were noted in the species cited by individual observers of the matched pairs. All 

 observers participated in training sessions involving ground and aerial identification 

 methods. 



ALTITUDINAL EFFECTS 



Due to the great disparity in size, coloration, and location of various species, it is 

 important to consider the effect of the elevation of the aircraft on the detection and 

 identification of individuals and groups of organisms. Flights were conducted at 91 and 

 228 m in equal frequency in order to evaluate altitudinal effects and maximize data 

 accuracy for all groups. 



The total number of groups sighted during low (91 m) and high (228 m) flights are 

 compared by season in Tables 33 through 41. In three of the eight seasonal samples 

 (37.5%) more terns were seen during low flights than during higher flights. The number 

 of turtles seen in low flights exceeded that of higher flights in three of the eight 

 samples, and approached that of higher flights by 95% in two others. More mammals 

 were seen in four of the eight seasonal samples (50%) at low altitudes. Thus, in terms of 

 total numbers of groups seen, flight altitude may have only a slight effect on 

 detectability depending upon the group concerned. 



Because of the small size and fast movements of birds, it was hypothesized that 

 low altitudes would be necessary to allow identification. When the number of birds not 

 identified to exact species is compared to the total number of birds sighted in a high or 

 low sample (Table 42) an increase in unidentified birds at high altitudes is evident in 

 seven of the eight seasonal samples. Thus, even if detectability of birds remains the 

 same at both survey altitudes, the identifiability may be reduced at higher altitudes. 



In part, differences in detectability and identification may be related to the visual 

 scan techniques employed by the aerial observer. At higher elevations a broader field of 

 view is available which requires more deliberate eye and head movement. In fact, this 

 increase in area scanned affords less time for detection of animals in any one location. 

 However, because of the relatively slower speed of the aircraft in relation to the ocean's 

 surface, once an organism is detected it remains in view longer, giving the observer more 

 time to identify it. Such a time interval is in part nullified by the greater distance (both 

 horizontal and vertical increases are possible) from which the animal is viewed. In 

 practice, identifications of turtles, mammals, and most birds are often made 

 instantaneously, but deviation from the flight path to confirm identification is necessary 

 more often during high flights than during low ones. 



At present the practice of employing an equal number of flights at 91- and 228-m 

 altitude appears to be justified in providing opportunities to maximize detection and 

 identification of organisms of diverse sizes, habits, and group composition. It is probable 

 that alternating flights at low altitudes serve to reinforce indentifications at higher 

 altitudes. This would increase data quality at both altitudes. 



Actual effect of altitude is complex, with increased sightings often accompanied 

 by decreased identifiability. For example, in November-NFLA surveys only 10% of all 

 birds sighted during flights at 91 m were not identified to species whereas 34% of those 

 sighted in flights at 228 m were not identified to species. 



65 



