Fish populations in kelp forests without sea otters: effects of severe storm damage and destructive sea urchin grazing

Abstract

Southern Californian kelp forests without Enhydra lutris disappear and reappear with episodic events such as severe storms, outbreaks of sea urchin grazing, and widespread plant recruitment. The response of reef-fish assemblages to this variability was observed by monitoring species abundances off Santa Barbara, one site being transformed from forest to urchin-dominated barrens for 3 yr. Compared to a "control' group at the kelp-forested site inshore, the semi-isolated offshore assemblage sustained a net loss in total fish abundance during the barrens period and underwent a change in structure. While most of the change was due to kelp-forest loss, some was attributable to differential recruitment success at the onset of an El Nino episode of southern water intrusion. All but a few southern Californian reef fishes are adapted to local, unpredictable losses of kelp and other stands of macroalgae if rocky relief is high. Invading sea otters would tend to stabilize the offshore kelp-forest community by checking any destructive sea urchin grazing brought about by loss of drift kelp supplies or episodic urchin recruitment. Thus, the long-term effect of otters would be to decrease spatio-temporal variability of fish assemblages within the mosaic of inshore and offshore areas of reef and kelp. Locally, this effect would bring about a net increase in fish abundance and diversity, although some macroinvertebrate eaters might experience increased competition for available food. -from Authors