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



337 



harassment of harbor seals at a greater distance than 

 do operators of powerboats (Calambokidis et al.''); 

 therefore, as sea kayaking becomes more popular, 

 there is a greater potential for disturbance of harbor 

 seals ashore. 



The differences in occurrence of unknown causes 

 of disturbance among haul-out sites was possibly a 

 result of haul-out site topography. Harbor seals on 

 Clements Reef had a 360° view of potential sources 

 of disturbance compared with roughly a 270° view 

 for Puffm Island and 180° for Skipjack Island. A dis- 

 turbance of unknown origin at Skipjack Island would 

 often begin by several harbor seals looking toward 

 the rocky cliff of the island, then entering the water. 

 A high incidence of disturbances of unknown origin 

 have also been documented at Gertrude Island ( 7T7( ; 

 Moss, 1992) and Protection Island (43%; Kroll, 1993), 

 Washington. The relatively high occurrence of ha- 

 rassments by bald eagles at Puffin Island may have 

 been due to a nearby bald eagle nest and the high 

 percentage of harbor seal pups (immature eagles 

 were observed harassing female and pup pairs ). Skip- 

 jack Island also had an active bald eagle nest, but 

 eagles were not observed harassing harbor seals. 



Overall, only 39% of all harassments resulted in 

 full recovery, indicating seals often remained in the 

 water or moved to a different site. Allen et al. ( 1984) 

 reported that the number of harbor seals that re- 

 turned to a haul-out site after a disturbance in 

 Bolinas Lagoon, California, was always less than the 

 original number, and in most cases, harbor seals did 

 not move to a nearby reef Murphy and Hoover"^ re- 

 ported that harbor seals off the Kenai Qords, Alaska, 

 often searched for a new haul-out site after harass- 

 ment. Disturbance to harbor seals, therefore, may 

 have considerable impact where haul-out space is 

 limited (Murphy and Hoover'^). Although haul-out 

 sites in the San Juan Islands are numerous, alter- 

 nate sites for female and pup pairs, similar to Puffin 

 Island, may not be readily accessible (particularly 

 since there tended to be less recovery of seals at Puf- 

 fin Island). 



Terhune (1985) compared aggregation behavior 

 and vigilance of harbor seals with flocking behavior 

 of avian species — a behavior that allows individuals 

 to decrease their surveillance without decreasing the 

 probability of detecting a predator (Caraco, 1979; 

 Studd et al., 1983). Da Silva and Terhune ( 1988) iden- 

 tified group size as the only factor accounting for 

 variation in time taken to scan for predators. Renouf 

 and Lawson (1986) suggested that only males in- 

 creased scanning time as mating season approached, 

 and scans were related to important events in their 

 mating system, not predators. Results of our study 

 indicated that increased vigilance may be related 

 more to potential "predators" (loosely defined as any 

 source of disturbance, human or animal). We found 

 that seals at an area with a greater percentage of 

 female-and-pup pairs scanned more frequently than 

 those at other locations. Other researchers have de- 

 scribed increased vigilance of females with pups. 

 Stein ( 1989 ) reported female harbor seals rested alert 

 significantly more frequently when their pups were 

 one to nine days old than when pups were older 

 Newby (1973) reported a female harbor seal with a 

 pup is "constantly alert and nervous." 



The greater vigilance of harbor seals at Puffin Is- 

 land than at Clements Reef and Skipjack Island and 

 the lack of recovery from a harassment indicated that 

 seals at a pupping location were affected more by 

 disturbance. We therefore expected that seals at 

 Puffin Island would enter the water when powerboats 

 were farther away, in contrast with harbor seals at 

 Clements Reef. This was not the case, there were no 

 significant diff"erences between sites. We cannot, how- 

 ever, conclude that seals at pupping locations toler- 

 ated boats to approach just as closely as seals at 

 nonpupping sites without harassment because of dif- 

 ferences in geographic characteristics of haul-out 

 sites, unreplicated sites, and lack of power to detect 

 a statistically significant difference. 



Bishop (1967) observed that a nucleus of harbor 

 seals, which usually included several very large ani- 

 mals, remained ashore unless danger became immi- 



