Piatt and Naslund 



Chapter 28 



Abundance, Distribution, and Population Status in Alaska 



of Marbled Murrelets (Piatt and Anderson, in press). Unlike 

 short-term phenomenon such as El Nino events, this long- 

 term shift represents a more pervasive and persistent change 

 in the ecosystem and can potentially have long-term effects 

 on Alaskan murrelet populations. In the short term, evidence 

 suggests that murrelets and other seabirds will have difficulty 

 recovering from impacts of the 1989 Exxon Valdez oil spill 

 and other sources of adult mortality until conditions favorable 

 for seabirds are re-established in the Gulf of Alaska (Piatt 

 and Anderson, in press). 



Life History 



As a group, the Alcidae exhibit life history characteristics 

 typical of other seabirds. Laying only 1-2 eggs per breeding 

 season, they have a low capacity for production but this is 

 balanced by low adult mortality and long life (see review 

 by De Santo and Nelson, this volume). Compared to other 

 fish-feeding members (e.g., murres, puffins, auks) within 

 the family, however, it is clear that murrelets are extreme 

 in their adaptation for very low production (see below), 

 which must be balanced by very high adult survival rates. 

 This is important to consider when evaluating the long- 

 term impacts of anthropogenic and natural mortality factors 

 on populations in Alaska. Whereas murres (Una spp.), 

 with natural adult mortality rates of 8-1 2 percent per annum 

 and annual chick production rates of 0.5-0.9 chicks per 

 pair, may be able to compensate relatively quickly for 

 acute or chronic mortality losses of adults, increases in 

 mortality of adult murrelets may have more serious 

 demographic consequences. Thus, losses of 1-3 percent of 

 adult murrelets resulting from oil spills and gill nets (see 

 above) are cause for serious concern. 



No data are available on adult survivorship in murrelets, 

 but much evidence suggests that production is extremely 

 low and regulated largely by predation. Indeed, predation 

 pressure appears to have been a major ecological factor 

 influencing the evolution of murrelet life history strategies. 

 Excepting its close relative, the Kittlitz's Murrelet, Marbled 

 Murrelets are the only alcid with cryptic plumage and nesting 

 behavior. Breeding Marbled Murrelets fly silently to their 

 woodland nest-sites for incubation exchange or chick-feeding, 

 and like the even smaller Synthliboramphus murrelets, fly 

 mostly at dawn. dusk, or in darkness (Gaston 1992; Naslund 

 1993a; Nelson and H^mer, this volume a). Selection for 

 breeding in old growth forest by Marbled Murrelets may 

 have arisen because of the scarcity of predators relative to 

 second growth or disturbed habitat 



Despite their best efforts to avoid predation. Marbled 

 Murrelets suffer the highest nesting failure known for any 

 alcid. largely due to predation. Only 28 percent of 32 nests 

 with known outcomes have ever fledged young success- 

 fully (Nelson and Hamer. this volume b). In southcentral 

 Alaska, all nests (n = 8) failed where breeding success was 

 known (Marks, pers. comm.; Naslund and others, in press). 

 Abandonment and predation were implicated as factors 

 causing nesting failure. Adults also suffer from predation by 



raptors and possibly corvids (Marks and Naslund 1994, Singer 

 and others 1991). Being only slightly smaller and larger, 

 respectively, than Marbled Murrelets, Synthliboramphus 

 murrelets and Cepphus guillemots also suffer from high 

 levels of chick and adult predation. However, these species 

 have compensated through the evolution of 2-egg clutches 

 unique among the Alcidae. Thus, Marbled Murrelets stand 

 out among the Alcidae for having extremely low levels of 

 production, and a limited capacity for dealing with increased 

 predation pressure or unnatural sources of mortality. 



Population Tre nd s 



There are few quantitative data to assess population 

 trends of murrelets in Alaska. We analyzed 20 years (1972- 

 1991) of Christmas Bird Count (CBC) data in the northern 

 Gulf of Alaska (fig. 7). Totals for each year were calculated 

 as the sum of all murrelets seen on CBC's in Sitka. Juneau, 

 Glacier Bay, Cordova, and Kodiak Island. We could not take 

 the average of counts among sites (n = 5) because of missing 

 data (see below). There was considerable inter-annual 

 variation in total numbers, which we smoothed by taking 5- 

 year running averages of the annual data (fig. 7). Unsmoothed 

 data were extremely variable, and did not reveal a statistically 

 significant trend. However, the smoothed data suggest a 

 steady decline in abundance of about 50 percent from the 

 early 1970's to the early 1990's. This analysis is biased 

 because some years of CBC data are missing (16 out of a 

 possible 100 counts). As most (11) missing CBC counts 

 were from the first decade ( 1972-1980) of study, the downward 

 trend is greater than indicated in figure 7. 



Interpretation of CBC's is confounded by several effects 

 including survey conditions and observer effort (Arbib 

 1981, Bock and Root 1981). CBC data may be most 

 suitable for monitoring long-term trends in species (such 

 as the Marbled Murrelet) that occur regularly, are widely 

 distributed, and occupy easily-censused, discrete habitats 

 (Bock and Root 1981, Trapp 1984). We chose not to 

 standardize the CBC data by dividing murrelet numbers 

 by some measure of census effort (e.g., party-hours) because 

 this approach may not be appropriate for some species 

 likely to be well censused, regardless of how many people 

 participate in the census (Bock and Root 1981). If we had 

 standardized the data for effort, which increased by more 

 than 50 percent over the period of study (fig. 7), the 

 apparent decline in Marbled Murrelets would have been 

 even more pronounced. 



Compelling evidence for a major decline in murrelet 

 abundance is also provided by comparing results of surveys 

 that were conducted in Prince William Sound during 1972- 

 1973 with those conducted after (1989-1991) the Exxon 

 Valdez oil spill (Klosiewski and Laing 1994). Based on 

 randomly-selected transects censused throughout the entire 

 Sound, and on surveys conducted in both winter and summer, 

 populations of Brachyramphus murrelets apparently declined 

 by 67-73 percent between the early 1970's and late 1980's. 



I SDA Forest Service Gen. Tech. Rep. PSW-152. 1995. 



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