Varoujean and Williams 



Chapter31 



Abundance and Distribution in Oregon and Washington 



and flow periods each lasting 6-7 hours, and with the hourly/ 

 daily variations in sea state that occur during the summer 

 months along the Oregon coast, rip-current plume configuration 

 on the scale of hours changes quickly. Additionally, plume 

 development occurs in a sequential manner along various 

 stretches of shore, because of the consistent differences 

 between areas in the timing of the tide cycle. For example, 

 between Coos Bay and the Siuslaw River, a distance of about 

 80 km, the onset of tidal ebb or flow consistently occurs 20 

 minutes earlier off Coos Bay. Given the various factors that 

 can contribute to spatial and temporal variability in murrelet 

 abundance near tidal and rip-current plumes, Varoujean and 

 Williams (1987) felt that they could have been counting the 

 same birds in more than one location on different days. For 

 the same reasons, we presently feel that there is also the 

 possibility that birds can be counted more than once on any 

 one day when boat surveys are conducted over extensive 

 (>40 km) stretches of coastline, especially if both an onshore 

 and offshore line are being surveyed on the same day. For 

 example, to carry out a boat survey on both an onshore and 

 offshore line over a 45 km length of coast would require 

 about 6 hours at 15 km/hr, over which time tidal conditions 

 and sea state would change decidedly. It would be reasonable 

 to assume that during this period every murrelet encountered 

 on transect would be expected to take flight at least once, 

 and, if they landed on a yet to be surveyed segment of a 

 transect, counted again. At 80 km/hr (Varoujean and others 

 1989) a murrelet can take flight, and overtake a vessel that 

 has passed it, or fly offshore into the path of a returning 

 survey vessel in a matter of minutes. The probability of again 

 encountering birds behaving in this manner are high. For 

 example, onshore and offshore survey lines are typically 

 located within 500 m and between SCO m and 1,000 m from 

 shore, respectively. A transect width of 200 m (i.e., 100 m on 

 each side of the vessel) samples 70 percent of the zone out to 

 500 m, given that the first 100 m from shore, the surf zone off 

 a sandy beach is infrequently inhabited by murrelets. Similarly, 

 a 200 m wide transect located between 500 m and 1 ,000 m 

 would sample 50 percent of this zone. Consequently, the 

 probability of counting any one bird, including birds that 

 have already been counted elsewhere and moved into the 

 path of the boat again, is 0.7 in the onshore zone, and 0.5 in 

 the offshore zone. 



Two adjustments were made to compensate for the 

 potential sources of overestimation in calculating a population 

 estimate based on the 1980's boat survey data. First, only the 

 mean density estimate for transects out to and beyond 3 km 

 of river mouths (13.5 birds/km 2 ) was used in the calculation. 

 Second, a conservative estimate of 470 km 2 was used as the 

 areal extent of suitable nearshore habitat over which it was 

 applicable to apply the density estimate. This area was 

 calculated using a study area width of 1 km, even though 

 one-quarter of the survey transects were located 1.0-1.5 km 

 from shore, and a north-south, straight line distance of 470 

 km as a measure of the length of the Oregon coastline. A 

 population estimate based on the product of mean density 



( 2 s.e.) and 470 km 2 of habitat is 4,850-7,850 birds. Note 

 that the 1993 aerial survey estimate of 6,400-6,600 birds 

 falls within this range. 



In sharp contrast to the concordance between the 

 population estimates of the 1980's boat surveys and 1993 

 aerial survey, is the disparity between these estimates and 

 the nearly three-fold higher population estimate of 15,000- 

 20,000 murrelets by Strong and others (this volume). This 

 estimate is based on the results of boat surveys conducted in 

 Oregon during the summers of 1992 and 1993, and is subject 

 to the same overestimation errors just discussed in reference 

 to the 1980's boat survey data. Interestingly, if we do not use 

 a conservative approach in calculating a population estimate 

 for the 1 980' s boat survey data, but rather apply the unadjusted 

 density estimate of 23.2 birds/km 2 to a 750 km 2 study area 

 (500 km of actual coastline times a 1 .5-km-wide study area), 

 we get an estimate of 17,400 birds. 



Our aerial survey population estimates do not take into 

 account the possibility that murrelets may have been 

 distributed farther offshore than our offshore survey lines. 

 Both Strong and others (1993) and Ralph and Miller (this 

 volume) have shown that 20 percent of the murrelet 

 population may be located beyond 1,000 m from shore, i.e., 

 in waters deeper than 18m. Our data, however, indicate that 

 murrelet abundance declined sharply out to 1,000 m from 

 shore in Oregon and out to 2,000 m from shore in Washington. 

 But even if the 1993 aerial survey estimate for Oregon was 

 adjusted for the possibility that 20 percent of the murrelet 

 population was located offshore of our study area, the resultant 

 estimate of 8,200 birds would still be only about one-half 

 the estimate of 15,000-20,000 birds. So, in summary, we 

 suggest that 15,000-20,000 birds is an overestimate of 

 Marbled Murrelet abundance in Oregon. With reservations 

 associated with comparing the 1980's boat to 1993 aerial 

 survey results, we tentatively conclude that murrelet 

 population size has remained relatively stable in Oregon 

 over the last 10 years. 



Between 1978 and 1985, several censuses of Marbled 

 Murrelets were conducted along various routes along the 

 outer coast of Washington and the Strait of Juan de Fuca 

 (Speich and others 1992, Wahl and others 1981). Survey 

 platforms included the use of aircraft on flights "of 

 opportunity", and small boats. Even with differences in 

 sampling methodology, the density estimates (0.2-8.3 birds/ 

 km 2 ) reported by Speich and others (1992) for spring and 

 summer on the Washington outer coast do correspond to 

 those reported for the September 1993 aerial survey. But 

 their combined population estimate for the outer coast and 

 western Strait was no more than 2,600 birds, about 1,000 

 fewer birds than our September 1993 estimate of 3,400- 

 3,600 birds. This difference is likely attributable to the 

 conservative approach taken by Speich and others (1992) in 

 delineating the areal estimates over which they extrapolated 

 their density estimates. But it is also possible that the higher 

 1993 estimate is due to differences in the timing of the 

 respective studies. None of the surveys conducted by Speich 



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



335 



