Suryan and Harvey: Variability in reaction to disturbance among Phoca vitulina richardsi 



333 



^ 



r-i^' 



obs. site 

 Skipjack Is. a^, ^ 



tection Island (Kroll, 1993) and 

 Woodard Bay (Calambokidis et 

 al."*), Washington. Sources of dis- 

 turbance to harbor seals ashore 

 at Gertioide Island, Washington, 

 were mainly unidentifiable; how- 

 ever, of detectable causes, human 

 activities and coyotes were the 

 most common (Moss, 1992). 



Reaction to disturbance may 

 vary among harbor seal groups 

 within an area (Terhune and 

 Almon, 1983) and according to 

 disturbance sources (e.g. power- 

 boats vs. canoes and kayaks; 

 Calambokidis et al.'*). This vari- 

 ability may be attributed to dif- 

 ferent levels of tolerance among 

 age, sex, or reproductive status 

 of harbor seals. Reaction to dif- 

 ferent causes of disturbance may 

 vary with exposure to particular 

 sources, eventually resulting in 

 greater avoidance or tolerance. In 

 any case, results of previous stud- 

 ies indicate that reaction to dis- 

 turbances vary within and among 

 regions, although little quantita- 

 tive evidence exists. 



In this study, we collected data to evaluate the 

 extent of disturbance to harbor seals at haul-out sites 

 in the northern San Juan Islands. Our objectives 

 were to determine 1 ) if human-related activities were 

 the primary source of disturbance; 2 ) if recovery var- 

 ied between flood and ebb tides and was similar 

 among the three haul-out sites (one location was a 

 pupping area); 3) if vigilance characteristics differed 

 among haul-out sites; 4) if the response to harass- 

 ment was similar for pups and for adults and sub- 

 adults; and 5) if the mean distance between harbor 

 seals and a boat causing a disturbance varied within 

 and among haul-out sites and, if so, to determine 

 potential causes of this variability. 



Methods 



Study area 



The study area was located in the northern San Juan 

 Islands, Washington (Fig. 1 ). This area is character- 

 ized by numerous islands, a tidal range of 3.6 m, strong 

 currents (maximum of 7.7 km/h), and a rocky shore- 

 line. Haul-out sites of harbor seals, which include reefs 

 and rockv intertidal zones of islands, are numerous but 



123° 00 W 



%/t 



o/-^ 



^O/i 



2Va 



Patos Is. ^, „ ^ 



Clements Reef 



Sucia Is.nf^-^^^-s obs. sites 



\ 



"^ 5<^^^^ Ewing Is 

 Matia Is 



48° 45 N - 



Puffin Is. 



Figure 1 



Locations (*) of harbor seal haul-out sites observed at Clements Reef, Puffin Island, and 

 Skipjack Island during 1991 and 1992 pupping seasons in the San Juan Islands, Wash- 

 ington. Dots ( • ) indicate other haul-out sites in the area. 



typically are used by fewer than 100 animals. During 

 this study, observations were conducted at Clements 

 Reef, Puffin Island, and Skipjack Island (Fig. 1). 



Observations of haul-out sites 



Ground-based surveys of harbor seals were conducted 

 at Clements Reef (« = 13), Puffin Island («=9), and 

 SkipjackIsland(n=8)from2 July to 19 August 1991. 

 In 1992, surveys were conducted between 24 June 

 and 10 September 1992 at Clements Reef (/2=21) and 

 Puffin Island (n-l8). Skipjack Island was not surveyed 

 during 1992 to allow increased sampling effort at the 

 other two sites. Seals at each location were observed at 

 least twice a week (one observer per site). Seals at 

 Clements Reef were viewed ft-om Ewing Island (Fig. 

 1), approximately 0.55 km away. Seals on the north- 

 west end of Puffin Island were viewed ft-om the south- 

 east corner of Matia Island, 0.38 km away (Fig. 1). 

 Observer heights above zero tide level were 10 m at 

 Clements Reef and 13 m at Matia Island. The observa- 

 tion point on the northeast side of Skipjack Island was 

 directly above (23 m) the haul-out site (Fig. 1). 



Observations of harbor seals began one to three 

 hours before low tide and ended three to seven hours 

 after low tide (when <50% of the maximum number of 

 seals counted during that tide cycle remained ashore). 



