A series of SCUBA surveys of subtidal reefs in the mainland Southern California Bight were used to relate the density of giant kelp (Macrocystis), kelp-grazing fish, kelp-fouling organisms, and sea urchins to bottom relief. On natural and artificial reefs, adult giant kelp plants
were more common on low-relief substrates, i.e., on hard substrates lying less than 1 m above the surrounding sand. Conversely, juvenile kelp, kelp-fouling organisms, and kelp-grazing fish were more common on high-relief substrates. There was no statistically significant relationship between
the density of sea urchins and bottom relief. The effects of fish grazing, but not the effects of abrasion and sea urchin grazing, are probably much greater on kelp plants that are partially encrusted with fouling organisms. To study fish grazing, fouled sections of adult kelp plants were
exposed on high and low relief. To study abrasion and sea urchin grazing, unfouled sections of adult kelp plants were similarly exposed. On both artificial and natural reefs, fouled sections of kelp plants lost significantly more tissue on high relief; unfouled sections did not. The results
indicated that the relationship of giant kelp density to bottom relief was produced by differences in fish grazing which were, in turn, produced by the higher densities of kelp-grazing fish or kelp-fouling organisms on high relief. The data did not suggest that the relationship between kelp
density and bottom relief was produced by abrasion, sea urchin grazing, or kelp recruitment. Finally, the data suggest that a reef intended to support giant kelp should be designed to minimize bottom relief and the number of shelter crevices suitable for kelp-grazing fish.
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