FISHERY BULLETIN: VOL. 73, NO. 2 



Methods of estimating percent canopy cover 

 varied. The Alaria canopies represent visual es- 

 timates. The 100% covers were very thick and in 

 these cases the floating stipes seemed to form an 

 almost impenetrable wall in the water column. A 

 few photographs taken of the Alaria canopy in 

 areas where it had less than 100% cover suggest 

 that the visual estimates in these locations were 

 conservative. The other percent cover estimates 

 were made with the aid of 0.25 or 0. 16 m" quadrats 

 which, in larger areas, were placed haphazardly, 

 and in restricted experimental areas were placed 

 systematically in such a way that the entire 

 experimental area was sampled. The actual 

 measurements were usually taken planimetrically 

 from photographs as defined earlier (Dayton 

 1971). There were a number of cases in which 

 visual estimates were used because of camera 

 malfunction, running out of film, etc. I have com- 

 pared such visual estimates with planimeter 

 measurements and found that they are usually 

 within 5% and always within 10% of each other 

 (Dayton 1971, 1975). The data are presented as 

 means because the actual sample numbers varied 

 (but except where stated, were never fewer than 

 10); the variance is given as standard error. 



RESULTS 



Shallow Area 



This area is covered with an extremely thick 

 growth of algae and is generally characterized by 

 a conspicuous absence of herbivores (Estes and 

 Palmisano 1974). I was surprised to find sea 

 urchins^ among the Laminaria (especially L. lon- 

 gipes) haptera and holdfasts upon removing the 

 canopies for the experiments discussed below. The 

 sea urchins may exist in these sheltered refuges 



Opinions are divided whether the Amchitka sea urchin is 

 Strongylocentrotus drobachiensia orS. polyacanthus. 



because the canopy is both very dense and rela- 

 tively close (25-35 cm) to the substratum, thus 

 seriously reducing the foraging efficiencies of 

 their visual predators. This sea urchin-refuge 

 hypothesis was supported by the observation that 

 the sea urchins remained untouched in both clear- 

 ings from 3 and 6 July through 8 July, but all were 

 gone on 9 July. I suspect that they were taken by a 

 sea otter that found the cleared patches, as one 

 was observed foraging in the vicinity on the 

 morning of 9 July. However, predation by the 

 common eider, Somateria mollissima (Williamson 

 and Emison 1969), and emigration are other pos- 

 sible explanations. At any rate, the small size (< 15 

 mm) and scarcity of these sea urchins do not 

 seriously affect the contention that the herbivores 

 have largely been eliminated from this area. The 

 elimination of the grazing pressures makes the 

 competition-based hypotheses discussed below 

 more meaningful. 



Hypothesis I 



The Alaria fistulosa canopy excludes 

 Laminaria spp. This hypothesis was tested (a) by 

 cutting Alaria from several rocks and observing 

 whether Laminaria recruited in the absence of 

 Alaria and (b) by cutting Laminaria and observ- 

 ing potential Alaria recruitment. Alaria and 

 probably Laminaria spp. were fertile at the time 

 of the cutting. Significantly more Laminaria 

 recruitment into Alaria clearings than into 

 uncleared controls would support the hypothesis, 

 whereas significantly more Alaria recruitment 

 into Laminaria clearings than into the control 

 would negate the hypothesis and suggest the truth 

 of the converse hypothesis, that Alaria behaves as 

 an opportunistic or fugitive species (Dayton 1973, 

 1975) in the presence of competition with the 

 competitively dominant Laminaria spp. The 

 results of such clearings at a depth of 5 m (done 3 

 and 4 July 1971) are presented in Table 1. The 



Table 1. -Effects of canopies of Alaria fistulosa and Laminaria spp. on each other and on the cover of red algae in the nearshore 

 experimental area (25 m-) at 3-5 m depth. The data are presented as percent cover with the variance presented as the 95% confidence 

 interval about the mean. Data presented without variance were visual estimates. Control no. 1 suffered heavy algal loss from winter 

 storms. The mean density of A. fistulosa in the April 1972 Laminaria removal experiment was 14.7 ( ± 1.1, SE) in ten 100 cm- quadrats. 



'Signifies that the canopy was experimentally removed. 

 ^Canopy ripped out during winter storms. 



232 



