Ralph and others 



Chapter 1 



Overview of Ecology and Conservation 



1980s. Their estimate is lower than the ship-based survey 

 of Strong and others (this volume) by a factor of three, and 

 the causes of this difference are probably due to differences 

 in methodology, spatial coverage, assumptions, and survey 

 and model errors. 



Sources of Error 



Although we believe that at-sea surveys for estimation 

 of Marbled Murrelet population size is necessary, there is 

 still need for validation of the methodology. Few seabird 

 species have population estimates based on at-sea counts, 

 and the accuracy (as opposed to precision) of these techniques 

 is only now being established. Population estimates of 

 murrelets based on at-sea counts are subject to several 

 sources of error, and these sources and their magnitudes are 

 likely to vary with location and season. Three aspects of 

 surveys that can affect accuracy are the way in which 

 counts of flying birds are made, observation conditions, 

 and observer competence. The possibility exists for double 

 counting birds that are flushed from the survey track and 

 then settle on a portion of the track yet to be surveyed. This 

 problem is more severe for ship-based than aerial surveys, 

 because the speed of the plane is great relative to that of the 

 birds. Even in aerial surveys, double counting may occur if 

 adjacent survey lines are sufficiently close. Strong and 

 others (this volume) suggest that double counting, even on 

 ship-based surveys, may be only a minor problem, with an 

 estimated 5 percent of counted birds being vulnerable to 

 recounting. There is also the likelihood of not counting 

 murrelets because they are underwater, either foraging or 

 diving in response to a vessel or airplane (Strachan and 

 others, this volume; Strong and others, this volume). This 

 source of error is greatest for aerial surveys because a given 

 area is in sight only for an instant. The ability of an observer 

 on a boat or plane to see birds will vary with the speed of 

 the survey platform, height of the observer, use of binoculars, 

 the area for which each observer is responsible, and observer 

 competence. To minimize these sources of error, or 

 uncertainty, it is necessary to either limit observations to a 

 narrow band (e.g., Varoujean and Williams, this volume), 

 to correct for the diminishing visibility of birds at greater 

 distances (Ralph and Miller, this volume), to calibrate aerial 

 versus boat surveys, or to calibrate observers (i.e., use only 

 a limited number of persons). 



The patchy distribution of murrelets and their propensity 

 for large daily shifts in distribution (Burger, this volume b; 

 Rodway and others, in press; Speich and Wahl, this volume) 

 further complicate the interpretation of survey data. Throughout 

 their range, the largest numbers of Marbled Murrelets seen 

 on the water are within a few kilometers, and often less than 

 500 m, of shore. Data from British Columbia (Burger, this 

 volume b; Morgan 1989; Sealy and Carter 1994; Vermeer 

 and others 1983) suggest that along the outer, exposed coasts, 

 murrelets may forage closer to shore (out to 500 m) than they 

 do in sheltered bays and fjords where birds are often 1 to 5 

 km offshore. Large-scale surveys by ship or airplane that fail 



to thoroughly survey this narrow, inshore strip are likely to 

 underestimate local murrelet populations. Additionally, within 

 this nearshore zone, murrelets are found concentrated in 

 preferred foraging locales. A consequence of this small-scale 

 patchiness is that surveys on different days must cover the 

 same routes each time if they are to be comparable (they 

 provide an index, not a sample), or they must be carefully 

 stratified by foraging zone. In addition, variance of counts is 

 large, so precise estimates of abundance require large samples 

 (numbers of counts). 



Temporal variation in the use of marine habitats by 

 murrelets further complicates the assessment of annual or 

 decade-long changes in numbers. Data from Washington 

 (Speich and Wahl, this volume), British Columbia (Burger, 

 this volume b; Rodway and others, in press) and Alaska 

 (Kuletz and others 1994a; Piatt and Naslund, this volume) 

 show that murrelets exhibit considerable seasonal and daily 

 variation in their use of specific foraging areas. During the 

 breeding season, the portion of the population attending 

 nests will change with time. In order for surveys to be 

 strictly comparable, care should be exercised to conduct 

 surveys hi similar seasons and at the same time of day, or to 

 make appropriate corrections to account for these sources of 

 variation. These sources of error apply to all surveys in table 

 2. More research is required to validate census techniques, 

 to establish the accuracy of different survey methods, and to 

 determine the time of year when the most comparable surveys 

 should be done. 



Trends in Murrelet Populations 



Historical data for Marbled Murrelet populations are 

 few, and no estimates can be made for populations before 

 1900. It is at least possible that the murrelet was an abundant 

 bud, nesting hi old-growth forests all along the Pacific Coast 

 in numbers commensurate with the abundant nearshore small 

 fish it preys upon, and not limited, as it is today, by the 

 availability of remnant stands of old-growth forests in the 

 southern portion of its range. Circumstantial evidence to 

 support this argument is the existence of large numbers of 

 murrelets in very high densities where old-growth is still 

 abundant (i.e., the Gulf of Alaska), or where it is the most 

 abundant seabird in summer (i.e., Prince William Sound) 

 (Kuletz, pers. comm,). 



Although the total population of Marbled Murrelets still 

 appears large (table 2), there is reason for concern for the 

 continued viability of this species in some regions. Numbers 

 at the southern end of the range are small and concentrated 

 geographically, thereby leaving subpopulations vulnerable 

 to damage by stochastic events. More importantly, evidence 

 is mounting that population trends are downward where they 

 have been measured, even though short-term fluctuations in 

 climate and longer-term variation hi ocean currents can result 

 in apparent or temporary increases. 



In Alaska (Piatt and Naslund, this volume), and in 

 Clayoquot Sound, British Columbia (Burger, this volume b; 

 Kelson and others, in press), populations have apparently 



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



11 



