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Pacific harbor seal, Phoca vitulina richardsi, census in California during May-July 2004
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  • Description:
    An aerial photographic census of Pacific harbor seals (Phoca vitulina richardsi) was conducted in California 18 May-19 July 2004 to document the number of seals hauled out during the molt period. To compensate for latitudinal differences in the timing of the molt, the state of California was divided into three sections and each section was surveyed on different dates. The three sections were: (1) Channel Islands and mainland coast of southern California from the U.S./Mexico border to Pismo Beach, (2) central California from Pismo Beach to Point Reyes and San Francisco Bay estuary, and (3) northern California from Point Reyes to the California/Oregon border. Aerial photographic surveys at the Channel Islands and the mainland coast of southern California occurred during the afternoon high-low tide cycle on 18-20 May and 1-3 June at tide levels of 0.43 m to 0.88 m and 0.40 m to 0.73 m, respectively. Surveys in central California occurred during the low-low tide cycle at tide levels of -0.03 m to 0.49 m on 22-25 June. Surveys in northern California occurred during the low-low tide cycle at tide levels of -0.34 m to 0.43 m on 5-9 July and 18-19 July (except at South Humboldt Bay, Eel River, and Cape Mendocino where tide levels were 0.49 m to 0.52 m, 0.89 m, and 0.98 m, respectively). Color transparency photographs of harbor seals were taken with a 126-mm-format camera equipped with image motion compensation. The latitude and longitude of each photograph was recorded by linking the camera to a computer and Global Positioning System (GPS). Two to three observers searched for hauled-out seals. Seals were photographed as the aircraft was flown over the coastline at 213 m (700 ft). A total of 563 haulout sites within the state of California were found during complete surveys conducted at a mean tide level of 0.28 m (-0.46 m to 0.98 m). At the Channel Islands there were 4,344 seals counted onshore, along the mainland coast of California and San Francisco Bay estuary there were 21,989, and statewide there were 26,333. The mainland coast of California was stratified into nineteen 0.5-degree latitude segments with counts from the San Francisco Bay estuary making up the twentieth stratum. Stratum 37.50E to 37.99E had the most seals (n = 3,360) and the most seals at a single haulout site (n = 939). Stratum 40.50E to 40.99E had the highest median (104) and mean (178.4) number of seals per haulout. Of the eight Channel Islands in southern California, Santa Cruz Island had the most seals (n =1,102) and Santa Barbara Island the fewest (n = 12). Total 2004 counts were greater than 2002 counts at mainland sites, but counts at the Channel Islands were relatively stable.
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