446 



Fishery Bulletin 102(3) 



Figure 4 



Schematic diagram of simultaneous helicopter-and-boat 

 surveys for Hector's dolphins south of Bank Peninsula, 

 South Island, New Zealand. 



to the boat was determined with the boat's RADAR. 

 The absolute position of the boat was determined to 

 an accuracy of 2-5 m by differential GPS (Trimble 

 GeoExplorer; postprocessed). Distances to land were 

 obtained at the time of sighting with RADAR or during 

 analysis by using GIS coastline data and the computer 

 program "SDR Map" (Trimble Navigation, Christchurch, 

 New Zealand). 



Boat observers followed our standard sighting pro- 

 cedures (see above). On most occasions the helicopter 

 was outside the field of view of the observers' binoculars 

 because the observers were scanning the water surface, 

 and the helicopter was well above what the observers 

 could sec through the binoculars. When it was within 

 their view, observers made a conscious effort to remain 

 unbiased by the movements of the helicopter. On mak- 

 ing a sighting, the helicopter observer informed an 

 independent observer located in the cabin (observers 

 on the platform could not hear communications from 

 the helicopter observer and vice versa). The helicopter 

 then hovered briefly above the sighting while a range 



and bearing in relation to the boat was taken by RA- 

 DAR. The helicopter then ceased hovering but tracked 

 the group of dolphins either until the boat observers 

 had sighted the group, or the group had passed abeam 

 of the boat. A second range and bearing were then 

 taken. Sightings lost by the helicopter observer during 

 tracking were discarded in our analyses. The indepen- 

 dent observer, in liaison with the helicopter observer 

 and boat observers, determined whether the sighting 

 was a duplicate (i.e., made by both helicopter and boat 

 observers) by using information on location and group 

 size. These decisions were checked again in analysis by 

 inspection of plotted locations of sightings made from 

 either platform or both platforms. 



Following the approach of Buckland and Turnock 

 (1992), let 



gjy) = the probability that a group detected from the 

 helicopter at perpendicular distance y from the 

 trackline of the ship is subsequently detected 

 from the ship; 



fs(y) = SsW'H' with ,» = | M "< y)<b 



(area under helicopter detection function ). 



w - truncation distance for perpendicular distances 



y; 

 n h = number of helicopter detections; 

 n s = number of ship detections; 



n hs = number of detections made from both platforms 

 (duplicate detections); 

 f/Jy 1 = probability density function fitted to helicopter 



detection distances; 

 f'hJy' = probability density function fitted to duplicate 

 detection distances as recorded from the heli- 

 copter; 

 fix) = probability density function fitted to perpen- 

 dicular distances recorded from the ship; 

 L = length of transect line. 



A conventional estimate of density of groups, assuming 

 no responsive movement andglO) = 1 (all animals on the 

 trackline seen with certainty) is calculated as 



IK 



nj(0) 

 2L 



(5) 



A corrected estimate, allowing for responsive movement 

 and including an estimate of g(0) is given by 



where / s (0) = — 



IX 



g s (0) 



»,./,(()) 

 214(0)' 



\™gAy)d) 



Ja 



",/ 1 ) I 



(6) 



(7) 



(8) 



