Lowry and Forney: Abundance and distribution of Zalophus californianus 



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127° 126° 125° 124° 123° 122° 121 120 11 9 C 

 Longitude (°W) 



Figure 1 



Strip-transect lines (solid lines) within 

 the study area (dashed line I used for esti- 

 mating at-sea abundance of California sea 

 lions (Zalophus californianus) in central 

 and northern California. 



approximately 85 meters left and right of the transect 

 line (Fig. 2; Carretta, personal commun. 9 ). Therefore, 

 strip transect assumptions, that all individuals within 

 the observed strip are detected, were expected to be 

 valid within 85 meters left and right of the transect line. 

 In our study we lowered the altitude of the aircraft to 

 183 m to increase the detection probability for sea lions 

 in the water, especially in Beaufort 3-4 sea states. At 

 that altitude, the viewing area of a single observer view- 

 ing from the belly window extended from directly below 

 (90) to a declination angle of 65° on each side, resulting 

 in a total strip width of 170 m, or 85 m on each side of 

 the viewing window. 



Transects followed predetermined lines that system- 

 atically zig-zagged the study area (Fig. 1). Surveys 

 were conducted in Beaufort sea states of 0-4. The lines 

 were flown from south to north to take advantage of 



9 Carretta, J. 1998. Personal commun. Southwest Fisher- 

 ies Science Center, NMFS, La Jolla, California, 92037. 



sun angle and to minimize sun glare, except on a few 

 overcast days when southbound flights provided ample 

 visibility. Geographical positions were recorded at one- 

 minute intervals directly to a laptop computer by a se- 

 rial cable connected to the aircraft's global positioning 

 system (GPS). The following data were collected: num- 

 ber of California sea lions, GPS position, percentage of 

 cloud cover over the survey area, name of the observer 

 and data recorder, Beaufort sea state, transect num- 

 ber, and percentage of glare. Percentage of glare was 

 defined as the proportion of the viewing area in which 

 the observer could not see into the water because of 

 surface reflection caused by sun or cloud glare. During 

 the May-June survey we used a recorder, observer, and 

 a resting person — the resting person rotating with the 

 observer approximately every 30 minutes. During the 

 July, September, and December surveys, the resting 

 person was eliminated and the observer and recorder 

 rotated at approximately 30-minute intervals. 



Abundance at sea 



We used the nonparametric Kruskal-Wallis test for two- 

 way comparisons of the effects of glare and sea state on 

 California sea lion sighting rates. For these tests, each 

 transect segment with constant viewing conditions was 

 randomly assigned to one of five substrata, which served 

 as replicate samples for the tests. Viewing conditions 

 with significantly lower sighting rates were excluded 

 from the abundance analyses to reduce bias caused by 

 missed animals. 



Two a posteriori geographic strata were created, 

 inshore (50,546 km 2 total surface area) and offshore 



