Chapter 34 



Offshore Occurrence Patterns of Marbled Murrelets in 

 Central California 



David G. Ainley 



Sarah G. Allen 2 



Larry B. Spear 3 



Abstract: We assessed the occurrence patterns of Marbled 

 Murrelets (Brochyromphus marmoratus) offshore of Waddell 

 Creek, in central California. Data were derived primarily from 

 cruises during the height of the murrelet breeding season, in June, 

 between 1986 and 1994, as well as some cruises during the pre- 

 breeding period, February to early April. The large majority of 

 sightings occurred within 10 km of Point Afio Nuevo, directly 

 offshore the species' breeding area. Only three sightings occurred 

 farther offshore (12-24 km). The physical factors that explained a 

 small but significant portion of variability in murrelet occurrence 

 were: ( 1 ) inverse relationships related to distance to breeding area 

 (including ocean depth); and (2) relationships to recently up- 

 welled water. Murrelets were least abundant during periods of El 

 Nino-Southern Oscillation. An analysis of the availability of 

 potential prey species indicated that murrelets were most abun- 

 dant when more euphausiids were found from the coast to well 

 offshore. Murrelets were absent in years when a large prey con- 

 centration occurred only close to shore, indicating that the birds 

 were too close to shore for us to census. We suggest that the 

 critical habitat of this population should include the nearshore 

 marine waters within 10 km of Afio Nuevo Island. 



The Marbled Murrelet (Brachyramphus marmoratus) nests 

 in old-growth forests along the Pacific Coast of North America 

 from central California to southern Alaska and the Aleutian 

 Islands. In this region, the birds are unevenly distributed at 

 sea, occurring in distinct clumps, often in bays at the mouths 

 of coastal rivers (Carter and Erickson 1992, Nelson and others 

 1992. Rodway and others 1992, Speich and others 1992, 

 Strong and others 1993). These concentrations may be 

 associated with the river valleys in which the birds nest, or 

 they may represent areas of good foraging. Both staging and 

 foraging areas comprise "critical habitat," the designation of 

 which is defined and required under the Endangered Species 

 Act (U.S. Fish and Wildlife Service, 1993b). 



Other than surveys to estimate abundance, little work at 

 sea on the biology of Marbled Murrelets has been undertaken 

 since the studies of Sealy (1972, 1975c), Carter (1984), and 

 Carter and Sealy (1990), who investigated the species in the 

 inside passage waters of British Columbia. Information on 

 factors affecting distribution or clumping remains largely at 

 the broadest scales (Briggs and others 1987, Piatt and Ford 

 1993). Repetitive at-sea surveys designed to understand at- 



1 Marine Program Director. Point Reyes Bird Observatory, 4990 

 Shoreline Highway, Stinson Beach, CA 94970 



2 Ecologist Western Regional Office, National Park Service, U.S. 

 Department of the Interior, San Francisco, CA 94107 



3 Research Biologist, Point Reyes Bird Observatory, 4990 Shoreline 

 Highway. Stinson Beach, CA 94970 



sea biology and marine factors as they affect murrelet 

 distribution from a mesoscale perspective, i.e. 30-50 km, 

 remain non-existent. The smaller scale surveys would be 

 more pertinent to localized populations. 



The most disjunct population of Marbled Murrelets is 

 in central California, the southern limit of the species' 

 breeding range. Murrelets aggregate along the coast of 

 northern Santa Cruz and southern San Mateo counties, in 

 the vicinity of Point Ano Nuevo and Afio Nuevo Island 

 (hereafter referred to collectively as Ano Nuevo), about 350 

 km south of the next closest murrelet nesting area; see 

 figure 1A (Briggs and others 1987). This aggregation is 

 associated with Waddell Creek, the last remaining near-to- 

 pristine coastal watershed in the lower two-thirds of California 

 (see, for example, Shapovalov and Taft 1954, an analysis 

 still pertinent to recent times). The watershed drains Big 

 Basin Redwoods State Park and adjacent, private, forested 

 lands. About 250 birds are thought to breed here (Carter and 

 Erickson 1992). 



We collected information on Marbled Murrelets in the 

 vicinity of Ano Nuevo on ship-based surveys designed to 

 elucidate the physical and biological factors that organize 

 seabird communities off central California during the years 

 1986-1994. Overall our work was not directed specifically at 

 Marbled Murrelets. Moreover, our large vessel was usually 

 not able to cruise in waters shallower than 20 m, where 

 Marbled Murrelets often occur (Ralph and others 1990, Sealy 

 and Carter 1984, Strong and others 1993). We were able to 

 direct the ship's course at times, however, and whenever in 

 the vicinity of Ano Nuevo, we pointedly surveyed for 

 murrelets. We collected data on sea-surface temperature and 

 salinity, thennocline depth, bottom depth, distance-to-shelf 

 break, distance-to-shore, and distance-to-nesting area, which 

 we recorded at frequent, regular intervals while we surveyed 

 continuously for birds. We also obtained information on 

 availability of potential prey in the study area. 



Methods 



Our research extended from Bodega Bay to Carmel, 

 California, in waters from 20 to 2,000 m deep. For purposes 

 of this report, our study area includes the middle third of this 

 region, specifically, from Half Moon Bay to Santa Cruz (fig. 

 1). We censused seabirds from the flying bridge of the NOAA 

 Ship David Starr Jordan during the 9 cruises in the first two 

 weeks of June 1986-1994 (referred to hereafter as late spring), 

 and in 5 cruises during 7-27 April 1987, 16-22 April 1988, 21 

 February-6 March 1992, 2-15 March 1993, and 1-14 March 

 1994 (early spring). Two observers simultaneously counted 



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



361 



