FISHERY BULLETIN: VOL. 73, NO. 2 



DISCUSSION 



The pattern emerging from these and other 

 (McLean 1962; Lowry and Pearse 1973; Estes and 

 Palmisano 1974) studies of sea otter-dominated 

 communities is that by consuming the populations 

 of invertebrate herbivores, the sea otter has an 

 extremely important role in maintaining the 

 structure of shallow algal communities. In this 

 study, high densities of sea urchins are found 

 below 18-20 m, suggesting that this depth is the 

 lower limit of effective sea otter foraging in this 

 area. It is interesting to note that this depth is 

 much more shallow than the 30-fathom profile 

 speculated by Kenyon (1969). In addition, this 

 seems to be a much more shallow limit to efficient 

 foraging than is exhibited by the California 

 population of sea otters, as I have seen evidence of 

 their foraging to at least 30 m in the Carmel Bay 

 region. 



Strong competitive interactions between 

 species of benthic algae appear well expressed in 

 the shallow nearshore waters of the Aleutian 

 Islands which have sea otters. The shallower (3-5 

 m) waters, subject to severe storm disturbance, 

 are functionally dominated by Laminaria species. 

 When the larger Laminaria spp. (L. groenlandica, 

 L. dentigera, and L. yezoensis) are removed, either 

 experimentally or by natural storm disturbance, 

 their space is quickly utilized by Alaria fistulosa. 

 In contrast, the rhizomelike holdfast with multiple 

 meristems of L. longipes appears to be an effective 

 adaptation to disturbance, as it allowed quick 

 regrowth of stipes and fronds after their 

 experimental removal. In deeper water (12-20 m), 

 where there are many sea urchins, Agarum 

 cribrosum is one of the dominant algal species. 

 Agarum, however, loses in competition for light to 

 solid canopies of Laminaria spp., which have erect 

 stipes supporting their fronds above the nearly 

 prostrate Agarum. When freed from Laminaria 

 competition, Agarum significantly increases its 

 cover and abundance. When both Laminaria and 

 Agarum are removed, there is a bloom of red algal 

 turf and of Alaria fistulosa. These tests of 

 competition-based hypotheses are probably valid 

 despite the various depth-related changes in the 

 physical environment because each was compared 

 to immediately adjacent controls. 



It is interesting to note that despite having po- 

 tentially long-lived individuals and the competi- 

 tively superior adaptation of a floating canopy. 



Alaria fistulosa behaves as a fugitive species with 

 its densest distribution in the highly disturbed 

 immediate offshore area, occurring farther 

 offshore only in areas where two understory 

 canopy levels are removed or by growing on 

 Laminaria stipes. This is surprising because quite 

 the opposite situation seems to exist in the 

 southern California kelp community, where 

 Macrocystis pyrifera forms a heavy surface 

 canopy which may inhibit the growth of the un- 

 derstory species (North and Shaef f er 1964; Dayton 

 unpubl. data). Although Alaria was observed in 

 depths of over 25 m, its lower distribution appears 

 to be restricted primarily by sea urchin grazing. 



Other research (Estes and Palmisano 1974; 

 Palmisano in prep.) contrasts the nearshore and 

 intertidal communities of Amchitka with nearby 

 otter-free islands and convincingly demonstrates 

 the powerful role the sea otters have in structuring 

 the nearshore community. This paper has 

 experimentally demonstrated competitive trends 

 between different canopy guilds in an algal com- 

 munity which contains an unusually high number 

 (four) of Laminaria species which have semirigid 

 stipes. It is tempting to speculate an evolutionary 

 hjrpothesis in which the sea otters reduce the her- 

 bivore pressure and thus allow a competitive 

 differentiation of niches of these large stipitate 

 kelps. Such hypothetical evolutionary thought has 

 the common and serious flaw of ignoring the roles 

 of extinct species, many of which may have left 

 large and important "vacant niches" (such as those 

 left by the mammal extinctions of the late Pleis- 

 tocene discussed in Martin and Wright 1967). This 

 problem is particularly acute in the Bering Sea, as 

 Steller in 1751 (reference in Card et al. 1972) 

 reported the giant sea cow, Hydrodamalis gigas 

 (Zimmermann 1780), eating algae in the nearshore 

 and tidal beaches of the Komandorskiye Islands. 

 The large populations reported by Steller and 

 various Russian and German sailors of this huge 

 (ca. 10 tons, Scheffer 1973) kelp-eating (Stejmeger 

 1936) sirenian surely had important consequences 

 to the kelp populations that weaken any present 

 day speculation of the evolutionary consequences 

 of kelp competition. It may be reasonable, 

 however, to pose the hypothesis that by consuming 

 invertebrate herbivores, particularly sea urchins, 

 the sea otter was indirectly responsible for the 

 high productivity of large algae necessary to 

 maintain the sea cow populations. Such an 

 hypothesis is supported by the overlap of the otter 



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