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Fishery Bulletin 102(3) 



Figure 2 



Map of New Zealand's South Island, showing transect lines and sightings 

 of Hector's dophins (dots) 1997-2000. 



Figure 3 



Example of transect layout in harbors 1 1997- 

 98 Akaroa Harbor transect lines and sight- 

 ings, showing three replicates I. 



Buckland et al.'s (1993) target of 60-80 detections for 

 robust ESW estimation, in the 1997-98 survey we con- 

 ducted replicate surveys (with a new set of lines each 

 time) in the harbors and bays stratum (e.g., Fig. 3). 

 Low sighting rates in the area surveyed in 1999-2000 

 would have required unrealistic effort levels to reach 

 this target; therefore we gained extra sightings from 

 areas with the same exposure but higher sighting rates 

 (e.g., data used to calculate ESW for the Marlborough 

 Sounds were supplemented by data gathered in Akaroa 

 Harbour by the same observers, in the same summer). 

 Hence different sample sizes were available to estimate 

 density and ESW (Table 2). Because observers changed 

 between surveys, we did not pool sightings across years 

 for estimating ESW. Strata areas were measured from 

 nautical charts with a digital planimeter. 



Data analysis 



Within each stratum, Hector's dolphin abundance (N s ) 

 was estimated as (Buckland et al., 1993): 



N s =^^. (1) 



2LESW 



where A = size of the study area; 

 n = number of groups seen; 

 S = expected group size; 

 L = length of transect line surveyed, and 

 ESW = the effective half strip width. 



Because there was no significant relationship between 

 group size and detection distance, expected group size 

 was estimated as a simple mean group size. 



