BARLOW ET AL.: AERIAL SURVEYS OF HARBOR PORPOISE 



STRIP TRANSECT 



300 400 

 DISTANCE (meters) 



500 



600 



700 



E 



CO 



o 



H 



100 200 300 400 



DISTANCE (meters) 



500 



600 



700 



Figure 1.— Configuration of transect strip widths and distributions of perpendicular sighting distances for aerial 

 surveys in 1984 and 1985. Angles are given as declinations from horizontal. Histograms indicate the relative number 

 of porpoise seen in the given distance interval. 



four swaths (inside and outside on both sides of the 

 aircraft). In severe sun glare conditions, searching 

 was discontinued for one or both swaths on one side 

 of the aircraft. Additional data included individual 

 observer numbers, date, time, and position (meas- 

 ured to lOths of minutes of latitude and longitude). 

 All of these data were recorded at the beginning and 

 end of continuous transects and whenever condi- 

 tions changed or a sighting was made. Additional 

 data were recorded for marine mammal sightings, 

 including a code for the species of animal seen, an 

 estimate of the number of individuals, a code in- 

 dicating on which side of the aircraft the animals 

 were seen, and a code indicating in which of the 

 swaths the animals were found (inside, outside, or, 

 if the animals were not within the designated strips, 

 neither). 



An attempt was made to gather the above infor- 

 mation in a similar manner for both surveys. Some 



differences in subjective measures of sighting con- 

 ditions could, however, be expected because there 

 was no overlap in observers between years. In addi- 

 tion, there were some differences in design between 

 the 1984 and 1985 surveys. In 1984, the inside and 

 outside swaths were from 123 to 305 m and from 

 305 to 620 m, respectively (as measured from the 

 midline of the transect). The margins of these 

 swaths corresponded to declinations angles of 19°, 

 35°, and 60° (Fig. 1). In 1985, the swaths were 

 91-294 m and 294-503 m and corresponded to 

 declination angles of 23°, 36°, and 67° (Fig. 1). In 

 1984, effort was concentrated on the inside swath 

 and the outside swath was not intended for abun- 

 dance estimation. The change in swath size was in- 

 tended to reduce the total area being searched, thus 

 potentially allowing the outer swath to be used for 

 density estimation. In 1985, effort was divided 

 equally between the two swaths. During the 1985 



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