Hopkins Marine Station Student Paper

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Title: Haul-out effects of the Pacific harbor seal (Phoca vitulina richardsi) on the rocky midtidal community
Student Author(s): Horng, George
Hayhurst, Susan
Faculty Advisor(s): Watanabe, James
Pages: 30
Location: Final Papers Biology 175H
Date: June 1996
Abstract: This study, conducted at the Hopkins Marine Refuge in Pacific Grove, CA examined haul-out effects of the Pacific harbor seal (Phoca vitulina richardsi) on the rocky intertidal community. The study employed a nested sampling design, consisting of three replicate plots each within three haul-out intensity sites, low, medium and high. Plots were sampled using 20 X 30 cm quadrates haphazardly placed within a vertical range of 1.5 to 3 feet above MLLW.
Percent cover analysis of sessile species revealed no significant differences among haul-out intensities. Mastocarpus papillatus cover, however noticeably decreased as haul-out intensity increased. Conversely, Mazzaella affinis cover increased as haul-out intensity increased. Average algal biomass and frond length measurements also failed to demonstrate significant differences among haul-out intensities. However, trends of increasing biomass and decreasing frond length with increasing haul-out intensity were observed.
Due to the high level of heterogeneity in the rocky midtidal, only large differences among haul-out sites could be detected by our experimental design. A lack of significant diferences among haul-out sites does not rule out the possibility of actual impact by harbor seals. Observed trends indicate that haul-out of harbor seals may have a moderate impact on the rocky midtidal community.
Notes: Published in The Stanford Biologist, 1997 2:33-38