Ainley and others 



Chapter 34 



Offshore Occurrence Patterns in Central California 



O 



o 



LU 

 O 



or 



ui 

 CL 



11 21 24 4 1 2 



11 19 13 1 1 



20 - 



10 - 

 5 - 







10 



r~ 

 15 



_ssa. 



20 



i 

 25 



o 

 o 



239 206 221 122 114 77 70 31 35 63 35 25 58 

 244 118 109 41 43 93 109 148 74 71 85 98 



DISTANCE FROM SHORE (KM) 



Figure 2 Frequency distribution of murrelet sightings by distance to shore by 1 000-m (1 - 

 km) intervals: (A) percent of sightings in the raw data (number of murrelet sightings along 

 the top); and (B) sightings normalized by search effort (sightings divided by number of 15- 

 min transects in the distance intervals and the number of transects along the top). 



Results of multiple regression analyses, using data from 

 all seasons and only census segments on which murrelets 

 were seen (hence, sample size is low), show distance-to- 

 land to be the most important explanatory physical factor, in 

 this case a negative one (higher densities occur closer to 

 land; table /). Using data from early spring, distance-to- 

 land remains important (and becomes statistically significant), 

 but depth and distance-to-nesting-area are important as well. 

 This is logical: a correlation analysis (table 2) shows that 

 distance-to-land, distance-to-nest, and depth are all closely 

 correlated: a point moving closer to land also moves closer 

 to nesting sites (to some degree) and to shallower water. 

 Analyses using only late spring data show that densities are 

 affected most strongly by waters influenced directly by 

 upwelling, i.e. those of low temperature and high salinity 

 (see below; table 1). 



In the logistic analysis, which considers only presence- 

 absence on each 15-minute transect and therefore uses all 



transects, the same factors, but more of them together, 

 explained murrelet distribution (table 3). In addition, more 

 of the variance was explained. Considering both seasons, 

 temperature, salinity, distance-to-land, distance-to-nesting- 

 area, and distance-to-shelf-break were all important (and 

 statistically significant). These distance parameters were 

 related to one another as was temperature to salinity (table 

 2). Murrelets were found in waters of low salinity during 

 early spring. At this time, freshwater runoff is at maximum 

 extent and enters the study area from the fresh water plume 

 that passes south from the Golden Gate (just to the north of 

 figure 1 boundaries), as well as from Pescadero and Waddell 

 creeks. Thus, lowest salinities at this time of year occur 

 close to shore but in a broad band. During early spring, 

 distance-to-shelf-break was not a significant variable. 

 Considering only late spring data, depth, distance-to-shelf- 

 break and -to-nesting-area, as well as salinity, became the 

 explanatory variables. The role of salinity was reversed from 



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



365 



