Douglas et al Seasonal distribution and abundance of cetaceans off Southern California 
203 
r 
0 
500 
— i 1 1 
1000 1500 2000 
Distance (m) 
B 
Distance (m) 
c 
Distance (m) 
Figure 2 
Detection function plots by species group ([A] large whales, I B] delphinids, and |C] 
Dali’s porpoise [ Phocoenoides dalli ]) were created to visualize the correct detection 
functions to estimate density and abundance for the species most commonly encoun- 
tered in the study area for line-transect surveys conducted off Southern California in 
2004-08 during 16 quarterly California Cooperative Oceanic Fisheries Investigation 
cruises. The points are the probability of detection for each encounter dependent on 
its perpendicular distance and chosen covariate(s) for the best fit. The sighting data 
showed some evidence of heaping ( i.e. , rounding to certain distances) because of the 
limitations of the use of reticles to estimate distance. Therefore, sightings were binned 
to facilitate data analysis (Buckland et al., 2001). For detection function covariates 
and summary statistics, see Table 2. 
egory. Selection of a detection function model was based 
primarily on the Akaike’s information criterion (AIC) 
value (generated with Distance) and then confirmed by 
visual examination of detection plots (Burnham and An- 
derson, 2002). Half-normal and hazard-rate key func- 
tions often provide a good fit to data used to model 
detection functions (Thomas et ah, 2010). Although both 
were considered in the models tested, the hazard-rate 
