Preliminary assessment of haulout behavior of harbor seals (Phoca vitulina) and sources of disturbance at the Spider Island reefs, Glacier Bay National Park

Mathews, E; US National Park Service, Glacier Bay National park and Preserve

Haulout patterns of harbor seals (Phoca vitulina richardsi) during the August molt (shedding) at a major terrestrial haulout in Glacier Bay National Park were markedly different in 1997 compared to previous years. This extreme change in haulout use, coupled with other observations made in 1996 and apparent increases in human activities near the haulouts, suggests that human disturbance may have altered harbor seal behavior in 1997. Between 1992 and 1996, three reefs west of Spider Island in the Beardslee Islands in Glacier Bay National Park were occupied by an average of 1,000 (n = 16; SD = 362) harbor seals during standardized aerial surveys conducted close to low tide in mid-August (Mathews 1995). This area is routinely used by more than half of the harbor seals found at terrestrial haulouts in Glacier Bay during both pupping (May-June) and molting (August) periods (Mathews 1992, Mathews 1997), and harbor seals have used these reefs for at least 15 years (Calambokidis et al. 1987, Lentfer and Maier 1989). During five of six surveys in August, 1997 I observed no seals at these reefs, and only about 100 seals were counted on the final day of the survey. Harbor seals traditionally exhibit strong year-to-year site fidelity for specific haulouts within a season (Pitcher and McAllister 1981; Brown and Mate 1983; Jeffries 1986; Thompson 1989). Peak visitation in Glacier Bay occurs primarily from June through August, exactly the months of sensitive phases in reproduction and molting for harbor seals. Harbor seals at haulouts are easily flushed into the water (disturbed) by humans. In order to reduce disturbance to seals and nesting shorebirds, the National Park Service (NPS) prohibits foot traffic and camping on Spider Island from May 31 to August 31. In addition, all vessels are supposed to remain at least ¼ nm (0.46 km) from the Spider Island reefs. Compliance with NPS wildlife regulations is incomplete and the effectiveness of NPS wildlife regulations is not known. This report summarizes preliminary observations made in 1996 and 1997 on haulout use near Spider Island during the annual molt in August. I also identify potential sources of disturbance of harbor seals at haulouts near Spider Island and suggest methods for further clarification of the sources of disturbance. If boaters are frequently flushing seals from haulouts, disturbance could be reduced through several mechanisms, including improved education of visitors, increased enforcement of existing regulations, and, if necessary, re-evaluation of existing regulations.