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Shellfish Mariculture in Drakes Estero, Point Reyes National Seashore, California
(Adams et al., 2006). Trends in the abundance of harbor seals across the whole California coast are also monitored less frequently by state and federal agencies (Lowry and Carretta, 2003).
The data used to assess annual trends in abundance and distribution of harbor seals within Drakes Estero are from surveys made during the peak pupping season (March 15–June 1) and molting seasons (June 1–July 30) (Hester et al., 2004). During each two-hour survey, hauled-out seals were counted every 30 minutes. The timing of these surveys was determined primarily by the need to provide the most robust estimates of abundance trends. The time and source of all disturbances were also recorded throughout the observation period. Disturbances are listed as head alerts, flush (seals move toward but not into water), and flush to water (seals leave haul-out and enter water). Although a head alert indicates a potential for a more serious response, it is less likely to represent an action that depresses fitness or has a negative population-level consequence, and may be less informative as a response variable (Jansen et al. 2006). Assessment of population trends relies upon the assumption that observer bias and individual haul-out behavior and, thus detectability, have not changed over this period. Haul-out behavior could change as a function of prey availability or the level and types of disturbance.
Collectively, the data from these observer programs suggest that seals using the eight subsites within Drakes Estero are best considered a single unit, within which individual subsite choice may be influenced by factors such as disturbance (Allen, 1988). Mixing occurs between these seals and those at other local colonies, and there is more limited exchange among colonies outside the region. Consequently, any changes in abundance within the Drakes Estero/Limantour colony will result from a complex interaction between broader-scale drivers and local factors, such as disturbance. As examples, during the 1998 El Niño, adult and pup counts were depressed throughout the entire Point Reyes region, and in 2003 a northern elephant seal (Mirounga angustirostris) disrupted the Double Point harbor seal colony in Point Reyes, killing about 40 seals and probably inducing emigration of others, judging from temporally corresponding increases in pup counts at Drakes Estero and Bolinas Lagoon (NPS, 2006a; Becker et al., 2009). Abundance trends within Drakes Estero should therefore be considered in relation to wider-scale population trends, but the time series of data is limited to the past 11 years (1997 through 2007), which is not sufficient to make a robust comparison with trends at other sites in the Point Reyes region with even fewer years of standardized count data.
There has been one statistical modeling study that tested for potential impacts of mariculture activity on harbor seals. Becker et al. (2009) examined how oyster mariculture activities are related to both interan-