Variability in reactions of Pacific harbor seals, Phoca vitulina richardsi, to disturbance

Disturbances to harbor seals, Phoca vitulina richardsi, during 1991 and 199 2 pupping seasons were observed at Puffin Island, Clements Reef, and Skipja ck Island in Washington state. Harassment (greater than or equal to one sea l entering the water) of seals ashore was common (greater than or equal to 71% of survey days) and primarily caused by powerboat operators approaching to observe seals. Recovery (number of seals on a haul-out site returned to preharassment levels) following a harassment was less at Puffin Island (19 %) than at Clements Reef(54%) and Skipjack Island (45%). Additionally, seal s were more vigilant (P<0.003) at Puffin Island than at the other two locat ions. These results indicated that seals at Puffin Island were less toleran t of disturbance than seals at other sites. This could possibly be attribut ed to a greater (P<0.05) percentage of pups ashore (17%) than at Clements R eef (3%) and Skipjack Island (3%). Because of this, we expected that powerb oats would disturb seals from greater distances at Puffin Island. To test t his, we used a theodolite to determine distance between seals and an approa ching vessel at Puffin Island and Clements Reef. There was, however, no sig nificant (P>0.05) difference in distances at which disturbances occurred. T he most notable difference in distance of disturbance was between initial a nd subsequent harassments during a haul-out period. Those seals remaining o r returning to shore after a harassment were more tolerant of powerboats, a llowing significantly (P<0.05) closer approaches than those initially haras sed. Seals detected (head raised and oriented toward the potential disturba nce) a powerboat at a mean distance of 264 m, and harassments occurred when boats approached, on average, to within 144 m. Results of this study exemp lify the variability in reaction to disturbance and the necessity for consi dering these differences for minimizing disturbance.