Rocky intertidal community monitoring at Channel Islands National Park: 2017–2018 annual report

Whitaker, Stephen

doi:10.36967/2299633

Channel Islands National Park includes the five northern islands and the surrounding waters out one nautical mile off the coast of southern California: San Miguel, Santa Rosa, Santa Cruz, Anacapa, Santa Barbara. There are approximately 176 miles of coastline around the islands, about 80% of which is composed of rock. The diversity and undisturbed nature of the tidepools of this rocky coastline were recognized as special features of the islands in the enabling legislation. To conserve these communities unimpaired for future generations, the National Park Service (NPS) has been monitoring the rocky intertidal communities at the islands since 1982. Sites were established between 1982 and 1998. Sites were selected after considering visitation, accessibility, presence of representative organisms, wildlife disturbance, and safety. This report summarizes the 2017–2018 sampling year efforts from November 2017 to January 2018, and findings of the Channel Islands National Park (CHIS)/Mediterranean Coast Network Rocky Intertidal Community Monitoring Program. Specific monitoring objectives are to: 1) determine the long-term trends in percent cover of key sessile organisms in the rocky intertidal ecosystem, and 2) determine population dynamics of black abalone (Haliotis cracherodii), owl limpets (Lottia gigantea), and ochre sea stars (Pisaster ochraceus). Objectives were met by monitoring percent cover of target species in target intertidal zones using photoplots and transects, and measuring size frequency and abundance of black abalone, owl limpets, and sea stars using fixed plots or timed searches. Twelve target species or assemblages, as well as the tar substrate, have been monitored twice per year at 21 sites on the five park islands as part of the rocky intertidal community monitoring program. Fixed photoplots were used to monitor the percent cover of thatched and acorn barnacles (Tetraclita rubescens, Balanus glandula/Chthamalus spp., respectively), mussels (Mytilus californianus), rockweeds (Silvetia compressa, and Hesperophycus californicus), turfweed (Endocladia muricata), goose barnacles (Pollicipes polymerus) and tar. Point-intercept transects were used to determine the percent cover of surfgrass (Phyllospadix spp.). Information about size distribution (i.e., “size frequency” data) was collected for owl limpets in circular plots. Size distribution and relative abundance of black abalone and ochre sea stars were determined using timed searches. The maximum number of shorebirds and pinnipeds seen at one time were counted at each site. The number of concession boat visitors to the Anacapa Island tidepools was collected and reported. This was the second year that we officially reduced our sampling interval from twice per year, spring and fall, to one sampling interval in the fall. This was changed to streamline the program and allow for the implementation of additional protocols (Raimondi et al. 2018). Additionally, not all sites were monitored for all species in 2017–2018. Weather conditions during the site visits were satisfactory, but high wind coupled with strong swell and surge limited or prevented the completion of some of the abalone and sea star searches. The percent cover for most key species or assemblages targeted in the photoplots was highly variable among sites. Mussel, Mytilus californianus, cover remained below average at Anacapa Island and Santa Barbara Island. Record or near record-low abundances for M. californianus were measured at the following sites: Anacapa Middle West, East Point on Santa Rosa Island, Harris Point on San Miguel Island, Prisoner’s Harbor and Scorpion Rock on Santa Cruz Island, and Sea Lion Rookery on Santa Barbara Island. The only sites that appeared to have above average Mytilus cover were Cuyler Harbor and Otter Harbor on San Miguel Island, Johnson’s Lee on Santa Rosa Island, Landing Cove (slightly) on Santa Barbara Island, and Willows Anchorage on Santa Cruz Island. All other sites had mussel cover near or below the long-term mean. Qualitatively, Mytilus cover appeared to be increasing at most locations as newly recruited mussels were common last year and most appear to have persisted this year. Both rockweed species, Silvetia compressa and Hesperophycus californicus, continued to decrease markedly in abundance this year at most sites compared to combined averages for previous years. Fossil Reef on Santa Rosa Island was again the only site that exhibited above average cover of rockweed. Most sites exhibited marked declines in S. compressa abundances beginning in the early 2000s with little recovery observed for the rockweed through this year. Barnacle (Chthamalus and Balanus spp.) cover fell below the long-term means at all islands except Anacapa whereby barnacle cover was slightly above average. Endocladia muricata abundance remained comparable to the grand mean calculated for previous years at Santa Barbara Island, while cover of the alga decreased slightly below the long-term means at Anacapa, San Miguel, Santa Cruz, and Santa Rosa islands. Black abalone (Haliotis cracherodii) abundances at the islands remain less than one percent of 1985 population levels. Zero abalone were found throughout the entire site at Landing Cove on Santa Barbara Island and South Frenchy’s Cove on Anacapa Island. Above average abundances relative to the long-term mean generated from post-1995 data were observed at all but four sites. Juvenile black abalone were seen at all islands except Santa Barbara. Ochre sea star (Pisaster ochraceus) populations crashed in 2014 at all monitoring sites due to sea star wasting syndrome, an illness characterized by a suite of symptoms that generally result in death. The mortality event was widely considered to be the largest mortality event for marine diseases ever seen. Beginning in June 2013, the disease swiftly and significantly impacted P. ochraceus and other species of sea star populations along the North American Pacific coast from Alaska to Baja California, Mexico. By the beginning of 2014, P. ochraceus abundances had declined by >95% at nearly all Channel Islands long-term intertidal monitoring sites, in addition to numerous other locations along the West Coast. At various times during the past decade, extremely high abundances about 500 P. ochraceus) have been observed at multiple sites, and most locations have supported >100 sea stars counted during 30-minute site-wide searches. This year, abundances ranged 0–28 individuals per site with the majority having fewer than 10 P. ochraceus seen during routine searches. Insufficient numbers of sea stars were seen to accurately estimate the size structure of P. ochraceus populations. Giant owl limpet densities in 2017–2018 were comparable or slightly above the long-term mean at all sites except Ford Point and Johnson’s Lee on Santa Rosa Island. Exceptionally high densities were measured at Fossil Reef on Santa Rosa Island and Otter Harbor on San Miguel Island. The sizes of L. gigantea this year varied among sites and islands. The smallest L. gigantea were observed at Johnson’s Lee followed closely by Willows Anchorage and Fraser Cove on Santa Cruz Island and the largest were seen at Northwest-Talcott on Santa Rosa Island. Temporally, the mean sizes of L. gigantea in 2017–2018 decreased below the long-term mean at each island, particularly Santa Rosa. Surfgrasses (Phyllospadix spp.) are typically monitored biannually at two sites each on Santa Cruz and Santa Rosa Islands. This year, all transects at each of the monitoring sites except East Point on Santa Rosa Island were sampled during the fall/winter season. At East Point, the conditions were not conducive to sampling the surfgrass transects, but qualitatively, percent cover of surfgrass appeared to be near 100% on all three transects. Relative to past years, cover of surfgrass increased above the long-term mean at the three sites that were sampled. Overall, the abundances and diversity of shorebirds in 2017–2018 at all sites appeared similar to observations made in recent years with the exception of slightly elevated numbers of cormorants (Phalacrocorax spp.) observed at Crook Point on San Miguel Island, and East Point and Fossil Reef on Santa Rosa Island. Black oystercatchers (Haematopus bachmani) were the most ubiquitous shorebird seen at all sites. Black turnstones (Arenaria melanocephala) were not common relative to past years. Pinniped abundances remained comparable in 2017–2018 to historical counts for all three species that are commonly seen at the islands. Harbor seals (Phoca vitulina) were seen at in the vicinity of six sites this year. As in past years, harbor seals were most abundant at Otter Harbor on San Miguel Island. Elephant seals (Mirounga angustirostris) were seen at three sites during the year whereby abundances ranged 1–3 individuals per location. California sea lions (Zalophus californianus) were common at Santa Barbara Island where 140 individuals, mostly juveniles, were observed. One to two sea lions were also seen nearby Crook Point, Cuyler Harbor, Otter Harbor, and Harris Point on San Miguel Island, and Johnson’s Lee on Santa Rosa Island. Relative to past years, abundances this year were considered average at other locations.

Rocky intertidal community monitoring at Channel Islands National Park: 2017–2018 annual report

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