Six Years of Harbor Seal Censusing at Yaquina Estuary, Oregon
1985; Pacific Northwest Bird and Mammal Society; Volume: 66; Issue: 2 Linguagem: Inglês
10.2307/3533765
ISSN2325-713X
Autores Tópico(s)Maritime Navigation and Safety
ResumoHarbor seals (Phoca vitulina) regularly hauled out at Yaquina Estuary only during tides lower than about +0.3 m. Few seals (and no pups) were present during the pupping season in May and June. The greatest numbers hauled out in August or September; a smaller secondary peak appeared in February in two of six years. Seal abundance was about the same from 1977-1983, with the summer maximum ranging from 40 to 72 seals. There have been many studies of harbor seals (Phoca vitulina) hauling out in the Pacific Northwest (e.g. Johnson and Jeffries 1977, Calambokidis et al. 1979, Everitt et al. 1980, Brown 1981, Graybill 1981, Brown and Mate 1983, Roffe and Mate 1984). But with the exception of Brown and Mate's 4.5-yr project these studies were brief (i.e. two years or less) or discontinuous. As a result, the yearly variation in numbers for a particular haul out site is poorly understood. The purpose of this paper is to examine the within-year and yearly variation of harbor seals at the major haul out site of Yaquina Estuary, Oregon during a 6.4-yr continuous period and to contrast the seasonality of seals at Yaquina Estuary with that of other Oregon estuaries. STUDY AREA AND METHODS The 15.8-km2 Yaquina Estuary is located on the mid-Oregon coast in Lincoln County. It is comprised of a narrow mouth, several embayment areas, and an upper estuarine channel area (Fig. 1). Low tides were divided into three categories: low-low, intermediate, and high-low. Low-low tides were +0.3 m or less above mean lower low water (MLLW, 0.0 m), intermediate low tides were between +0.3 and +0.6 m above MLLW, and high-low tides were from +0.6 to +1.4 m above MLLW. Yaquina Estuary was divided into four regions (II-V) with a fifth region (I) just outside the estuary mouth (Fig. 1). Censuses consisted of single sweeps of each region with a 20 x telescope from roadside vantage points adjacent of each region with a 20 x telescope from roadside vantage points adjacent to the estuary. There was one census per day (when wind speeds were less than about 20 km/hr) that was made within 1 hr of the time of predicted low tide. Regions I and II were censused during 22 low-low tides, 14 intermediate low tides and 28 high-low tides from April through September 1982 and April through August 1983. Region II was also censused during 28 low-low tides and 9 high-low tides from December 1976 through August 1977. Region III was censused during 170 low-low tides and 31 high-low tides from October 1980 through October 1983. Region IV was censused during 393 low-low tides from June 1977 to October 1983, and region V was censused during 118 low-low tides; both regions were also censused during 31 highlow tides. RESULTS AND DISCUSSION Haul-out sites.-There were eight sites where seals hauled out in regions I-V (Fig. 1). Five of these sites (A, B, F, G, and H) were adjacent to the main Yaquina Estuary channel. Site A was on the tip of the largest rock finger perpendicular to the south jetty west of the Highway 101 bridge, sites B and G were intertidal mudflats, site F was composed of three rocks that were only exposed during low tides and could hold 1-3 seals, and site H consisted of oyster rafts. In contrast, sites C, D, and E were on intertidal mudflats adjacent to two subtidal channels whose banks were not exposed at tide heights above about +0.3 m above MLLW. Site C was near a channel about 70 m wide that flowed 44 THE MURRELET 66:44-49 SUMMER 1985 This content downloaded from 207.46.13.28 on Wed, 31 Aug 2016 05:21:06 UTC All use subject to http://about.jstor.org/terms BAYER: HARBOR SEAL CENSUSING IN OREGON FIGURE 1. Yaquina Estuary study regions. southwest into the Yaquina channel, and sites D and E were along a separate channel about 140 m wide that also flowed southwest into the Yaquina channel. Log booms and rafts were present upstream of H but were not used by seals. Seals did not haul out evenly throughout the estuary. They never hauled out at region I. At region II, two seals were observed once at site A during only one of 31 high-low tides, and no seals were at A for any intermediate low or low-low tide counts. At region III, only one seal at a time was at site B in one of 31 high-low tide censuses and in only 4% of 170 low-low tide censuses. At region V, two seals hauled out in one of 31 highlow tide censuses, and one seal hauled out in 2% of 118 low-low tide counts. There were no intermediate low tide censuses at either region III or V. When seals hauled out at Yaquina Estuary, they usually did so at region IV, where they were observed in 73% of 393 low-low tide censuses. Prior to the second time seals were caught at C (i.e. researchers from Oregon State University captured and tagged seals there on 28 July and 22 August 1983), seals hauled out at C in 71% of 135 low-low tide counts; seals were at sites D, E, F, or G in 1% or less of these censuses. After the second tagging operation, seals were still at C in 64% of 11 censuses (which was not significantly different than before tagging, Fisher's exact test, p > 0.10), and they were not at F or G. But they hauled out significantly more often after than before the second tagging at D (i.e. in 55% of 11 counts, Fisher's exact test, p < 0.001) and E (in 18% of 11 counts, Fisher's exact test, p < 0.001). During high-low tides, five or less seals were found in this region only at site F in 13% of 31 censuses; there were no intermediate low tide censuses at region IV. Frequency of hauling out.-The frequency of seals at region IV during low-low tides changed seasonally (Table 1). They were least likely to be present during the JanuaryMarch and April-June periods, when an average of 43-47% of the counts were without 45 SUMMER 1985 This content downloaded from 207.46.13.28 on Wed, 31 Aug 2016 05:21:06 UTC All use subject to http://about.jstor.org/terms
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