FISHERY BULLETIN: VOL. 73, NO. 2 



On 11 and 12 July 1971, a study site was chosen 

 and the data in Figure 2 labelled July 1971 were 

 collected. The differences between these data and 

 those given in the preceding paragraph give an 

 idea of the variation in this area. The inverse 

 relationship between the percent cover of 

 Laminaria and Agarum, in which the Laniinaria 

 decreases and the Agarum increases with depth 

 and sea urchin density, suggests that in shallow 

 water Laminaria competition suppresses the 

 growth of Agarum, but that Agarum, which has 

 been demonstrated to be highly distasteful to 

 Strong ylocenirotus drobachiensis (Vadas 1968), is 



Lominano spp 



i\ Agarum cribrosum 



o^A 



1 ^Aprll 1972 



,'T-<a Red olgal turf 



Depth (metere) 



Figure 2. -Mean sea urchin densities and percent covers of 

 Laminaria, Agarum, and red algal turf canopies at increasing 

 depths before (July 1971) and after (April 1972) the Laminaria 

 canopy was removed. Figure 2A contrasts the decreasing 

 Laminaria cover with increasing sea urchin density in July 1971. 

 Note that the>4 .9ar?/wi canopy cover at that time shown in Figure 

 2B is nearly complete only at those depths at which there is 

 reduced Laminaria coverage and relatively low sea urchin den- 

 sity. After removal of Laminaria, the Agarum canopy increased 

 dramatically at the shallower depths. The increase of red algal 

 cover after Laminaria removal is shown in Figure 2C. Variance 

 is presented as the 95% confidence interval around the mean. 



more successful in the presence of a moderate 

 density of grazers. Finally, Agarum itself may 

 also have an important competitive effect against 

 Alaria and the foliose red algal turf. Grazing 

 pressure and limiting light conditions probably 

 cause the severe reduction of foliose algae in 

 deeper water. These data demonstrating high 

 densities of sea urchins at depths below 20 m agree 

 with the observations of Barr (1971), Estes and 

 Smith (1973), and Estes and Palmisano (1974). This 

 suggests that sea otters at Amchitka do not forage 

 effectively below 18-20 m. 



That the experimental area could not be con- 

 tinuously monitored meant that it was not possible 

 to manipulate the sea urchin density, but compet- 

 itive effects of the algae at this depth were readily 

 testable by selective removal of algal species. 



Hypothesis III 



The presence of Laminaria spp. has no effect on 

 other algae. This hypothesis was tested by remov- 

 ing a 2-m wide strip of Laminaria from the area 

 where the data in Figure 2A were collected. The 

 hypothesis was negated as both Agarum and the 

 foliose red algae canopies significantly increased 

 their covers (Figure 2B, C). The spectacular 

 increase in the cover of the Agarum canopy cer- 

 tainly resulted partially from growth of the 

 fronds; however, samples taken in April 1971 and 

 repeated in July 1972 at approximately the same 

 spots along the experimental Laminaria removal 

 strip, showed that the mean Agarum density 

 increased significantly from 4.1 (± 0.6, SE; ten Vi 

 m^ samples) plants to 15.6 plants per Va m^ (was 

 calculated from ten 1/16 m- samples with a mean 

 of 3.9; +_ 0.4 SE). The increase in canopy cover of 

 the red algal turf was less spectacular, but a one- 

 tailed Wilcoxon matched-pairs signed-ranks test 

 of mean percent canopy cover at all depths con- 

 sidered shows a significant (P<0.005) general 

 increase after the Laminaria were removed, this 

 despite the fact that April may be early in the 

 season for red algal growth. Thus the Laminaria 

 canopy in the presence of an Agarum canopy has 

 an important effect on other algal species. 



Hypothesis IV 



The Agarum cribrosum canopy alone has no ef- 

 fect on the other algae. This hypothesis was tested 

 by clearing 45-85% covers of Agarum from 4 m- 



234 



