670 



Fishery Bulletin 100(4) 



140 mm in diameter were incorrectly classified as over- 

 size). Both actions would tend to minimize effects of our 

 experimental size-selective harvest. Commercial hai-vest- 

 ers take a much smaller proportion of over-size sea ur- 

 chins (11.2% of catch) and harvest a higher proportion of 

 legal-size sea urchins (Pfister and Bradbury, 1996). Thus 

 commercial harvest should produce larger differences in 

 size distribution than observed in our study. 



Effect of harvest on sea urchin density 



Commercial hai"vest in nonresei-ve habitats of the San 

 Juan Islands for over two decades has significantly de- 

 creased sea urchin densities (Pfister and Bradbury, 1996). 

 Both harvest treatments in our study reduced sea urchin 

 densities dramatically and immediately, as expected. The 

 accumulation of older larger individuals in the SJC popu- 

 lation led to high yields in selective harvest sites in 1997. 

 Yields in selective hai"vest sites declined by more than two 

 thirds in 1998 owing to the fishing down of the population 

 (Hilborn and Walters, 1992), low immigi'ation of legal- 

 size sea urchins, and the presence of few under-size sea 

 urchins in the population with a potential of growing to 

 legal size. 



Recolonization of harvested sites 



Recolonization of harvested sites occurred primarily by 

 immigration of adult red sea urchins. Recolonization 

 began within one month of harvest and continued for the 

 next 18 months. In northern California sites at 11 m in 

 depth, red sea urchins also immigrated into harvested 

 sites within a short time period (Rogers-Bennett et al., 

 1998). Complete and selective han'est sites in California 

 were recolonized to 32% and 86% of their original densi- 

 ties, respectively, within nine days. These recolonization 

 rates are much higher than those observed in SJC, pos- 

 sibly because of high sea urchin movement (up to 10 m/h, 

 Rogers-Bennett et al., 1995, 1998) and smaller study plots 

 (64 m- per site) in the northern California location. Sea 

 urchins move less (~1 m/day) in areas with abundant food, 

 such as in SJC, than in areas with little food (Mattison et 

 al., 1977; Carney, 1991). 



Seasonally high recolonization rates observed each 

 spring in both harvest treatments may be related to feed- 

 ing activity. Spring and summer are seasons when sea 

 urchins feed more frequently (Vadas, 1968) and densities 

 of algae preferred by sea urchins in SJC (e.g. Laminaria, 

 Nereocystts, Ala/'ia, and Costana; Vadas, 1968, 1977) are 

 highest (Carter, 1999). Both factors may have stimulated 

 movement of sea urchins from deeper waters (with lower 

 algal densities) into our sites. 



On a monthly basis, selective harvest sites were re- 

 colonized at about half the rate at which complete harvest 

 sites were recolonized. Sea urchins recolonizing complete 

 harvest sites were removed monthly, whereas those recolo- 

 nizing selective harvest sites were not. The presence of sea 

 urchins, however, did not inhibit immigration in another 

 study (Watson, 199.3). Differences in sea urchin densities 

 in adjacent habitats or food availability may have contrib- 



uted to the observed differences in recolonization rates 

 both within and between treatments. 



Recolonization of selective harvest sites was insufficient 

 to maintain sea urchin populations at the densities and 

 size distributions observed prior to harvest under an an- 

 nual harvest scenario. A site in Washington was recolo- 

 nized to preharvest density and size distribution after 2.5 

 years (Bradbury, 1991). Recolonization of SJC sites varied 

 substantially in the first year following harvest (range 

 1-202 sea urchins per site). One year, after harvest, sites 

 varied between 37% and 69% of their original densities. 

 Such variability in recolonization between sites may be 

 common and should be considered when estimating re- 

 colonization rates for commercially harvested areas. 



Recolonization rates observed in this study should be 

 considered maximum estimates for recolonization of com- 

 mercially harvested areas. SJC study sites were small, 

 and sea urchins were relatively abundant adjacent to 

 sites. Commercial fishermen harvest entire beds of sea 

 urchins before moving on to the next bed and prefer to 

 harvest at shallow depths (Pfister and Bradbury, 1996; 

 Kalvass and Hendrix, 1997; Bradbury'). Thus sea urchins 

 in adjoining habitats at the same depth would likely be 

 harvested, and only sea urchins inhabiting deeper waters 

 would be available for recolonization. 



Juvenile recruitment 



The unimodal size distribution of red sea urchins and the 

 rarity of juvenile red sea urchins in SJC suggest variable and 

 infrequent red sea urcliin recruitment in northern Washing- 

 ton. In central California, red sea urchin recruitment was 

 also rare; no major recruitment events occuiTed during a 

 ten-year period ( Pearse and Hines, 1987 ). Size structures of 

 other sea urchin populations north of Point Conception also 

 suggest low red sea urchin recruitment (Table 6). 



Commercial harvest may negatively affect juvenile re- 

 cruitment in several ways. Commercial harvest decreases 

 sea urchin densities in northern Washington (Pfister and 

 Bradbury, 1996; our study), potentially decreasing fertil- 

 ization rates and larval supply (Levitan et al., 1992). A 

 reduction in adult sea urchin density might also nega- 

 tively affect juveniles in benthic habitats because adult 

 sea urchins may provide associated juveniles with protec- 

 tion from predators (Duggins, 1981; Breen et al., 1985) 

 and provide an increased food source (Tegner and Dayton, 

 1977, but see Andrew and Choat, 1985). In southern and 

 northern California, 81% and 73%, respectively, of juve- 

 nile red sea urchins were found under the spine canopy 

 of adults (Tegner and Dayton, 1977; Rogers-Bennett et 

 al., 1995). In southern California, juvenile sea urchins 

 were less abundant in harvested sites than in control 

 sites (Tegner and Dayton, 1977). In northern California, 

 juvenile red sea urchins were rare or absent in completely 

 harvested sites but were present in similar numbers in 

 selectively harvested and control sites (Rogers-Bennett et 

 al., 1998). In British Columbia, 69% of juvenile red sea 

 urchins were associated with adults (Breen et al., 1985). 

 Adults may be less important to juveniles in SJC than in 

 other west coast areas because few (4.3%) juvenile green 



