EFFECTS OF BOTTOM RELIEF AND FISH GRAZING ON THE DENSITY OF THE GIANT-KELP, MACROCYSTIS

A series of SCUBA surveys of subtidal reefs in the mainland Southern C alifornia Bight were used to relate the density of giant kelp (Macrocy stis), kelp-grazing fish, kelp-fouling organisms, and sea urchins to b ottom relief. On natural and artificial reefs, adult giant kelp plants were more common on low-relief substrates, i.e., on hard substrates l ying less than 1 m above the surrounding sand. Conversely, juvenile ke lp, kelp-fouling organisms, and kelp-grazing fish were more common on high-relief substrates. There was no statistically significant relatio nship between the density of sea urchins and bottom relief. The effect s of fish grazing, but not the effects of abrasion and sea urchin graz ing, are probably much greater on kelp plants that are partially encru sted with fouling organisms. To study fish grazing, fouled sections of adult kelp plants were exposed on high and low relief. To study abras ion and sea urchin grazing, unfouled sections of adult kelp plants wer e similarly exposed. On both artificial and natural reefs, fouled sect ions of kelp plants lost significantly more tissue on high relief; unf ouled sections did not. The results indicated that the relationship of giant kelp density to bottom relief was produced by differences in fi sh grazing which were, in turn, produced by the higher densities of ke lp-grazing fish or kelp-fouling organisms on high relief. The data did not suggest that the relationship between kelp density and bottom rel ief 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.