Ainley and others 



Chapter 34 



Offshore Occurrence Patterns in Central California 



Table 1 Results of multiple regression analysis to explain variation in murrelet density: dependent variable = murrelet 

 density; independent variables = water depth, sea-surface temperature and salinity, wind speed, distance-to-land, distance-to- 

 shelf-break, and distance-to-nesting-area, as well as interaction terms presented in the Methods 



Table 2 A matrix showing correlation coefficients among independent variables used in the regres- 

 sion analyses 



the early spring, however; murrelets were found where salinity 

 was highest, which at this time also happened to be close to 

 shore. This was consistent with the onset of coastal upwelling, 

 which reaches maximum in May and June and which brings 

 cold, high salinity water to the surface adjacent to the beach 

 especially to the south of Point Ano Nuevo (fig. 1). 



The relationship of murrelet occurrence to prey 

 availability during late spring (fig. 3) could be analyzed 

 only qualitatively because of an inconsistency of scale 

 between the trawls and the censuses. Patterns were apparent, 

 however, and high inshore prey abundance appeared to 



result in fewer birds offshore. When murrelets were scarce 

 in June surveys (1986, 1988, 1989, 1992, and 1993), prey 

 abundance was disproportionately high (>1000 prey per 

 trawl) on the shallowest (and next shallowest in the case of 

 1993) trawl station, as compared to the adjacent trawls in 

 deeper waters out to the shelf break (i.e., waters <280 m). In 

 contrast, during cruises when murrelets were abundant, 

 particularly 1990, 1991, and 1994, potential prey were also 

 abundant, and equally so among all or almost all the trawls 

 throughout the shelf waters. The exception was 1987, when 

 trawl catches were low, but murrelets were abundant. In 



366 



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



