Growth and predation by the ochre sea star,Pisaster ochraceus(Brandt), in Monterey Bay, California

Abstract

The growth rate of one laboratory and three field populations of the rocky intertidal sea star Pisasfer ochraceus from Monterey Bay, California, is considered. Both arm-length and weight were used to assess growth, but the latter measurement proved a more reliable one. The mean monthly increase in size for laboratory sea stars (on an unrestricted diet of Mytilus calijornianus) weighing less than 50 grams was 34.4 % while those weighing 800-1200 grams increased 7.1 %. Laboratory controls starved for 18 months lost 35.2 % of their body weight. Sea stars in the field (on a restricted diet both qualitatively and quantitatively) grew much more slowly than laboratory individuals. Most gained weight during the year, but much of this was associated with gonad development. Some sea stars in the field showed a net loss in weight during the year after spawning. The mean size of sea stars living primarily on mussels in the field was greater than that of animals living on acorn barnacles. The relationship between sea-star arm length and weight is presented. Although well-fed laboratory animals increased in both arm-length and weight, the same was not true for field sea stars. In the latter, increase in one measurement did not always accompany an increase in the other. An assessment of predation of P. ochraceus on M. californianus is made. It is calculated that P. ochraceus may consume up to 80 mussels (300 grams of soft parts) per sea star per year. The general predatory impact of t.he sea star on organisms in its environment is considered. A population of juveniles was examined, and was found to grow 0.4 grams in a three-month period. It is suggested that continual monitoring of a juvenile population of. known temporal origin may be the only reliable means of aging P. ochraceus.