FISHERY BULLETIN: VOL. 82. NO. 1 



Vertical Patterns of Species, 

 Abundances, and Sizes 



Several of the commonly occurring species within 

 the Habitat Reef kelp forest were far more abundant 

 in the canopy than in the lower zones. Ampithoe plea, 

 Hyale frequens,Acanthomysis sculpta, and Hippolyte 

 clarki all displayed this type of distribution, and 

 other investigators have noted the canopy prefer- 

 ences of these species. Limbaugh (1955) described a 

 large canopy-dwelling amphipod {Ampithoe sp.) that 

 formed a tube by rolling and "stitching" the edge of a 

 Macrocystis blade. Several investigators working in 

 kelp forests off San Diego and at Habitat Reef have 

 noted the canopy occurrence of Aconthomysis 

 sculpta (Limbaugh 1955; Clutter 1967; Clarke 1971; 

 Hobson and Chess 1976) and H. clarki (Hobson and 

 Chess 1976). Lowry (unpubl., cited in Lowry et al. 

 1974) observed large numbers off/, californiensis, a 

 close relative of//, clarki, in the canopy of kelp forests 

 off central California. 



The canopy contained larger gammarids, mysids, 

 and shrimps as well as proportionately greater num- 

 bers of large individuals of these groups than in either 

 of the lower zones. Size-selective predation by fishes 

 frequently has been documented to be a major factor 

 in structuring aquatic communities (Brooks and 

 Dodson 1965; Archibald 1975; Vince et al. 1976; 

 Macan 1977; Nelson 1979) and may account for the 

 size distributions of invertebrates observed at 

 Habitat Reef. The interdigitating fronds of the 

 canopy greatly increase the structural complexity in 

 this zone and may offer more spatial refuge for motile 

 invertebrates than provided by the middle and bot- 

 tom zones. As increased structural complexity has 

 been demonstrated to decrease effectiveness of prey 

 capture by fishes, particularly larger prey (Vince et al. 

 1976; Brock 1979; Coen et al. 1981; Heck and Tho- 

 man 1981; Savino and Stein 1982), the canopy com- 

 plexity may discourage extensive foraging by 

 fishes. 



Relatively few fishes forage within the kelp 

 canopies off southern California. The most abundant 

 fish is the kelp perch, Brachyistius frenatus, a small 

 diurnal species that forages preferentially in the 

 canopy and preys extensively on small gammarids 

 and copepods (Hobson 1971; Bray and Ebeling 

 1975; Hobson and Chess 1976). Other fishes are 

 observed in the kelp canopy, but the large-mouthed 

 species are much less abundant than the kelp perch 

 and forage more often in other areas of the kelp 

 forest, and the small-mouthed species capture small 

 planktonic prey or utilize small invertebrates 

 attached directly to the kelp surfaces (Bray and E bel- 



ing 1975; Hobson and Chess 1976; Bernstein and 

 Jung 1979). Consequently, predation pressure on 

 larger individuals of motile prey in the canopy may be 

 reduced relative to the lower zones, resulting in a pro- 

 portionately greater abundance of larger individuals. 

 For example, the mysid S. pacifica was much more 

 abundant in the lower zones than in the canopy, yet 

 the largest individuals consistently were present in 

 the canopy. 



Alternate hypotheses may explain the size 

 stratification of some species. Intraspecific behav- 

 ioral interactions may confine certain size classes to 

 specific zones, as demonstrated experimentally for 

 an amphipod (Van Dolah 1978). Larger individuals 

 may be more abundant in the canopy simply in re- 

 sponse to the presence of preferred food types and/ 

 or sizes, although this hypothesis has not been 

 examined. 



The size distribution of invertebrates in the lower 

 zones resembled the size distribution of insects in 

 temperate terrestrial forests (Schoener 197 1), in that 

 both areas supported large numbers of small, and few 

 large, individuals. The size distribution in the 

 canopy, however, was somewhat similar to the insect 

 size distribution of tropical terrestrial forests where 

 there are proportionately greater numbers of large 

 insects (Schoener and Janzen 1968; Schoener 1971). 

 The presence of larger insects in the tropical forests 

 effectively expands the food size dimension relative 

 to the temperate forests (assuming equal abundance). 

 The expansion has been hypothesized to account for 

 some of the increased diversity of bird species in the 

 tropics, as much of this increase is due to the addition 

 of insectivorous birds adapted to capture large 

 insects (Schoener 1971). 



In contrast to the tropical forests, the higher propor- 

 tion of large prey items in the Habitat Reef kelp 

 canopy apparently did not attract additional species 

 of fish predators. Nevertheless, it may be useful to 

 examine the size distributions of important prey 

 items in other kelp forests to determine whether a 

 relationship exists between prey size distributions 

 and fish species diversity. 



Seasonal Patterns of Species, 

 Abundances, and Sizes 



The kelp-associated invertebrates as a group did 

 not exhibit seasonal cycles. Numbers and biomass 

 generally were highest during winter 1975, with the 

 marked increase in biomass due primarily to 

 increased abundances of the relatively large canopy 

 mysid A. sculpta and shrimp H. clarki. Gammarid 

 amphipods, particularly M. litotes, were largely re- 



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