Sea urchins, El Ninos, and the long term stability of Southern California kelp forest communities

Southern California kelp forests experienced major losses during the El Nino of 1957-1959. The proximal reason for the decline was ascribed to sea urchin grazing, and the eventual recovery of some forests followed sea urchin control efforts, kelp restoration, and improvements In sewage disposal practices. The very strong El Nirio of 1982-84 allowed us to reexamine hypotheses regarding the interactions between kelps, sea urchins, and warm water, and to speculate about what happened during the earller El Nino Algal drift abundance, urchin recruitment rates, and changes in urchin density were followed at 5 sltes in the Point Loma kelp forest near San Diego from 1983 to 1987. Sea urchin recruitment rates were very low during 1982-84 and the apparent reasons for thls, namely decreased reproduction, depauperate planktonic conditions, and altered current patterns, probably prevailed during the earher El Nlrio as well. Algal drift did not become limiting during the recent event, but urchin grazing was triggered at one slte in 1987 after an amphipod infestation had reduced kelp biomass, and urchin recruitment and migration had increased grazing pressure. The Point Loma kelp forest showed significant recovery the year after the 1982-84 El Nirio, something that took over 5 yr in the 1960s. Decline in the density of red urchins Strongj~locentrotus franciscanus since the mid 1970s suggests that the fishery for this species, which became extensive in the late 1970s. was an important contributor to the faster kelp recovery. In the absence of evidence for increased recruitment or temperature effects on grazing demand, it appears that the destructive grazing observed during and after the El NiAo of 1957-59 resulted from reductions in kelp standing stock and productivity below levels necessary to satisfy the existing grazing demand.