Faunal patterns and dispersal on kelp rafts in
Southern California

Alistair J. Hobday, Ph.D.
Pacific Fisheries Environmental Laboratory

Wednesday, April 14, 1999
3:00 p.m.—Pacific Forum

This dissertation examines the faunal patterns on drifting Macrocystis pyrifera (kelp) rafts in the Southern California Bight. Kelp rafts form when kelp detaches from the substrate. Rafts retain some of the kelp forest fauna and provide a potential transport mechanism between the isolated kelp forests. In addition, rafts are colonized by both "pelagic" and larval kelp forest fauna as they drift. The patterns of faunal abundance change with time spent drifting.

An assemblage of fish, molluscs and crustaceans is found on kelp rafts. Some of this fauna has direct development and limited dispersal due to poor swimming ability. As a result, these species cannot disperse between kelp forests alone. The persistence time of these species on kelp rafts influences the connection distances between kelp forests. Forests separated by distances that rafts cannot travel, or by long drift times, will become isolated and may lose species that are only transported by kelp rafts. While on the rafts, some species are successful and increase in abundance, while other species are unaffected, or decline in abundance with raft age. A combination of biotic and abiotic factors can explain these changes. When a raft arrives at a kelp forest, the influx of new animals may be important in restocking.

Loss of kelp forests along the coast, either naturally during El Niņo events, or through human activities such as dredging, may increase the separation between kelp beds. This will reduce the frequency of the connections that may be maintaining the integrity of the kelp forest community.

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