Ralph and others 



Chapter 1 



Overview of Ecology and Conservation 



may have been reduced. However, this type of demographic 

 data provide perhaps our best hope of assessing this aspect 

 of the species' life history. 



State of Knowledge of Marbled Murrelet Demography 



The rate of production of young by Marbled Murrelets 

 appears to be one of the lowest of all alcids (De Santo and 

 Nelson, this volume). Of the 32 nesting attempts for which 

 we know the reproductive outcome, only 28 percent resulted 

 in the fledging of young (Nelson and Hamer, this volume b). 

 Data were gathered over several years throughout the 

 murrelet's range from the Gulf of Alaska to California. In 

 Washington, Oregon, and California, the success rate was 

 somewhat higher, 36 percent of 22 nests fledged young. 

 Most of the known causes of nest failure were related to 

 predation of nest contents. Analysis of counts of young at 

 sea in the early stages of the species' fledging period in 

 relation to numbers of adults indicate that the reproductive 

 rate in recent years has been less than that needed for a 

 sustainable population (Beissinger, this volume). 



The data used for determining productivity are based 

 on a number of assumptions and may be biased. First, some 

 of the data on nest success were gathered in years when 

 ocean temperatures have been unusually high and prey 

 availability may have been reduced. For most alcid species, 

 breeding failures in warm water years are the result of 

 adults forgoing breeding or chick starvation (Ainley and 

 Boekelheide 1990). If warm water conditions during recent 

 years depress the number of adults attempting to breed, the 

 age ratio at sea would not be typical of years with high food 

 availability. Secondly, the data on age ratios determined at 

 sea are also based on assumptions about the ability of 

 observers to separate adults from juveniles on the water 

 (Carter and Stein, this volume). This can be further 

 complicated later in the season when, as Hamer and Nelson 

 (this volume a) indicate, young can be leaving the nest as 

 late as September, and many adults have molted to a plumage 

 indistinguishable from that of the young. Thus, the number 

 of young may be underestimated. The data that Hamer and 

 Nelson compiled can be used to correct for the proportion of 

 young fledged at any given date (Beissinger, this volume; 

 Ralph and Long, this volume), giving a more accurate picture 

 of the proportion of young. 



We think it unlikely that a reduction of murrelet prey, if 

 such was the case during the recent studies, would be 

 responsible in some way for the high rate of predation of 

 nest contents. The ratios of juveniles to adults at sea would 

 be influenced by the proportion of the adult population that 

 bred, a proportion that is likely to be sensitive to prey 

 availability. In contrast, nesting success may be depressed 

 due to the possible attraction of nest predators to activities of 

 researchers (see below). 



Adult mortality rates are unknown. However, evidence 

 is accumulating that fouling by oil and bycatch in gill nets 

 may be locally significant (Carter and Kuletz, this volume; 

 Carter and others, this volume; Fry, this volume). 



Inferences from Other Species 



In the absence of adequate data on most aspects of 

 murrelet breeding, we must try to infer many of the 

 population parameters of demography from more detailed 

 studies of other alcids. We know that other small (150-500 

 g), fish-eating alcids (three Cepphus guillemot species and 

 four Synthliboramphus murrelet species) naturally suffer 

 high juvenile and adult losses from predation. 



These species produce two eggs and often fledge two 

 chicks. Thus, because they have high rates of reproduction, 

 these species can experience high levels of adult mortality 

 and still maintain stable populations (Hudson 1985). The 

 larger (500-1000 g), fish-eating alcids (four puffin species 

 [Fratercula sp.], two murre species, and the Razorbill Alca 

 tordd) produce only one egg, but under normal conditions 

 have higher levels of chick production than most other alcids, 

 due to low levels of juvenile and adult mortality and the long 

 lifespans for some of these species (De Santo and Nelson, 

 this volume). 



The relatively small (ca, 230 g) Marbled Murrelet differs 

 from the other small fish-eating alcids by producing a 

 single-egg clutch and having, at least in recent years, very 

 low success in raising young to fledging. If these patterns of 

 reproduction are typical, the Marbled Murrelet must have as 

 high or higher levels of adult survival, compared to other 

 alcids, if the murrelet populations are to be stable. The 

 Marbled Murrelet may be more sensitive than other alcids to 

 factors that increase adult mortality (Beissinger, this volume). 

 In the absence of hard data, we must infer that murrelet 

 demography is likely to be relatively more impacted than 

 that of other alcids by adult losses to predation, oil pollution, 

 gill nets, etc. Certainly, there is evidence of the pervasive 

 influence of predation in shaping the breeding biology of the 

 species (e.g., cryptic breeding plumage, crepuscular nest 

 attendance, behavior at the nest, and nesting in trees) (Nelson 

 and Hamer, this volume b; Ydenberg 1989). 



Factors Affecting Murrelet Demography 



The demography of Marbled Murrelets is influenced by 

 age of first breeding, the proportion of the adult population 

 that breeds, the rate of production of young that survive to 

 breeding age, and adult and subadult mortality rates. In this 

 section we evaluate these factors and their potential for 

 influencing the population dynamics of Marbled Murrelets. 



Limits on the Proportion of Adults Breeding 



Limitation of Nesting Habitat There is circumstantial 

 evidence, including both distributional and observational 

 data, that Marbled Murrelet populations are limited by the 

 availability of suitable nesting habitat and that the habitat 

 presently available is already occupied by breeding murrelets 

 (at least south of Alaska). This evidence includes the 

 following: 



(a) Concentrations at sea near suitable nesting habitat 

 Marine resources do not seem to determine the at-sea 

 distribution of murrelets in the breeding season, at least in 



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



13 



