Chapter 32 



Distribution and Population Estimates of Marbled Murrelets at 

 Sea in Oregon During the Summers of 1992 and 1993 



Craig S. Strong 



Bradford S. Keitt 



William R. Mclver 



Clifford J. Palmer 



Ian Gaffney 1 



Abstract: We used standardized transect techniques to count Marbled 

 Murrelets and other seabird species at sea from a boat and from a 

 low-flying light aircraft along the length of the Oregon coast. The 

 focus of effort was on vessel surveys of the central Oregon coast. In 

 both years, Marbled Murrelets were most abundant in central 

 Oregon, between Cascade Head and Cape Arago. They were con- 

 centrated much closer to shore in 1992 than in 1993. Different 

 distribution patterns in the two years was likely a consequence of 

 El Nino oceanographic conditions which severely impacted Oregon's 

 seabirds in 1993. New population estimates for the state ranged 

 from 2,500 (shore-based) to 22,250 birds (boat). Estimates gener- 

 ated from vessel surveys were considered far more reliable than 

 estimates from air or from shore counts due to more thorough 

 coverage, proximity to birds, more observers, and longer scanning 

 time. Vessel estimates using both strip and line transect analyses 

 for two years with very different distribution characteristics each 

 produced state population totals between 15,000 and 20,000 birds, 

 after accounting for some assumptions. There is a strong possibility 

 that a large proportion of these birds may not be nesting success- 

 fully due to limitations of nesting habitat and other factors. 



In the past 6 years, research effort on the Marbled Muirelet 

 (Brachyramphus marmoratus) has increased in response to 

 an apparent dramatic decline in their numbers on the west 

 coast south of British Columbia (Carter and Erickson 1992; 

 Marshall 1988; Nelson and others 1992; Ralph, this volume). 

 Their recent listing as a federally threatened species (U.S. 

 Fish and Wildlife Service, 1992) adds a further imperative to 

 learn more of this bird's nesting and at-sea biology, population 

 size, and reproductive parameters so that meaningful 

 management and recovery plans may be developed. 



Historically, Marbled Murrelets were described as 

 'common' and 'abundant' in the vicinity of the Columbia 

 River and in Tillamook county, and near the Yaquina River 

 mouth in central Oregon (Gabielson and Jewett 1940, Taylor 

 1921). Currently, sightings from shore are infrequent in 

 these areas (Nelson and others 1992, Strong and others 

 1993), indicating a decline in the northern half of the state. 

 Presently Marbled Murrelets are seen regularly from shore 

 only between Seal Rock, Lincoln County, and Cape Arago, 

 Coos County (Strong, unpubl. data). Unfortunately, there 

 are no quantified historical data to compare with recent 

 shore counts or vessel surveys in order to determine to what 

 extent the population has declined in central Oregon. There 

 are no records to indicate the historic abundance of murrelets 

 south of Cape Arago. Even current shore observations are 

 few and inconclusive (Nelson and others 1992, Strong and 

 others 1993). 



1 Wildlife Biologists. Crescent Coastal Research, 7700 Bailey Rd., 

 Crescent City, CA 95531 



This project was initiated to fill a gap in knowledge about 

 the abundance, distribution, and at-sea biology of Marbled 

 Murrelets along the Oregon coast. Previous murrelet research 

 at sea in Oregon consisted of observations from shore and 

 limited vessel surveys, summarized in Nelson and others ( 1 992), 

 though more recently aerial surveys have been undertaken 

 (Burkett, pers. comm.; Varoujean and Williams, this volume). 



We surveyed Marbled Murrelets and other seabird species 

 in the Oregon coastal waters from Washington to California 

 during the summers of 1 992 and 1 993 to address the following 

 objectives of this report: 



(1) Compare behavior, distribution, and abundance patterns 

 of murrelets between the two years in each of four regions. 



(2) Compare and evaluate population estimates between 

 the three survey methods (aerial, vessel, and shore-based) 

 and between line and strip transects. 



(3) Qualitatively assess the feasibility and reliability of 

 the three methods for monitoring distribution and abundance 

 of murrelets. 



Methods 



The Oregon coast was divided into three regions with 

 distinctly different characteristics of murrelet abundance 

 (Nelson and others 1992, Strong and others 1993). The northern 

 region extended from the Columbia River to the north end of 

 Cascade Head (155 km of coastline). The central region 

 extended from Cascade Head to Coos Bay (209 km), though 

 the southern 75 km of this region, from Florence to Coos 

 Bay, was analyzed separately as a fourth region because of 

 ambiguity of survey results. The southern region went from 

 Coos Bay, south to the California border (195 km). 



Vessel Surveys 



A 20 foot Boston Whaler powered by two 70 hp outboard 

 motors was used for all surveys. It was operated from a 

 console in the middle of the boat. A driver and two observers 

 manned the boat. Each observer scanned a 90 arc between 

 the bow and the beam continuously, only using binoculars to 

 confirm identification or to observe plumage or behavior of 

 murrelets. All species of birds within 50 m of the boat and on 

 the water were recorded, and plunge divers (terns, pelicans) 

 were also recorded when flying. Marbled Murrelets sighted 

 at any distance were recorded along with the time of sighting, 

 distance from the vessel, group size (defined as birds within 

 2 m of each other), side of vessel, behavior and plumage 

 notes. Distance was not reported until murrelets had either 

 responded to the boat by flying or diving, or had been passed 

 by the boat. A bright float was deployed periodically at 50 m 

 behind the vessel to aid in distance estimation. 



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



339 



