Unlike the California coast immediately to the north, very deep water lies within a few kilometers of shore near Moss Landing and Davenport, as a result of the presence there of the submarine Monterey and Ascension canyons, respectively. This habitat is the most prevalent in terms of total surface area of Sanctuary waters. Surface waters overlying these depths (>2000 m deep) are populated with "pelagic" birds, such as Black-footed Albatross, Ashy Storm-Petrel and Xantus' Murrelet during summer and fall, and Northern Fulmars and Black-legged Kittiwakes during winter and early spring (Ainley 1976, Briggs et al. 1987, Stallcup 1990, Allen 1994). Owing to the persistent upwelling plume that emanates southward from Año Nuevo Point, to overlie the two canyons and bring nutrients to surface waters (Schwing et al. 1991), the food web is rich and these birds are relatively numerous compared to other portions of the west coast. These species prey on oceanic squid, lanternfishes (Family Myctophidae) and Pacific Saury (Cololabis saira; Ainley and Sanger 1979). At times, immense numbers of migrant phalaropes, both the Red-necked and Red, also can be found in Sanctuary offshore waters (Briggs et al. 1984).
Along the continental shelf break (200-1999 m), a relatively narrow habitat, seabird densities, in conjunction with an alongshore front, are also substantial. These waters are dominated by Sooty shearwaters, during spring and summer, and by fulmars and gulls during winter; other characteristic species are Pink-footed and Buller's Shearwaters, Black-Storm Petrels and Rhinoceros Auklets (Briggs et al. 1987, Stallcup 1990, Allen 1994). Along the shelf break this group of birds preys heavily on euphausiids, juvenile rockfish (Sebastes spp.) and a smattering of other fish species, including juvenile black cod (Anoplopoma fimbria; Ainley and Boekelheide 1990).
Inshore of slope waters (those greater than 200 m deep), the avifauna is composed largely of Sooty Shearwaters, Western Grebes, Pacific Loons, Brown Pelicans, Brandt's and Pelagic cormorants, Western Gulls and Common Murres (Briggs et al. 1987, Stallcup 1990, Allen 1994). In the shallow waters, where seabird densities compare to the highest anywhere else in the world ocean (Figure 2; Ainley 1976, Briggs et al. 1987), market squid (Loligo opalescens), anchovy (Engraulis mordax), juvenile rockfish (Sebastes spp., sardines (Sardinops sagax), surfperch (Embiotocidae) and smelt (Osmeridae) are the dominant prey (Baltz and Morejohn 1977, Ainley and Boekelheide 1990). Very close to shore, usually near the breakers, are Surf and White-winged scoters and Marbled Murrelets (Briggs et al. 1987, Stallcup 1990). The scoters feed on benthic invertebrates and the murrelets on euphausiids and small fishes (Vermeer and Bourne 1984, Burkett 1995).
Among the assemblages of species that comprise the MBNMS marine avifauna, a mixture of zoogeographic affinities is the general rule. The few breeding species that occur in the MBNMS typically have subarctic affinities. Very little breeding habitat exists; therefore, locally breeding species typically occur in very small numbers. These are the Pelagic and Double-crested cormorants, Western Gulls, Caspian Terns, Common Murres, Pigeon Guillemots, Rhinoceros Auklets and Marbled Murrelets (Carter et al. 1992, Roberson 1985). The Brandt's Cormorant is an exception, as it breeds in relatively large numbers (nearly 7000 pairs in Monterey County; Roberson and Tenney 1993) within the MBNMS. Three of these breeding species are not subarctic in affinity: the Brandt's Cormorant and Western Gull, which are endemic to the California Current region, and the Double-crested Cormorant, which is found throughout North America.
In contrast to the few breeding seabirds in the MBNMS, the visiting species of the avifauna rank among the most dense in the world (Ainley 1976, Briggs et al. 1987). Most species are seasonally resident and come in large numbers to "winter" in Sanctuary waters from temperate areas of New Zealand and Chile as well as Hawaii, Mexico and Alaska. They exploit the rich prey resources of the area, which is in the central upwelling region of the California Current system. This current system, one of five eastern boundary currents, is among the most productive in the world (Glantz and Thompson 1981). As a result of the rich waters and the replacement of summering species by wintering species, relatively little seasonal fluctuation in total seabird biomass is apparent (Figure 3).
The prevalence of marine birds using Sanctuary waters changes from year to year due to fluctuation in marine conditions, especially related to El Niño (Ainley 1976, Allen 1994). In a longer term perspective, the numbers of marine birds using Sanctuary waters have been declining during the past two decades (Ainley et al. 1994, Ainley et al. 1995a; Figure 4). This is due largely to a shift in ocean climate, which has resulted from a relaxation of upwelling (Bakun 1990) and an increase in sea surface temperature (Roemmich and McGowan 1995). In turn, the prevalence of zooplankton (Roemmich and McGowan 1995) and mid-water fishes (Ainley et al. 1994) has been reduced.
Sanctuary waters are among the most important in the world as measured by the sheer numbers of seabirds supported year-round, but are also important to several species of special concern because of their small world populations (see Special Status Species Table). Among these species are the endangered Brown Pelican (Baldridge 1973, Jaques and Anderson 1988, Jaques 1995); the threatened Marbled Murrelet (and the MBNMS population is the smallest, most disjunct and, therefore, most precarious breeding population of this species; Ainley et al. 1995b); and several species being considered for listing, such as Black Storm-Petrel, Ashy Storm-petrel and Xantus' Murrelet (Stallcup 1990, Ainley et al. 1995). The world's largest known concentration of Ashy Storm-Petrel can be found in Monterey Bay during fall (Stallcup 1990). The Brown Pelican once bred at sites in the Sanctuary (Baldridge 1973), and with return of the sardine, may do so again (MacCall 1984).
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Avifauna of the MBNMS