Differences in population structure and growth rate of the stalked barnacle Pollicipes polymerus between a rocky headland and an offshore oil platform

Population structure and growth rate of the stalked barnacle Pollicipes polymerus Sowerby differed between a rocky headland along protected outer coast (Goleta Point, Santa Barbara, California) and the supports of an 18 yr old oil production platform (Holly, ARCO) located 3 km offshore. The population structure at Goleta Pt. was positively skewed, with a maximum individual size of 26 mm capitulum height (CH). The population structure at Holly was distinctly bimodal, with a maximum individual size of 36 mm CH. The differences in maximum size were attributable to the much faster growth rate of offshore barnacles. A P. polymerusat 10 mm CH grew 0.114 mm d-' at Holly compared with 0.033 mm d-' at Goleta Pt. The skewed size distribution at Goleta Pt. was due to poor long-term sunrival, in part because of erosion of the soft shale rock substratum. The age of the platform, the stability of the pipe substratum, and intraspecific competition contributed to the bimodal size distribution of the offshore population. The factors which influence the growth and survival of P. polymerus at Holly may similarly influence these characteristics in other invertebrate species at the same offshore location.