Strong and others 



Chapter 32 



Distribution and Population Estimates in Oregon 



nest sites are not included in the above estimates. Marbled 

 Murrelet chicks are left on their own soon after hatching 

 (Marshall 1988), so the largest period of absence from the 

 water is during incubation. Information on breeding chronology 

 and breeding status were not adequate to adjust for this factor. 



The above estimates provide no information on the size 

 of the breeding population in Oregon. A relatively large 

 proportion of some alcid populations do not breed for lack 

 of a nest site or other reasons, constituting a 'floater 

 population' (Ainley and others 1990a, 1990b; Divoky and 

 others 1974; Manuwal 1974). The proportion of non-breeding 

 adults probably varies somewhat by year, as it does for other 

 alcids, depending on such variables as oceanographic 

 conditions (affecting prey availability) and weather. The 

 proportion of non breeding adult murrelets may be 

 considerable for the Oregon population if loss of nesting 

 habitat has left many pairs without nest sites. 



We have no data to account for Marbled Murrelets 

 which may occur at greater than 6 km from shore. However, 

 other researchers have recorded Marbled Murrelets in offshore 

 waters of the west coast as very scarce (Ainley and others, 

 this volume; Wahl 1984) or entirely absent (Briggs and 

 others 1989, 1992; Nelson and others 1992). For lack of 

 better data, we assumed that an insignificant number of 

 murrelets occurred beyond 6 km and that birds in that area 

 were unlikely to be part of the breeding population. 



Conclusions 



Distribution 



The different offshore distribution pattern between 1992 

 and 1 993 was likely due to differences in prey species and/ 

 or prey availability in the two years, although data to support 

 this assertion is sparse and indirect. In 1992, when Marbled 

 Murrelets were so concentrated inshore, they and other seabird 

 species were only seen to eat smelt. When they dispersed 

 farther offshore late in 1992, all prey seen were sand lance 

 (Strong and others 1 993). In 1 993 murrelets and other species 

 were all farther offshore than in 1992, and the few prey 

 items seen appeared to be sand lance. Murres suffered a 

 dismal nesting failure on the Oregon coast in 1993 (unpubl. 

 data; Lowe, pers. comm.). Pigeon Guillemots also fared 

 poorly, as indicated by the complete lack of guillemot 

 fledglings seen on the water in 1993. Although both years 

 were reported as El Nino years, water temperatures in Oregon 

 were higher in the summer of 1993 (NOAA Coastal Ocean 

 Program 1992-1993), and the effects of the ongoing El Nino 

 event on seabirds were much more apparent in that year. 



The higher numbers of birds encountered in northern 

 Oregon in 1993 (table 3) and the more northerly distribution 

 within the central region in 1993 (fig. 3) cannot be easily 

 interpreted. In 1992 when birds moved farther offshore late 

 in the season, they also moved farther north (Strong and 

 others 1993). The very high densities of birds recorded on 

 the July 21, 1993 survey, relative to all other data for the 

 region (Nelson and others 1992) were interpreted as post 



breeding or non-breeding birds which may reflect fewer 

 nesting attempts in that more severe El Nino year. Additional 

 years of data are needed to characterize distribution along 

 the coastline of both northern and southern Oregon. 



Population Measures 



These are the first estimates of the Oregon Marbled 

 Murrelet population which used extensive, repeated, and 

 standardized vessel transect data to quantify abundance 

 patterns parallel and perpendicular to the coast. Given this, it 

 is not surprising that estimates presented here are far higher 

 than previously given for Oregon (Nelson and others 1992, 

 using shore-based observations; Varoujean and Williams 

 1987, using a small sample of vessel observations; and 

 Varoujean and Williams [this volume] using aerial surveys). 

 The consistency of our estimated totals in the 15,000 to 

 20,000 range using different analyses and between very 

 different years, is supportive of their general validity. 

 Individual daily estimates of the central and north coast 

 regions were also consistent around the mean values (see 

 coefficient of variation (C. V.) in table 3), with the exception 

 of the July 1993 north coast transect mentioned above. The 

 few surveys of the south coast took place in conditions and 

 locations too variable to characterize a central tendency. 

 Greater survey effort of the southern Oregon coast and offshore 

 sampling of the northern and southern coasts, are urgently 

 needed to strengthen these estimates. 



Aerial transects have systematic problems (high flight 

 speed, missed scanning time, diving avoidance behavior) 

 and great sensitivity to conditions (glare, wind, banking on 

 turns, density of other species) which make estimation results 

 weak and certainly conservative (every factor listed has the 

 effect of potentially reducing detections). Improved data 

 recording methods can increase scanning time, which is 

 probably the greatest factor affecting detections (Varoujean 

 and Williams, this volume), but estimates still may only 

 provide an index of abundance, rather than an absolute 

 measure. It may be possible to develop a correction factor 

 between aerial and vessel detections if the difference is 

 consistent. Aerial surveys do provide an instantaneous 

 ' snapshot' measure of distribution over large areas of coastline 

 not obtainable by other methods. 



Shore-based surveys appear inadequate to measure 

 population, and even presence-absence information for a 

 given location could require repeated surveys through a 

 season. An intensive, daily shore survey effort could possibly 

 produce useful population assessments, probably by weighting 

 high count surveys and otherwise statistically accounting for 

 their patchy distribution. The main strength of shore-based 

 surveys may be in studying behavior, since there is minimal 

 possibility of interfering or disturbing the bird. Information 

 on grouping, foraging, dive times, diurnal activity patterns, 

 and social interaction are some areas of research that are 

 easily accomplished from shore. Shore based observation is 

 also likely to be the least expensive and logistically easiest 

 means of studying Marbled Murrelets at sea. 



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



351 



