Carter and VanBlaricom: Effects of experimental harverst on Strongylocentrotus franciscanus in northern Washington 



669 



circular areas). Over the entire study period, juvenile red 

 sea urchin densities in circular areas averaged 0.005/m- 

 in control sites (0.6% of the population), 0.00 1/m- in selec- 

 tive harvest sites (0.9% of the population), and 0.0006/m'- 

 in complete harvest sites. Sampling at the larger spatial 

 scale revealed similar results. Juvenile sea urchin density 

 averaged 0.002/m-' in control sites in September of both 

 1997 and 1998. Juvenile sea urchin density averaged 

 0.00 1/m- and 0.005/ni- in selective harvest sites in Sep- 

 tember of 1997 and 1998, respectively. 



Invasive sampling in September 1998 confirmed that ju- 

 venile sea urchins were rare in SJC sites during the study. 

 One juvenile ( 13 mm) red sea urchin and 45 juvenile (2-12 

 mm) green sea urchins were found in a search of 6.4 m^ of 

 substrate in each site (57.6 m^ total). Density of juvenile 

 green sea urchins sampled did not differ between treat- 

 ments (P=0.79; average 0.79/m- |±0.86|). Only 4.3% of the 

 juvenile green sea urchins were associated with (found 

 within 8 cm of) an adult red sea urchin. 26.7% of juvenile 

 green sea urchins were found in cryptic microhabitats 

 (algal holdfasts), and the remainder were found on algal 

 blades or on top of cobble, boulders, or bedrock. 



Discussion 



Size distribution of sea urchins in SJC and SJDF 



The modal size of sea urchins in SJC is larger than ob- 

 served in most other locations on the west coast (Table 6). 

 The size distribution of red sea urchins in SJC sites 

 is strongly skewed toward large individuals. The slow 

 growth rate of sea urchins in Washington (Lai and Brad- 

 bury, 1998), combined with the fact that SJC was, at the 

 time of our study, a reserve area that has never been sub- 

 ject to significant harvest, indicates that the sea urchin 



population in SJC is composed primarily of older individu- 

 als. Fewer juveniles are present in SJC and SJDF than 

 in most other areas on the west coast (Table 6). Large 

 recruitment events appear to be rare for red sea urchins 

 in SJC, evidenced by the lack of peaks in the size distribu- 

 tion at small test diameters. 



Effect of harvest on size distribution 



The first annual size-selective harvest only slightly 

 affected the size distribution of sea urchins in SJC. The 

 cumulative effect of two annual size-selective harvests 

 was significant, however, significantly reducing the den- 

 sity of legal*-size sea urchins in the population and in- 

 creasing the modal size of the population. Commercial 

 fisheries elsewhere along the west coast have also affected 

 the size distribution of sea urchin populations. Three com- 

 mercial harvests in the San Juan Islands (outside the SJC 

 reserve) over a nine-year period decreased the proportion 

 of legal-size sea urchins by about 73% (Pfister and Brad- 

 bury, 1996). In southern California, two years of harvest 

 reduced the proportion of legal-size sea urchins in shallow 

 waters by 15% (Tegner and Dayton, 1981). In northern 

 California, the modal size of sea urchins in unharvested 

 reserve sites was 108 mm, whereas the modal size of sea 

 urchins in nearby heavily harvested sites with a 76-mm 

 minimum size limit was 73 mm (Smith et al., 1998). On 

 the east coast of Vancouver Island, however, the modal 

 size of sea urchins remained above the legal size limit 

 after 3-1- years of significant harvest (Sloan et al., 1987). 



The modal size of the sea urchin population and the 

 upper size limit for legal harvest coincided in SJC. Small 

 errors in the measurement of sea urchins by divers led to 

 an incomplete harvest of legal-size sea urchins and inad- 

 vertent "poaching" of many over-size sea urchins (41% of 

 the catch, likely a slight overestimate because sea urchins 



