FISHERY BULLETIN: VOL. 86, NO. 3 



A principal assumption of strip transect methods 

 is that all individuals within the designated strip are 

 counted. Assuming that the fraction of diving 

 animals does not vary with sighting conditions, any 

 effect of sighting conditions on apparent density is 

 likely due to missed animals. In any instance where 

 we show that poorer sighting conditions result in 

 a significant decrease in apparent density (one-tailed 

 for paired tests, tw^o-tailed for ANOVA tests), we 

 eliminate the category of sighting conditions which 

 resulted in that lower estimate. If it is not possible 

 to predict which category would be worse a priori 

 in paired tests (e.g., right swath vs. left), two-tailed 

 probabilities are used. Although we cannot be sure 

 of eliminating all biases using these methods, this 

 pattern of data paring should avoid much of the bias 

 due to missed animals. 



RESULTS 



In 1984, 247 groups of harbor porpoise (680 in- 

 dividuals) were seen within transect strips which 

 covered a linear distance of 5,763 km. In 1985, we 

 saw 119 groups (384 individuals) in surveys of 3,715 

 km. Mean group sizes were 2.75 and 3.23 in- 

 dividuals, respectively, for 1984 and 1985. For 1984, 

 the relative frequencies of individuals seen within 

 the inside and outside swaths are illustrated in 

 Figure 1. For 1985, the perpendicular distances 

 from the trackline to the animals were calculated 

 from declination angles, and the relative distribu- 

 tion of sightings is shown in Figure 1 as a function 

 of perpendicular distance. 



Inside vs. Outside Swath 



For 1984 data, only the inside swaths were used, 

 but for 1985 both inside and outside swaths were 

 considered for density estimation. For 1985, we 

 tested whether the density in the inside swaths was 

 greater than the density of the outside swaths. We 

 only considered cases when the water surface pene- 

 tration codes were equal in both the inside and out- 

 side swaths. Data for inside and outside were thus 

 paired, with all other sighting factors equal. For 

 1985, the density in the inside was greater (0.09 vs. 

 0.06 porpoise/km^), but this difference was not 

 significant (P > 0.10). 



Surface Penetration 



Observers used a subjective coding system to 

 describe their ability to see through the sea surface. 

 Cloud cover, haze, and water turbidity contributed 



to poor surface penetration. In 1984, observers used 

 codes to indicate good and poor conditions; in 1985, 

 observers used codes for excellent, good, and poor. 

 There were frequent cases when observers recorded 

 different codes for the inside swaths on opposite 

 sides of the plane; hence, paired tests were again 

 appropriate. For 1984, we tested whether density 

 in the "good" category was higher than density in 

 the "poor" category. When surface penetration was 

 different in the inside swaths on opposite sides of 

 the plane, mean density in the "good" category was 

 higher than in the "poor" category (0.16 vs. 0.12 

 porpoise/km^), but this difference was not signifi- 

 cant (P > 0.25). For 1985, no tests were necessary 

 because the mean density in "excellent" category 

 was lower than in the "good" category, and like- 

 wise, density in the "good" was lower than in the 

 "poor" category. All categories of water surface 

 penetration were included in subsequent analyses. 



Effects Due to Observers and 

 Side of the Plane 



Sightings were classified based on which observer 

 made the sighting and on whether the sighting was 

 on the inshore or offshore side of the aircraft. In 

 fact, these two classifications were confounded in 

 1985 because the two principal observers were sit- 

 uated on the same sides of the aircraft for most of 

 this survey. Effects of these classifications on den- 

 sity estimation were considered together. Survey 

 teams were defined as pairs of observers who 

 worked together. There were two such teams for 

 1984 and three for 1985 (Table 1). Only one of the 

 teams in 1985 had sufficient numbers of sightings 

 to be considered here. Statistical tests were based 

 on paired cases during which both members of the 

 sighting team were searching. 



For 1984, porpoise density on the offshore side 

 of the airplanes was greater than on the inshore side 

 for both team A and team B (Table 2). The differ- 

 ence in density estimates between observers was 

 less than the difference between inshore and off- 



Table 2.— Relative harbor porpoise densities (km"^) for teams 

 of observers. Density estimates are stratified by inshore and 

 offshore sides of the aircraft and by individual observers. 



Observer 

 team 



Inshore Offshore 



Observer 

 1 



Observer 

 2 



A 

 B 

 D 



0.275 

 0.168 

 0.281 



0.425 

 0.244 

 0.164 



0.320 

 0.232 

 0.300 



0.380 

 0.179 

 0.146 



438 



