Ainlev and others 



Chapter 34 



Offshore Occurrence Patterns in Central California 



Table 3 Results of logistic analysis to explain presence and absence of Marbled Murrelets on transect segments 



most years of high prey abundance, euphausilds dominated 

 most of the trawl catches. 



Discussion 



Our results indicate that in the designation of "critical 

 habitat." at least for Marbled Murrelets of the Waddell Creek 

 nesting population, the coastal waters within 10 km of Point 

 Ano Nuevo should be included. It is not surprising to us that 

 during the nesting season, murrelets in this population do 

 not travel far from the nesting area because, as explained 

 below, food availability is predictably high. The juxtaposition 

 of nesting and feeding areas should also be studied among 

 vulnerable, i.e. isolated, murrelet populations to the north. 



The small percentage of variation in murrelet distribution 

 explained by physical habitat variables is partly due to the 

 small regional scale of our study and the fact that certain 

 oceanographic features are quite ephemeral (see below). 

 Within the mesoscale perspective that our study area provided, 

 the availability of prey was likely the factor that best explains 

 murrelet occurrence (see below). 



The specific prey of this population are unknown, but 

 the three most likely candidates are euphausiids, juvenile 

 rockfish. and young-of-the-year anchovies, the three most 

 important prey to all other coastal seabirds in the vicinity 

 (Ainley and Boekelheide 1990). Euphausiids, shown to be 

 important to murrelets in British Columbia (Sealy 1972, 

 1975c), probably also comprise a significant proportion of 

 the diet among adults in the Waddell Creek population. The 



effect of euphausiid abundance on murrelet distribution, as 

 indicated in the analysis above, is circumstantial evidence 

 for this. Euphausiids are especially abundant in this region 

 (see below) and are easily caught by diving seabirds. 



Similar to many other alcids (see Bradstreet and Brown 

 1985), while the adults eat euphausiids, only fish are fed to 

 the nestlings. It is much more efficient for the parents to 

 carry fish to their young, because a single fish represents 

 much more food value than a single euphausiid. In this 

 region, juvenile rockfish and anchovies are likely candidates 

 as dominant species in the chick diet The undersea Ascension 

 Canyon, extending out from Waddell Creek (see fig. 7), is an 

 area where the high prevalence of juvenile rockfish, 

 specifically Sebastesjordani, is more consistent than anywhere 

 else along this coast (Woodbury, pers. comm.). The principal 

 prey species of central California's seabirds during the nesting 

 season is S.jordani (Ainley and Boekelheide 1990). Anchovies 

 are also consistently abundant in the Ano Nuevo area and 

 northern Monterey Bay during late spring and summer 

 (Woodbury, pers. comm.). Euphausiids, too, are consistently 

 abundant in the Ascension Canyon region at this time of 

 year and more so than in adjacent ocean areas. 



When euphausiids are abundant throughout shelf waters 

 in the vicinity of Ano Nuevo, adult murrelets need not forage 

 far from Waddell Creek, but can remain within 3-5 km of the 

 nesting area throughout the day. This was the pattern revealed 

 in our analysis. Not inconsistent with this pattern is the fact 

 that we saw few murrelets when potential prey were only 

 abundant at the shallowest trawl station and sparse farther 



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



367 



