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

Abstract

Algal drift abundance, urchin recruitment rates, and changes in urchin density were followed in the Point Loma kelp forest near San Diego from 1983-1987. 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 an earlier El Nino (1957-1959). 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 increased grazing pressure. The Point Loma kelp forest showed significant recovery the year after the 1982-84 El Nino, something that took >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. The destructive grazing observed during and after the El Nino of 1957-59 thus apparently did result from reductions in kelp standing stock and productivity below levels necessary to satisfy the existing grazing demand. -from Authors