Ralph and Miller 



Chapter 33 



Offshore Population Estimates in California 



400 800 1400 2000 



4000 



5000 



6000 



DISTANCE FROM SHORE (m) 

 Figure 3 Distribution of Marbled Murrelets at distances from 1 00 m to 6,000 m from the shoreline. 



Extrapolation and interpolation of population estimates for survey strips based on known counts (shaded 

 bars) of the 400-, 800-, 1400-, 2000-, 3000-, and 5000-m transect distances. 



extensive survey of coastal sections where only two offshore 

 distances (800 m and 1,400 m) were surveyed. 



For the 800 m and 1,400 m distances of each coastal 

 section (fig. 2), we calculated the mean count, the standard 

 deviation of the count, and the correlation coefficient of the 

 two distances for all paired counts (2-km segments with 

 counts at both distances on the same day). These summary 

 statistics were used with the regression equation from the 

 intensive distribution to estimate the total number of 

 murrelets in each section (table 1) and the standard error of 

 that estimate. 



Summary Statistics: 



H 800 = number of counts in a section at 800 m 

 * 800 = mean count at 800 m 

 S 8oo = standard deviation of counts at 800 m 

 /I 1400 = number of counts in a section at 1400 m 

 Jc 1400 = mean count at 1400 m 



J 1400 



= standard deviation of counts at 1400 m 



r = correlation coefficient for pairs of 800-m and 



1400-m counts 



n = number of pairs of counts at 800 m and 1400 m 

 L = number of 2-km coastal segments 



The total numbers for all the sections are then summed 

 to obtain an estimate of murrelets in California and the 

 standard error of the total estimated. 



Section total = 



L (6.758 + 4.610 x goo + 4.624 x , 400 ) 



Results 



Coastal Distribution 



We estimate the total state population to be approximately 

 6,500 birds (table 1). The distribution of birds in the north 

 and central parts of the state were disjunct (fig. 2). The 

 highest densities of birds were found in the northernmost 

 part of the state, from the Oregon border to Trinidad in 

 Humboldt County. In most of this area, there was a density 

 of more than 4 birds/km 2 (48 birds per 2-km coastal segment, 

 12 km 2 ). This population includes approximately 4,000 birds. 

 Most of these birds were adjacent to, and contiguous with, 

 the old-growth forests in Del Norte and northern Humboldt 

 counties. These forests are largely on state and federal 

 parks, and composed primarily of coast redwood (Sequoia 

 sempervirens). From Trinidad south to False Cape 

 Mendocino, murrelet densities were generally less than 2.5 

 birds/km 2 . This population was adjacent to the old-growth 

 forests of Humboldt Redwoods State Park and the private 

 lands of Pacific Lumber Company, all in Humboldt County. 

 South of False Cape Mendocino, the densities of birds again 

 declined from 1.5 to 0.67 birds/km 2 in the area of Fort 

 Bragg and Albion. No birds were observed during surveys 

 between Albion and Half Moon Bay, several hundred 

 kilometers to the south. 



The central California population, comprising about 12 

 percent of the state's population, was estimated at 763 

 individuals and was located between Half Moon Bay in San 

 Mateo County and Davenport in Santa Cruz County. This 

 population was found primarily between Pigeon Point and 

 the mouth of Waddell Creek and was also offshore of old- 

 growth redwood forests, mainly in state parks. 



Discussion 



Censusing murrelets from boats is preferable to censusing 

 from the shoreline. During 2 years of surveys conducted 

 from observation points on the shoreline with 30 x telescopes, 



358 



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



