Varoujean and Williams 



Chapter 31 



Abundance and Distribution in Oregon and Washington 



determinations of presence/absence of a species. Data on 

 species abundance were not used. 



Aerial surveys, because of the speed of the aircraft, may 

 underestimate bird abundance through sighting error, i.e., 

 observers not seeing birds that are present on transect. In an 

 attempt to document how many birds observers miss seeing 

 at these speeds, tests of aerial observer sighting abilities 

 were conducted over fields of Marbled Murrelet decoys laid 

 out along three 50 m wide by 2 km long courses in Humboldt 

 Bay in February 1992. During the tests, a single engine 

 Cessna was flown at a speed of 175 km/h (110 mph) and an 

 altitude of 60 m ASL over the courses, while a single observer 

 recorded the number of sighted murrelet decoys. On each 

 course, six overflights were made, three with the observer on 

 the glare side of the aircraft, and three with the observer on 

 the non-glare side of the aircraft. The sighting error on the 

 non-glare side of the aircraft resulted in 9-30 percent of the 

 decoys being missed, but there were problems associated 

 with the layout of the courses (some were not straight, which 

 resulted in the plane flying over decoys), and with the density 

 of decoys in the courses. This density was ten times higher 

 than murrelet densities commonly encountered in coastal 

 waters, resulting in an unknown bias. Regardless of the test 

 shortcomings, it was felt that using a slower moving survey 

 aircraft would further reduce sighting error. For this reason, 

 the Partenavia, which is capable of flying at a speed of 145 

 km/h (90 mph) was used for the 1993 survey. Unfortunately, 

 decoy tests at this ground speed were not conducted. We feel 

 that sighting error was low (<10 percent), however, based on 

 the excellent viewing conditions that prevailed during the 

 survey period. Accordingly, the population estimates are not 

 increased because of sighting error. 



Potentially murrelets are also missed because they dive 

 in response to the approach of the survey aircraft. Our 

 experience indicates that murrelets were not avoidance diving 

 in response to the approach of the Partenavia. When surveying 

 in Oregon and Washington, the calmness of the sea's surface 

 and the water clarity allowed us to see birds below the 

 water's surface that had dived as, or just before, the plane 

 passed over them. Additionally, we noted the presence of a 

 concentric pattern of wavelets and, frequently, white 

 excrement at the point where the bird dove. Most of these 

 sightings were determined to be of Red-throated Loons (Gavia 

 stellata) and Western/Clark's Grebes (Aechmophorus 

 occidentalis/clarkii). No diving murrelets or murrelet-sized 

 alcids, and no concentric patterns of wavelets on a scale of a 

 diving murrelet-sized seabird were observed during either 

 survey. Therefore, the aerial survey population estimates are 

 not adjusted for bird avoidance-diving. 



An onshore and an offshore survey line, each running 

 approximately parallel to the coastline were flown in each 

 segment of the Oregon study area. The onshore line was 

 positioned so that the inboard observer was looking at water 

 just offshore of the breaking wave zone. This placed the 

 onshore survey line approximately 100 m from the shoreline. 

 At times, however, in both Oregon and Washington the 



plane was flown farther offshore (typically 300 m from 

 shore) to avoid disturbing seabirds on nesting colonies, and 

 pinnipeds on hauling-out areas. The pilot, who used a map 

 catalog of these sensitive areas as a reference, was made 

 aware of his approach to these areas by the position-data 

 recorder. Since the pilot would fly the plane in a half-circle 

 arc around these areas, portions of the habitat along the 

 onshore survey line lying just offshore of the wave zone, 

 were undersampled. The offshore survey line was located 

 1,000-1,200 m from shore, far enough offshore to include 

 the 18-m (10- fathom) bathymetric contour line. Given this 

 survey coverage, it was appropriate to consider the study 

 area for the Oregon coast to be a 1,000-m-wide band of 

 coastal water. This area extended from Point Saint George/ 

 Crescent City California north to Tillamook Head, an area of 

 approximately 500 km 2 . When reflected sun glare covered 

 20 percent or more of the field of view, only data from the 

 non-glare side was used (reducing the survey track from 

 100-m to a 50-m-wide band). Even with this restriction, 

 approximately 65 km 2 (13 percent) of the 500 km 2 study 

 area was surveyed. 



On the outer Washington coast, three sets of survey 

 lines running approximately parallel to the coastline, were 

 flown in each segment of the study area. As in Oregon, the 

 onshore line was positioned so that the inboard observer was 

 looking at water just offshore of the breaking wave zone. 

 The nearshore (i.e. middle) line was located 1,600-2,000 m 

 from the shoreline. As compared to the Oregon coast, the 

 continental shelf on the Washington coast is relatively broad; 

 hence, the offshore survey line had to be flown farther 

 offshore than in Oregon to include the 18-m (10-fathom) 

 bathymetric contour line. Consequently, the offshore line 

 was located 3,500-4,500 m from shore. Given this survey 

 coverage, it is appropriate to consider the study area for the 

 outer Washington coast to be a 4,000 m wide band of coastal 

 water extending from Tillamook Head north to Cape Flattery, 

 an area of approximately 1 ,065 km 2 . With sun glare at times 

 precluding surveying on one side of the aircraft, approximately 

 80 km 2 (7.5 percent) of the 1,065 km 2 outer Washington 

 coast study area was surveyed. 



Aerial surveys were also flown inside Grays Harbor, 

 Willapa Bay and the Columbia River during the Washington 

 study. The study area for each of the embay ments was based 

 on the size of an irregular-shaped polygonal area as defined 

 by the survey flight track. Only one complete survey was 

 flown inside each embayment. With such poor temporal 

 coverage of these tidally dominated waters, we feel it is 

 inappropriate to project population estimates for these specific 

 areas. Instead, the murrelets seen in and the survey effort for 

 these embayments are applied to the offshore coastal segment 

 adjacent to each embayment. 



Due to restrictions on the availability of the survey 

 aircraft, there was time to survey only one onshore line 

 along the British Columbia shore, and one onshore and one 

 offshore line on the Washington shore of the western Strait 

 of Juan de Fuca. The study area for the Strait was considered 



328 



USDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



