Monster Seminar JAM - Implications of ecosystem restoration and novel disease for conservation of abalones in California
Dr. Glenn Van Blaricom, School of Aquatic and Fishery Sciences, University of Washington
Sustainability of harvest rates in exploited marine populations may be confounded by two factors, consumption by apex predators and increased mortality due to disease. In recent decades both factors have influenced the status of fisheries for abalones in California. Sea otters are known abalone predators off central California and alter the density, size distribution, and microhabitat distribution of abalones. Abalone populations outside the current sea otter range have higher density, are composed of larger individuals, and occur in more open microhabitats compared to populations in locations with sea otters. Demographic effects of sea otters are stronger than effects of harvest by people based on available data.
The translocation of sea otters to San Nicolas Island (SNI), California, in the late 1980s was intended to reduce population-level vulnerability of sea otters to oil spills, and to contribute to ecosystem restoration at SNI by reestablishing a top-level carnivore. The translocation project led to predictions of significant demographic effects on abalone populations and damage to abalone harvest rates at SNI. Long-term studies were focused on black abalones because of their extreme abundance and accessibility, and because of an extensive time series of data prior to release of sea otters at SNI. Black abalone population data have been collected periodically since 1981, and include numbers, sizes, and microhabitat use in permanent plots at nine shoreline study sites.
Release of sea otters had no measurable effect on abalone demography or harvest rate at SNI. However, beginning in 1992 black abalones at SNI experienced elevated mortality rates across all size classes as a result of an exotic microbial pathogen. The pathogen causes a fatal condition termed withering syndrome (WS). The periodic survey data indicate a 99% reduction in population size since WS appeared at SNI. Two indices of spatial isolation were derived from survey data, based on the premise that isolation is a reasonable surrogate for depensatory demographic effects in sessile or near-sessile invertebrates that have separate sexes and are broadcast spawners, such as abalones. Proportions of isolated individuals were lower than expected based on the degree of population reduction. There was a trend of increasing proportionate use of crevice microhabitats following the appearance of WS. Movement into crevice habitats may have been a behavioral response to reduced density, facilitating the preservation of aggregations despite reduced population size.
Based on the available data it is suggested that abalone populations are robust to intensive predation and disease mortality by virtue of strong behavioral preferences for aggregations of conspecifics in cryptic microhabitats, and as a result of intensive selection for disease resistance. For abalones and other species with similar life histories, the most effective conservation strategies may be those that facilitate the protection of aggregations in crevice habitats. Efforts to control predation or mitigate disease effects may in fact undercut natural selective processes that favor disease resistance and aggregative behavior in secure microhabitats. However, conservation strategies patterned after natural selective processes may result in drastic reduction of expected harvest rates for subject populations.
Date and Time:
January 15, 2004,
11:00 am - 12:30 pm