



»XW*. 



(2) 



where s t 



n 



fis,) 



= the spatial location (x,y) on an ASCII grid; 



= the number of grids squares; 



= the TIN surface and corresponds to a z value 



for each grid cell; and 

 = the grid cell size, which was 1.72 hectares 



for area 1 and 1.82 hectares for area 2. 



Fishable biomass is defined as the biomass of all 

 legal-size sea urchins and is simply the subset of the 

 total biomass corresponding to legal-size sea urchins. 

 Exploitable biomass corresponds to the legal-size sea 



urchins that are available to the fishery. Some areas 

 included in this study may not be subject to fishing 

 pressure because of geographic isolation or low sea 

 urchin densities. Because information on historical 

 fishing grounds is insufficient, exploitable biomass was 

 estimated by using a threshold density value. Only 

 areas with densities greater than the threshold were 

 included in the exploitable biomass estimates. 



Two different types of threshold values were tested: 

 1) a threshold based on total sea urchin density and 2) 

 a threshold based on the density of legal-size sea ur- 

 chins. The threshold values make different assumptions 

 about the fishery: method 1 assumes that fishermen 

 target areas based on total sea urchin density, whereas 

 method 2 assumes that fishermen target areas based 

 on the density of legal-size sea urchins. Interviews 

 were conducted with state sea urchin biologists and 

 fishermen to determine an appropriate threshold value. 

 The reported threshold values, the minimum total sea 



