Carter and VanBlancom Effects of experimental harverst on Stiongyloccntiotus fiancisconus in norttiern Wasfiington 



665 



130-139 mm). Because of the measurement incTement 

 used, we could only estimate proportions of under-, legal-, 

 and over-size sea urchins in SJC and SJDF. Asterisks (' ) 

 indicate estimated size classes. Estimated size classes are 

 as follows: SJC — under'-size: 0-95 mm, legal*-size: 100- 

 135 mm. and over*-size: 140-175 mm; SJDF — under*- 

 size: 0-80 mm, legal*-size: 85-110 mm, and over*-size: 

 115-175 mm. Sea urchins less than 50 mm are less than 

 two years old and were classified as juveniles (see Pfister 

 and Bradbury, 1996). 



Harvest and recolonization 



Recolonization was determined for three periods: summer 

 1997 (March 1997 Ipostharvestl-September 1997), winter 

 1997 (September 1997-March 1998 Ipreharvestl ), and 

 summer 1998 (March 1998 [postharvest] -September 

 1998). In complete harvest sites, monthly recolonization 

 for each time period was calculated as the sum of the 

 number of sea urchins removed each month from each site 

 during the time period divided by the number of months 

 in the time period. A few sea urchins that divers did not 

 harvest in March 1997 because they could not safely be 

 removed from the substrate were excluded from the cal- 

 culation of recolonization for the first time period. Size 

 of recolonizers was based on sea urchins harvested from 

 May 1997 to September 1998. In selective harvest sites 

 where sea urchins were removed only once each year, we 

 calculated monthly recolonization for each time period as 

 the difference between the number of sea urchins counted 

 in each site at the beginning and end of the time period, 

 divided by the number of months in the time period. 



At the end of the study (September 1998), divers sam- 

 pled destructively for juvenile sea urchins in one 0.80-m- 

 wedge within each circular area within a sampling site. 

 Prior to this time, divers may not have seen very small sea 

 urchins concealed underneath rocks or large sea urchins 

 because sampling was not destructive to avoid disturbing 

 other experiments. We include results on both red and 

 green juvenile sea urchins sampled because of the very 

 low number of red sea urchins sampled and to provide an 

 indication of the microhabitats inhabited by juvenile sea 

 urchins in general in SJC. 



Data analysis 



Size-frequency data were grouped into discrete size classes 

 and compared by using chi-square analysis. Sea urchin 

 data collected at each site were correlated over time. 

 Therefore, we used a paired ^test to analyze the effect of 

 harvest on the total number of sea urchins in sites and on 

 the density of sea urchins in each size class. Similarly, we 

 analyzed sea urchin densities in permanent circular areas 

 over time by using repeated measures analysis of vari- 

 ance. In the analysis, site was treated as a random factor 

 and was nested within treatment, which was treated as a 

 fixed factor. Densities of juvenile sea urchins in Septem- 

 ber 1998 were analyzed by using analysis of variance. 

 Sea urchin densities were log-transformed to improve the 

 variance structure for analysis (Zar, 1984). Mean values 



50 -, 



Test diameter (mm) 



Figure 3 



Size-frequency distribution of red sea urchins 

 sampled in the three control sites combined in 

 San Juan Channel (SJC) in September 1997 

 (n=5.37l and in the western Strait of Juan de 

 Fuca (SJDF) in August 1997 (n=405). Dashed 

 lines indicate minimum and maximum size 

 limits for commercial harvest in each location. 



generally are followed by one standard deviation in the 

 text. The level of significance for all tests was a = 0.05. 



Results 



Size distribution of sea urchins in SJC and SJDF 



Size distributions of red sea urchins in SJC were strongly 

 skewed to the right and had a modal size of 140 mm (range 

 20-175 mm. Fig. 3). 4.8% (±3.1%) of sea urchins in circular 

 areas in all sites in early March were <100 mm (under*- 

 size), 50.0% (±13.0%) were 100-139 mm (legal*-size), and 

 45.2% (±12.5%) were >140 mm (over*-size). Juveniles rep- 

 resented 0.3% (±0.4%) of the population. 



The modal size of red sea urchins in SJDF was 100 mm 

 (range 30-175 mm. Fig. 3). 12.1% of the red sea urchin 

 population was under*-size, 35.6% was legal*-size, and 

 52.4% was over*-size. Juveniles represented 2.5% of the 

 population. 



Effect of harvest on size distribution 



Sea urchins harvested in the initial size-selective harvest 

 in 1997 were 90-160 mm in diameter (mode 135 mm. 



