Abundance and Distribution of Harbor Seals (Phoca vitulina) in Oregon, 1975-1983

Abstract

Harbor seals were observed on 32 haulout sites in Oregon during aerial counts conducted from 1975 to 1983; 90% were seen on 14 sites. The greatest number of seals seen on a haulout was 985 recorded at Cape Arago in July 1982. Counts of harbor seals in 1982 and 1983 were 38.6% greater than counts from 1975 to 1980. These data indicate an increase in numbers of harbor seals in Oregon, an increase corroborated by other information, namely, lower counts made before 1975 and increased use of new haulout sites since 1975. Between 1975 and 1983, numbers within bays increased, whereas numbers on most offshore rocks remained somewhat constant. Decreased harassment and mortality since implementation of the Marine Mammal Protection Act of 1972 doubtless allowed harbor seals to increase in numbers and to reoccupy protected haulout sites in bays. The harbor seal (Phoca vitulina richardsi) is one of five pinniped species common in Oregon. It is the most abundant and ubiquitous pinniped in the state, found in most bays and estuaries, and on many offshore rocks. Harbor seal abundance has been determined at specific haulout sites along the Oregon coast (Graybill 1981; Brown and Mate 1983; Roffe and Mate 1984; Bayer 1985), although there is only one published account of their state-wide distribution and abundance (Pearson and Verts 1970). From 1967 to 1968, Pearson and Verts (1970) conducted four counts from land and one aerial count along the Oregon coast and concluded that there were fewer than 500 harbor seals in the state. More than 3800 harbor seals were killed in Oregon between 1925 and 1972 by statehired and bounty hunters (Pearson 1969). Harbor seals were harassed at inshore areas where they rested and pupped, and their number in Oregon was probably reduced. Harassment and killing of harbor seals were greatly reduced after 1972 when the Marine Mammal Protection Act became effective. Protection probably has resulted in an increase in abundance and redistribution of animals into bays and estuaries. The objectives of this study were to determine the location and use of haulout sites and to document any changes in abundance and distribution of harbor seals in Oregon from 1975 to 1983. METHODS Aerial photographic surveys of seals on land along the entire Oregon coast were performed annually from a high-winged, single-engine aircraft (Cessna 172 or 182) during 1 month in the summer (June, July, or August) from 1975 to 1983 (Fig. 1). Surveys were begun approximately 2 hr before low tide and ended approximately 2 hr after low tide to maximize the number of seals visible on haulout sites (Schneider and Payne 1983; Terhune and Almon 1983). The entire Oregon coastline was scanned during annual surveys to determine locations of harbor seals ashore and to locate new haulout sites. When more than five seals were ashore, the group was photographed from an altitude of 215-305 m using a hand-held 35 mm camera and 300 mm or 70 to 210 mm zoom lens. Photographs were taken within 15° of vertical. Photographic images of seals on land were projected onto a white surface, and each seal was counted. Photographic surveys required 2 days; flights were made from Newport to the north or south on consecutive days. Surveys of some haulout sites were not possible because of inclement weather.