Strong and others 



Chapter 32 



Distribution and Population Estimates in Oregon 



for birds sighted off the bow. We also truncated observations 

 to within 160 m from the vessel, which eliminated very few 

 observations and improved modeling capability 



This approximated the undisturbed distribution of birds 

 and allowed satisfactory fit of the models. Transect data for 

 each day were fined to a model and an independent population 

 estimate was derived for each day. Daily estimates were 

 averaged within a region for population estimates in each 

 year, and variance of daily estimates was used to construct 

 confidence intervals using a normal approximation (Zar 1984: 

 103). Where lower confidence intervals approached zero 

 (due to few sample days), the lower limit was taken as the 

 actual count times 2. 



Strip Transects 



For strip transects, we summed all Marbled Murrelets 

 occurring within the designated strip (excluding flying birds) 

 for each day's transect, and divided that sum by the length of 

 the transect (in km) for a density within the strip. This was 

 multiplied by the appropriate factor to obtain a density measure 

 in km 2 and by the length of the region to obtain a population 

 estimate for the day. These estimates were averaged the same 

 way as for line transects to obtain regional population estimates 

 and confidence intervals. Strip width was selected at 50 m 

 out from the vessel (100 m total) after study of frequency 

 histograms of reported distances and iterations of density 

 calculations at different strip widths (figs. 1 and 2). 



18 



16- 



3 12H 



2 io- 



LLJ 

 Q. 



4 



2H 

 



.fUnJI 



15 35 55 75 95 115 135 155 175 >200 

 METERS FROM BOAT 



1992 (n = 4882) 



1993 (n = 3346) 



Figure 1 Distances at which murrelets were reported from observing vessel. Arrow at 50 m 

 indicates distance from vessel within which all birds were assumed to be detected for strip transects. 





80 



100 130 160 



STRIP WIDTH (m) 



320 



1992 



1993 



Figure 2 Abundance estimates for the state extrapolating for five different 

 strip widths, without addition of birds > 500 m offshore (central region) or birds 

 1000 m offshore (north and south regions, see text). 



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



341 



