Ralph and Miller 



Chapter 33 



Offshore Population Estimates in California 



we found the highest numbers of birds were seen between 

 400 m and 800 m from shore, depending on the height of the 

 observer above the water. On surveys conducted from boats 

 at the same locations, we found that most birds were 800 m 

 or farther from shore, apparently beyond the effective range 

 of the shore-based observers. 



Factors Which Might Affect the Estimate 



There are five factors which might cause over- or 

 under-estimate of the populations (1) a small number of 

 birds occur at distances greater than 6,000 m from shore, 

 beyond our surveys; (2) during incubation a portion of the 

 birds are on nests and, therefore, not counted at sea; (3) a 

 fraction of the birds are foraging underwater when the boat 

 passes and are. therefore, missed by observers. (4) some 

 birds would be flushed and fly ahead of the boat and be 

 repeatedly counted, thus resulting in an overestimate, and 

 (5) observer error in estimating distances to the birds. We 

 feel that these sources of error, detailed below, in part 

 compensate for each other, and would account for only 

 perhaps as much as 10 percent error. 



Birds Outside Our Sampled Area 



The density of birds declined rapidly beyond 2,000 m 

 (fig. 3), but even at 5.000 m, the density of birds is appreciable. 

 About 10 percent of the total are estimated to occur between 

 3.500-6.000 m from shore. In extensive surveys off the 

 central California coast murrelets have only very rarely been 

 detected beyond 7,000 m (Ainley and others, this volume). 

 If we extrapolate our distribution to 7,000 m, approximately 

 4 percent of the population might occur beyond our sampled 

 area. A log-log plot of the data shows that birds could 

 theoretically be detected out to 20 km from shore, albeit in 

 extremely low densities. While birds regularly occur out to 

 60 km from shore off British Columbia and Alaska, there is 

 no evidence of this in California. 



Birds Missed During Incubation 



During the approximately 90-day breeding season (Hamer 

 and Nelson, this %olume a), incubation extends over about 

 30 days for each breeding pair. As the sexes alternate 

 incubation duties, half of the breeding population would be 

 on the nest during the incubation period. Estimates of the 

 proportion of the population breeding in any one year range 

 between 30 and 85 percent (Carter and Sealy 1987b; 

 Beissinger, this volume). Thus, at-sea censuses during 

 incubation would result in an underestimate of 5 to 14 percent 

 of the population. This is calculated by determining the 

 percent of birds that would be on the nest at any one time 

 during the breeding season: 



0.85 breeding x 0.33 breeding season x 0.50 birds = 



14 percent underestimate. 

 0.30 breeding x 0.33 breeding season x 0.50 birds = 



5 percent underestimate. 



Since the potential incubation period represents 43 percent 

 of our survey period from April through November, the 

 error estimates of 5 to 14 percent should be multiplied by 

 0.43, suggesting an underestimate of approximately 2 to 6 

 percent. Based on the proportion of young birds observed 

 offshore in recent years (see Beissinger, this volume; Ralph 

 and Long, this volume), the proportion actually breeding 

 could be substantially lower than 30 percent. 



Birds Missed While Diving 



We assume in this study that no birds were missed by 

 being underwater as the observers passed. Our data show 

 that the average dive time of murrelets is less than 1 7 seconds 

 ( Strachan and others, this volume), and the distance traveled 

 in that time at 12 knots is less than 100 m. Since we can 

 detect birds out to 100 m in front of the boat, most birds that 

 dive while foraging would resurface before the boat passed. 

 While we are certain that some birds are missed due to this 

 factor, we feel that the effect is minimal, and probably much 

 less than 5 percent of the total population. 



Repeated Counting of the Same Individuals 



Double counting by more than one observer might result 

 in an overestimate with some survey methods, but we used 

 only a single observer, aided by the driver. It is possible that 

 some birds would fly ahead of the boat and be repeatedly 

 counted, thus also resulting in an overestimate. Strong and 

 others (this volume), however, discount this, and present 

 data indicating a relatively small number of birds fly out 

 ahead of the boat. 



Distance Estimates 



One factor which could affect population estimates using 

 EAS for calculations is observer variation, or error in distance 

 estimates. Underestimation of the distance to birds would 

 reduce the transect width and would result in an overestimate 

 of the total population. Overestimating the distance would 

 have the opposite effect and the population would be 

 underestimated. Our use of a reference buoy towed at a 

 known distance from the boat helped decrease the variation 

 and error in distance estimates. 



Comparison with Previous Population Estimates 



The numbers of birds derived from the pioneering work 

 of Sowls and others (1980) and Carter and others (1990b) 

 were based on more limited data. Sowls and others (1980) 

 speculated that the population was about 2,000 birds, but 

 their murrelet data was collected opportunistically and did 

 not provide sufficient data for a population estimate. Carter 

 and others (1990b) assumed that birds could be detected out 

 to 250 m, and conducted a limited number of surveys during 

 one breeding season. Furthermore, they often surveyed inshore 

 of the area where we found the highest numbers of murrelets 

 (Carter, pers. comm.). Our surveys were more extensive, 

 sampled most of the offshore areas used by murrelets, and 



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



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