THE EFFECT OF DISTURBANCE ON HARBOR SEAL HAUL OUT 

 PATTERNS AT BOLINAS LAGOON, CALIFORNIA 1 



Sarah G. Allen, David G. Ainley, Gary W. Page, 2 

 and Christine A. Ribic 3 



ABSTRACT 



We studied harbor seals at Bolinas Lagoon, California, from May 1978 to June 1979. Field observation 

 and two time lapse motion picture cameras were used to monitor the numbers of seals and of distur- 

 bances, and to provide information on tidal height. Peak numbers occurred during the summer. During 

 nonbreeding seasons, high numbers occurred at low tides, and during the breeding season they 

 occurred in early afternoon except when haul out areas were flooded. Seals were disturbed by humans 

 on 71% of days monitored; people in canoes were the primary source of disturbance. Human activities 

 closer than 100 m caused seals to leave haul out sites more than activities at greater distances. 



Several studies exist on the haul out patterns of 

 harbor seals, Phoca uitulina, in undisturbed loca- 

 tions (Scheffer and Slip 1944; Venables and Vena- 

 bles 1955; Richardson 1975 4 ; Pitcher 1977 5 ; 

 Loughlin 1978), but the effects of human activities 

 on haul out patterns have been examined in- 

 frequently (Newby 1971; Paulbitsky 1975; Chap- 

 man 1979 6 ). We report here how daily and seasonal 

 haul out patterns of harbor seals can be modified 

 by human activity in a small estuary, Bolinas La- 

 goon, Calif. The data also provide a baseline 

 against which the effects of pending increased 

 levels of human activity could be compared. 



Since 1970, a state quarantine has reduced 

 human activities in the contaminated waters of 

 Bolinas Lagoon. Human use has been confined to 

 bird watching, some boating, illegal clam digging, 

 beach combing, and recreational bait fishing. 

 When the quarantine is lifted, many of these ac- 

 tivities will increase. Increased human activity 

 could also result from provisions included in the 

 General Management Plan of the Golden Gate 



Contribution No. 256 of the Point Reyes Bird Observatory. 



2 Point Reyes Bird Observatory, 4990 Shoreline Highway, Stin- 

 son Beach, CA 94970. 



3 Department of Ecology and Behavioral Biology, University of 

 Minnesota, Minneapolis, MN 55455. 



4 Richardson, D. T. 1975. Assessment of harbor seal and 

 gray seal populations in Maine. Contract report to the U.S. 

 Marine Mammal Commission, Washington, D.C., 37 p. 



5 Pitcher, K. W. 1977. Population productivity and food 

 habits of harbor seals in the Prince William Sound - Copper River 

 Delta area, Alaska. U.S. Department of Commerce, Final Re- 

 port to Marine Mammal Commission, Contract No. MM4AC009, 

 37 p. 



6 Chapman,D. 1979. The effects of recreational activities on 

 the harbor seal, Phoca vitulina. [Abstr.] American Mammal 

 Association Annual Meeting, Corvallis, Oregon, June, 1979, p. 

 80. 



National Recreation Area (June 1979) for a walk- 

 in camp site and parking area along the lagoon. 



Little information exists on harbor seals at 

 Bolinas Lagoon. Carlisle and Alpin (1966, 1971) 

 estimated numbers as part of a statewide aerial 

 count, but their figures for Marin County were low 

 compared with preliminary data collected by Gary 

 W Page (unpubl. data). More recently, Mate's 

 (1977) 7 monthly statewide counts failed to detect 

 any seals in Bolinas Lagoon. 



STUDY AREA AND METHODS 



Bolinas Lagoon, a 448 ha estuary 24 km north of 

 San Francisco, is a Marin County Nature Preserve 

 and is part of the Golden Gate National Recreation 

 Area. Triangular in shape, it is bordered by paved 

 roads, pasture land, the small community of 

 Bolinas, and a 3 km long sand spit covered with 

 houses. At the end of the spit there is a 60 m wide 

 opening to the ocean. The major channel used by 

 the seals passes by Kent Island (KI) and 

 Pickleweed Island (PWI) and cuts north along the 

 northeastern shore (Fig. 1). KI and PWI remain 

 above water when tides exceed 1.7 m above mean 

 low water level and are the two main seal haul out 

 areas. 



Movie cameras recorded the activity of seals and 

 humans in the vicinity of KI and PWI. A Canon 8 



Manuscript accepted February 1984. 

 FISHERY BULLETIN: VOL. 82, NO. 3, 1984. 



7 Mate, B. R. 1977. Aerial censusing of pinnipeds in the 

 eastern Pacific for assessment of population numbers, migratory 

 distributions, rookery stability, breeding effort, and recruit- 

 ment. U.S. Dep. Commer., N.T.I.S. PB-265 859, 67 p. 



8 Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



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