Effect of Giant Kelp on Local Abundance of Reef Fishes: The Importance of Ontogenetic Resource Requirements

The size and structure of populations of many rocky inshore fishes in southern California, USA, can be profoundly influenced by the canopy-forming giant kelp, Macrocystis pyrifera. Forests of giant kelp undergo substantial spatial and temporal variation which in turn can influence the local (among-reef) abundance of substrate-oriented fishes. The effects of kelp are strongly related to the resources required by different life history stages of fishes. Giant kelp has a positive, direct effect on local abundances of species that use it as a nursery ground and/or adult habitat. Kelp can also indirectly affect abundances of fish. These indirect effects, which can be positive or negative, result from the shading of understory algae by kelp. Reef fishes with similar ontogenetic resource requirements appear to share a common relationship with the presence and density of kelp.