NOTE Forney: Decline in the abundance of Phocoena phocoena 



743 



sighting information (number of sightings, number 

 of porpoise, and mean group size) is provided in Table 

 1 for each year and region. Survey coverage was com- 

 parable in most years, but owing to poor 

 weather throughout the 1991 field season, 

 only about half of the usual replication was 

 obtained during this year. Differences also 

 occurred in the proportion of survey effort 

 obtained under good sighting conditions, 

 defined as Beaufort sea states 0-2 and as 

 less than 25% cloud cover. Consistent dif- 

 ferences in encounter rates, measured as 

 the number of porpoise observed per kilo- 

 meter surveyed, are apparent between the 

 three defined areas (Fig. 2). Substantially 

 higher encounter rates occurred in north- 

 ern California than in central California, 

 and the lowest encounter rates occurred 

 south of Monterey Bay, in the southern end 

 of this population's range. 



The best model obtained by the stepwise 

 selection procedure for central California 

 data included area, Beaufort sea state, and 

 cloud cover as categorical variables, and 

 year as the covariate (Table 2). All four 

 factors were significant at a = 0.10, with 

 probabilities ranging from 0.0001 to 0.0798 

 (Table 3). With the exception of the inclu- 



sion of the covariate year in the model, these results 

 are qualitatively the same as those for the analysis 

 of the first five years of data (Forney et al., 1991). 



0.7 



1 0.5- 



0.3- 



0.2- 



0.1- 



AREA1 



-III 



III. lie 



AREA 2 



ll 



i.lll 



AREA 3 



L 



1 3 5 8 10 12 14 16 18 20 22 24 26 42 44 46 48 50 52 54 56 

 2 4 6 9 1113 15 17 19 21 23 25 41 43 45 47 49 51 53 55 



Transect number 



Figure 2 



Mean number of harbor porpoise, Phocoena phocoena, observed per 

 kilometer on each of the surveyed transects. Only data obtained under 

 good survey conditions (Beaufort sea state 0-2, <25% cloud cover) are 

 included. Transect and area numbers correspond to those shown in 

 Figure 1. 



