inclinometer, categorizing sightings by 90° to 40° 

 (< 200 m from the transect) or 40° to 20° (between 

 200 and 630 m from the transect). 



Species identification problems were not a factor 

 during the experiment since 1) all aerial and ship- 

 board observers were experienced in P. phocoena 

 observations; 2) land-based observers reported no 

 other cetacean species in the vicinity during the 

 entire week; 3) P. phocoena, although sometimes dif- 

 ficult to spot, once sighted has clear and unique field 

 marks that make it easy to identify. 



Results 



Triangulation of land station sightings resulted in 

 accurate plots of the harbor porpoise movements 

 through the area for every set of four transects. Air- 

 craft and shipboard sightings were then plotted using 

 the same methods over the same time periods. The 

 number of sightings made by aircraft and shipboard 

 platforms were then compared independently 

 against the number of sightings made by ground 

 stations to test sightability from each platform. A 

 " sighting" represents one or more porpoise. Analysis 



of observations by all platforms on all days show that 

 average "sighting" group size was 1.94 to 2.39 por- 

 poise per group (Table 2). Reported group size from 

 moving platforms was generally lower than observed 

 from the shore. 



Comparison between aircraft and ground counts of 

 the number of sightings of harbor porpoise groups 

 indicated that the aircraft observers consistently 

 sighted only 10 to 20% of the harbor porpoise groups 

 available in the passage (Table 3). The shore-based 

 observers, using aircraft sightings for comparison, 

 were estimated to sight about 80% of the available 

 harbor porpoises. 



Comparison between shipboard and ground obser- 

 vations was more inconsistent because of smaller 

 sample size (Table 4). Shipboard observers sighted 

 about 50% of the harbor porpoises in the area and 

 ground-based observers about 60%. 



Discussion 



Analysis of the data shows that shipboard observers 

 are more likely to see harbor porpoise than aircraft 

 observers. Although vessels may not be as efficient as 

 aircraft in terms of the amount of area covered, air- 

 craft observers tend to miss porpoise because of their 

 small size, the high survey speed, and limited effec- 

 tive survey width. Aircraft effectiveness appears to 

 rise in high-density porpoise areas. However, the 

 results suggest that shipboard surveys are the 

 superior method in estimating harbor porpoise dis- 



Table 3.— Numbers of groups of harbor porpoise observed by ground 

 observers and from the air in Head Harbor Passage, New Brunswick, 

 August 1980. 



Observed from aircraft 



Observed 

 from 



5 August 



7 August 



8 August 



calculated probability of sighting from aircraft. 

 i calculated probability of sighting from around 



Total 



TABLE 4. — Numbers of groups of harbor porpoise observed by ground 

 observers and from shipboard in Head Harbor Passage, New Brunswick, 

 August 1980. 



' S = calculated probability of sighting from shipboard. 

 2 G = calculated probability of sighting from ground. 



912 



