Behavioral ecology of young-of-the-year kelp rockfish, Sebastes atrovirens Jordan and Gilbert (Pisces: Scorpaenidae)

Young-of-the-year (YOY) kelp rockfish, Sebastes atrovirens, recruit initially to the canopy of the giant kelp, Macrocystis pyrifera (L.), along the coast of central California. During the summer of 1991, I observed major fluctuations in YOY kelp rockfish abundance in the canopy before the onset of a vertical migration downward to the Macrocystis holdfasts. Within the Macrocystis canopy, YOY kelp rockfish make a series of microhabitat shifts preparatory to their downward migration, forming smaller and more closely held aggregations. Experimental manipulations of Macrocystis architecture indicate that YOY kelp rockfish densities are reduced in areas with only small Macrocystis (few stipes) compared to unmanipulated areas with a range of Macrocystis sizes. Unusually large Macrocystis, created by binding multiple kelps together, did not compensate for a preponderance of small Macrocystis in the surrounding areas. In unmanipulated habitat, YOY kelp rockfish densities were correlated with the size of the Macrocystis (number of stipes). Habitat selection in the canopy and holdfasts appears to be behavioral and is closely tied to the architecture of the alga. The onset of the downward migration appears to be triggered by a combination of ontogenetic and environmental cues.