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

  • Tegner M
  • Dayton P
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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 Nino 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 earlier El Nino. Algal drift abundance, urchin recruitment rates, and changes in urchin density were followed at 5 sites 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 this, namely decreased reproduction, depauperate planktonic conditions, and altered current patterns, probably prevailed during the earlier El Nino as well. Algal drift did not become limiting during the recent event, but urchin grazing was triggered at one site in 1987 after an amphipod infestation had reduced kelp biomass, and urchin recruitment and migration had incrased grazing pressure. The Point Loma kelp forest showed significant recovery the year after the 1982-84 El Nino, something that took over 5 yr in the 1960s. Decline in the density of red urchins Strongylocentrotus 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 Nino of 1957-59 resulted from reductions in kelp standing stock and productivity below levels necessary to satisfy the existing grazing demand.

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  • M J Tegner

  • P. K. Dayton

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