Ralph and others 



Chapter 1 



Overview of Ecology and Conservation 



years. In Washington, Oregon, and California, population 

 trends are downward, but the magnitude of decline over the 

 past few decades is unknown. As a result of the small size of 

 remnant populations, the species has been listed by various 

 authorities as threatened or endangered in parts of its range. 



Counts of Marbled Murrelets at sea are currently the 

 best method of estimating the size of regional populations. 

 Nests are difficult to find, and although detections of calls at 

 inland sites provide indices to local activity, numbers of 

 detections can not be translated into absolute numbers of 

 birds present. In contrast, surveys of birds at sea can be done 

 from boats or airplanes, can cover large areas quickly, and 

 can be standardized to provide repeatability. It is also possible 

 to develop models for extrapolation of results from areas 

 that have been surveyed thoroughly, and apply them to 

 nearby areas that have received more cursory inspection 

 (Ralph and Miller, this volume). 



Estimates of Population Size 



Based on the at-sea survey data, our best estimate of the 

 Marbled Murrelet population in North America is on the 

 order of 300,000 individuals (table 2). The major portion of 



this population is concentrated in northern Southeast Alaska 

 and Prince William Sound. 



Population size diminishes rapidly north and west of 

 there. Populations are relatively small and fragmented 

 throughout Washington, Oregon, and California. 



The repeatability of survey results appears to vary 

 considerably between location, methods, and researcher. In 

 Alaska, overall population estimates were similar between 

 summer and winter counts within the same decade (Piatt 

 and Naslund, this volume). In contrast, population estimates 

 for Prince William Sound varied considerably between 

 those made in the 1970s and those made subsequent to the 

 Exxon Valdez oil spill; the disparity is greater than can be 

 explained by the oil spill alone, and probably is the result of 

 different sampling methods in the 1970s or changes in food 

 availability (Klosiewski and Laing 1994; Piatt and Naslund, 

 this volume). In Washington, counts made from 1978 to 

 1985 (Speich and others 1992), were similar in magnitude 

 to those made in 1993 (Varoujean and Williams, this volume), 

 with perhaps 5,000 in the entire state. Likewise, along the 

 Oregon coast, Varoujean and Williams (this volume), using 

 an aerial survey, found murrelet numbers in 1993 to be in 

 the same range as their estimates of population size in the 



Table 2 Estimated size of Marbled Murrelet populations by geographic regions 



1 Midpoints are usually used where ranges were given in the source 



10 



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



