Chapter 29 



Marine Distribution, Abundance, and Habitats of Marbled 

 Murrelets in British Columbia 



Alan E. Burger 1 



Abstract: About 45,000-50,000 Marbled Murrelets (Brochyromphus 

 marmoratus) breed in British Columbia, with some birds found in 

 most parts of the inshore coastline. A review of at-sea surveys at 

 84 sites revealed major concentrations in summer in six areas. 

 Murrelets tend to leave these breeding areas in winter. Many 

 murrelets overwinter in the Strait of Georgia and Puget Sound, but 

 the wintering distribution is poorly known. Aggregations in sum- 

 mer were associated with nearshore waters (<1 km from shore in 

 exposed sites, but <3 km in sheltered waters), and tidal rapids and 

 narrows. Murrelets avoided deep fjord water. Several surveys showed 

 considerable daily and seasonal variation in densities, sometimes 

 linked with variable prey availability or local water temperatures. 

 Anecdotal evidence suggests significant population declines in the 

 Strait of Georgia, associated with heavy onshore logging in the 

 early 1900s. Surveys made between 1979 and 1993 in Barkley and 

 Clayoquot sounds suggest 20-60 percent declines in densities. 

 These changes are correlated with intensive onshore logging, al- 

 though El Nino effects are probably also involved. 



British Columbia is second only to Alaska in the 

 population size of Marbled Murrelets and also in complexity 

 of marine habitats used by these birds. There have been 

 many marine studies of this species in British Columbia, 

 beginning with the pioneering work of Sealy in the Queen 

 Charlotte Islands (Sealy 1973b, 1974, 1975a,c), and the 

 work of Carter and Sealy in southwestern Vancouver Island 

 (Carter 1984, Carter and Sealy 1984, Sealy and Carter 1984). 

 Most studies have been short term (one season or less) and 

 highly localized. Previous province-wide reviews of marine 

 distributions and habitats relied primarily on data from sight 

 record cards, along with a few standardized censuses 

 (Campbell and others 1990, Rodway 1990, Rodway and 

 others 1992). The sight records are not reanalyzed here. This 

 chapter summarizes data on populations, distribution, habitats, 

 and basic biology of the Marbled Murrelet obtained largely 

 from boat surveys. 



Abundance and Distribution 



Regional and Range- Wide Population Densities 



Marbled Murrelets have been recorded from most of the 

 coastal waters of British Columbia (Campbell and others 

 1990, Rodway 1990, Rodway and others 1992). Current 

 estimates of the provincial population (45,000-50,000 breeding 



1 Associate Professor (Adjunct), Department of Biology, University of 

 Victoria, Victoria, British Columbia, V8W 2Y2, Canada 



birds) are extrapolations from relatively few data from 1972 

 to 1982, mainly counts in high-density areas and transects 

 covering a small portion of coastline (Rodway and others 

 1992). Between 1985 and 1993 many parts of the British 

 Columbia coast were censused, usually by shoreline transects, 

 although methods and dates varied, making comparisons 

 and extrapolation difficult. Much of the 27,000 km of coastline 

 remains uncensused (fig. 1). 



Appendix 1 summarizes censuses for the core of the 

 breeding season (1 May through 31 July). Murrelet densities 

 are given as birds per linear kilometer of transect. It was 

 necessary to convert density estimates from other units in 

 several cases, and this was done in consultation with the 

 original authors, using charts to determine distances travelled. 

 Several authors used strip counts, ranging in width from 300 

 m (e.g., 150 m on either side of the boat) to 1 km, whereas 

 others reported all birds visible from the boat. Relatively 

 few murrelets are likely to be detected at distances >200 m, 

 even in the sheltered inner waters of British Columbia (Burger, 

 unpubl. data; Kaiser, pers. comm.), and so these differences 

 in technique, while adding to the variability of the data, were 

 not considered to be a major source of error. 



Gaps in Distribution 



There are no obvious gaps in the marine distribution in 

 British Columbia, although low densities are associated with 

 several large areas (e.g., eastern Graham Island, eastern 

 Vancouver Island, and many of the large mainland fjords), 

 and this is discussed below. Many areas have not been 

 adequately sampled in the breeding season (much of NW 

 and NE Vancouver Island, the Strait of Georgia, Strait of 

 Juan de Fuca). 



Movements and Seasonal Variations in Density 



Rodway and others (1992) used an extensive data 

 base of sight record cards in the Royal British Columbia 

 Museum to demonstrate a post-breeding emigration from 

 areas which support large breeding concentrations on the 

 west coast of Vancouver Island and the Queen Charlotte 

 Islands. Habitat shifts were associated with these changes. 

 Apparent densities (mean numbers of birds per sighting 

 record) were higher in spring and summer than in fall and 

 winter in both exposed inshore waters (1-5 km from shore) 

 and nearshore waters (<1 km of shore) of the Strait of 

 Georgia, but the opposite was true for most fjords. This 

 suggests that murrelets leave the exposed outer coast and 

 more exposed areas of large straits in late summer and fall 

 to move into more sheltered waters, including some of the 

 many large fjords on the mainland. 



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



295 



