Graduate Thesis Or Dissertation
 

Population dynamics, annual food consumption, movements, and dive behaviors of harbor seals, Phoca vitulina richardis, in Oregon

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  • Number of harbor seals, counted on 32 haul-out areas in Oregon, increased by 6 - 8.8 % per year from 1975 - 1983. Percent of seals in bays has increased an average 1.8%/year, from 47% in 1975 to 61% in 1983. Along the central Oregon coast, harbor seals were most abundant during January and April. Four environmental factors, height of low tide, amount of rain, wind speed, and wave height, were correlated significantly with number of harbor seals on land. A greater precentage of radio-tagged harbor seals were found on land during summer and spring counts (X = 53.1%), than during autumn and winter (X = 9.2%). There were an estimated 9,023 - 20,018 harbor seals in Oregon during winter 1985. The results of experiments with captive harbor seals indicated that only 25 - 34% of some fish species (e.g. Engraulis mordax, Clupea harengus, and Thaleichthys Dacificus) was represented as otoliths in feces, whereas with other species it was 80%. Over 80% of the otoliths were excreted within 24 h of ingestion. Mean reduction in otolith length (X = 27.5%, SD = 10.3) was not significantly different among fish species. Harbor seals caught in Oregon (n = 214) were an average 126 cm in length and 55.7 kg in mass. Average dive duration for six individuals ranged from 1.0 min (SD = 0.7) to 3.1 min (SD = 1.8). Maximum dive duration was 11.4 min. Radio-tagged individuals moved as far as 280 km, but 92% of time were found within 8 km of the release site. Harbor seals fed primarily on fishes of 8 - 15 cm standard length. Generally they consumed smaller fish in spring and summer and larger fish during fall and winter. Juvenile fishes (e.g. Parophrys vetulus and Citharichthys sordidus) were probably consumed by harbor seals in estuaries, whereas adults were eaten in the ocean. In 1980, 5 species of fishes, Leptocottus armatus (721.4 metric tons), Clupea harengus (451.4 metric tons), Cymatogaster aggregata (440.8 metric tons), Parophrys vetulus (427.8 metric tons), and Glyptocephalus zachirus (332.6 metric tons), comprised 42.5% of a total 5,584.9 metric tons of fish estimated consumed by harbor seals in Oregon.
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