Chapter 33 



Offshore Population Estimates of Marbled Murrelets 

 in California 



C. John Ralph 



Sherri L. Miller 1 



Abstract: We devised a method of estimating population size of 

 Marbled Murrelets (Brachyramphus marmoratus) found in 

 California's offshore waters. The method involves determining the 

 distribution of birds from the shore outward to 6,000 m offshore. 

 Applying this distribution to data from boat surveys, we derived 

 population estimates and estimates of sampling error. We estimate 

 a total California population of approximately 6,500 birds (+ 450). 

 Lower previous estimates of the Marbled Murrelet population in 

 California were derived from surveys conducted for different pur- 

 poses. Possible sources of error in our estimates are birds occurring 

 farther offshore than our surveys, incubating birds missed while on 

 nests, birds foraging underwater when the boat passed, double 

 counting flushed birds, and observer error in estimating distance to 

 birds. We feel that these sources of error compensated each other 

 or were minimal. 



The widespread Marbled Murrelet (Brachyramphus 

 marmoratus) breeds inland along coasts of the North Pacific 

 and is fairly abundant in many portions of its range. In 

 California, the murrelet forages for small fish and invertebrates 

 (Burkett, this volume) in nearshore waters, primarily within 

 five km of the coast. 



Because of the murrelets' secretive nesting habits at 

 inland conifer forests, and the unknown relationship between 

 the number of detections at inland sites and the number of 

 birds present, population estimates must be based on censuses 

 of birds at sea. Previous estimates of the population in 

 California have been derived from incidental data collected 

 during surveys of seabird colonies. Sowls and others (1980) 

 recorded observations of murrelets opportunistically while 

 travelling by boat between colonies. Birds were counted in 

 narrow strips at variable distances, within 1 km of shore. 

 They speculated that the breeding population in California 

 could be about 2,000 birds. In 1989, Carter and others ( 1990b) 

 systematically recorded murrelets along certain coastal 

 sections. Boat transects were parallel to and between 200 m 

 and 600 m out from shore. They estimated a population of 

 1,821 breeding birds. Few birds were seen south of Humboldt 

 Bay and only 5 birds between Cape Mendocino (just south 

 of Eureka) and Half Moon Bay, in Central California. 



To effectively use offshore survey data to estimate a 

 population of murrelets, we first needed to determine how 

 the birds are distributed in relation to the shoreline. Are they 



1 Research Wildlife Biologist and Wildlife Biologist, respectively. 

 Pacific Southwest Research Station, USDA Forest Service, Redwood Sci- 

 ences Laboratory, 1700 Bayview Drive, Arcata, CA 95521 



found in pelagic or nearshore waters, and at what depth or 

 distance from the shoreline do they most often forage? 

 From this information, appropriate survey techniques can 

 be developed which optimally survey murrelet populations 

 in the marine environment. 



The objectives of our work were to ( 1 ) determine the 

 distribution of murrelets from the shore outward in these 

 waters; (2) determine the distribution of birds in the varied 

 marine habitat along the coastline of the state; and (3) from 

 these data, estimate the population for California. 



Methods 



For each bird(s) detected during the surveys described 

 below, we recorded the number of individuals, their 

 perpendicular distance from the transect line, and 

 characteristics of plumage or behavior. A 40-cm fishing 

 buoy attached to a 1 00-m line was towed behind the boat and 

 used by observers as a reference for distance estimates. All 

 birds detected by the observer were recorded, including 

 flying birds. The crew of observers changed from year to 

 year, but some observers surveyed in all years. During each 

 season observers usually participated in surveys in all areas, 

 thereby reducing the bias of observer variability. 



Detection Distance 



We assumed that all birds on the transect line were 

 detected, but that some birds were missed as the distance 

 from the transect increased (Dixon 1977, Gould and Forsell 

 1989, Weins and others 1978). We calculated the "effective 

 area surveyed" (EAS), which allows an unbiased estimate 

 from all detected birds. Using this distance and transect 

 width, it is then possible to calculate the density of birds on 

 the water. The EAS of murrelets varied little from about 

 100 m, probably because of the limited range of sea 

 conditions under which surveys were conducted. We 

 discontinued surveys when seas reached 25 - 35 cm and 

 frequent whitecaps appeared (Beaufort scale 3 or 4). The 

 transect width also appeared to remain relatively constant 

 regardless of the observer platform heights on boats we 

 used: 7 m and 1-2 m. A 100-m transect width was used for 

 calculations below. The area surveyed on each 2-km survey 

 segment described below is therefore 200 m wide by 2,000 

 m long or 0.4 km 2 (fig. 1). 



Intensive Surveys 



In order to determine the distribution of birds outward 

 from the shoreline, we conducted intensive surveys from 



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



353 



