No abstract

Disease outbreaks can have substantial impacts on wild populations, but the often patchy or anecdotal evidence of these impacts impedes our ability to understand outbreak dynamics. Recently however, a severe disease outbreak occurred in a group of very well-studied organisms–sea stars along the west coast of North America. We analyzed nearly two decades of data from a coordinated monitoring effort at 88 sites ranging from southern British Columbia to San Diego, California along with 2 sites near Sitka, Alaska to better understand the effects of sea star wasting disease (SSWD) on the keystone intertidal predator, Pisaster ochraceus. Quantitative surveys revealed unprecedented declines of P. ochraceus in 2014 and 2015 across nearly the entire geographic range of the species. The intensity of the impact of SSWD was not uniform across the affected area, with proportionally greater population declines in the southern regions relative to the north. The degree of population decline was unrelated to pre-outbreak P. ochraceus density, although these factors have been linked in other well-documented disease events. While elevated seawater temperatures were not broadly linked to the initial emergence of SSWD, anomalously high seawater temperatures in 2014 and 2015 might have exacerbated the disease’s impact. Both before and after the onset of the SSWD outbreak, we documented higher recruitment of P. ochraceus in the north than in the south, and while some juveniles are surviving (as evidenced by transition of recruitment pulses to larger size classes), post-SSWD survivorship is lower than during pre-SSWD periods. In hindsight, our data suggest that the SSWD event defied prediction based on two factors found to be important in other marine disease events, sea water temperature and population density, and illustrate the importance of surveillance of natural populations as one element of an integrated approach to marine disease ecology. Low levels of SSWD-symptomatic sea stars are still present throughout the impacted range, thus the outlook for population recovery is uncertain.

No abstract

In recent decades, the world has witnessed an array of harmful invasions by exotic marine organisms. To provide the public and policymakers with better information on the status of exotic species in southern California waters, and to assess differences between port and non-port areas, a Rapid Assessment Survey of selected habitat types in sheltered waters between San Diego and Oxnard was conducted in the summer of 2000. The objectives included comparing the prevalence of exotic species among habitats and regions and between recent and past surveys; obtaining reference data for future assessments of changes in invasion status and the effectiveness of prevention or control efforts; detecting new invasions; and documenting significant range extensions. Twenty-two sites were sampled to include the three major commercial port areas in southern California, non-port-area marinas and lagoon sites. Sampling included dock fouling and adjacent soft-bottom benthos, nearby intertidal sites, and selected subtidal lagoon habitats. Samples were collected by a variety of manual techniques. Sixty-nine of the species collected are exotic, including representatives from two algal divisions and six invertebrate phyla. Ascidians are especially well-represented (14 exotic species) and widely occurring, and some bivalves and bryozoans also occur very widely. The numbers and proportions of exotic taxa were not significantly greater in port areas than in non-port areas.

The size structures of Lottia gigantea populations were studied at 8 southern California sites, 4 of which were longstanding Marine Protected Areas (MPAs). Greater mean sizes and higher frequencies of larger individuals occurred at sites with lowest human visitation. Mean L. gigantea shell lengths (SLs) were negatively correlated with the number of visitors and collectors per 10 m of shoreline. Limpets never achieved SLs greater than 79.0 mm at any of our 8 sites, a size well below the maximum for this species. Mean SLs were comparable to sizes at other sites where L. gigantea is intensely collected. The presence of collectors, small mean SLs, and the absence of large limpets suggest that L. gigantea populations are affected by humans at our sites. Lower densities and higher frequencies of larger limpets were found in a subpopulation living on open-rock surfaces compared with a subpopulation occupying smaller, patch habitats within mussel beds. Larger limpets were mostly female and gonadal mass increased exponentially with SL in both subpopulations. Limpets < 40 mm SL exhibited highly variable growth rates; however, limpets in patch habitats grew slower and produced less gonadal mass than limpets inhabiting open rocks, despite the availability of more microalgal food. Ages of the largest limpets in the open-rock subpopulation were estimated to be >8 yr. Given the age of larger limpets and the potential for collectors to remove larger individuals and shift populations towards smaller size structures, effective MPAs may take decades to reverse the effects of human impacts on L. gigantea populations in southern California.