HEWITT DOLPHINS' REACTION TO A SURVEY VESSEL 



ter and detected by shipboard observers appeared 

 to react to the ship. Spotted dolphins, Stenella 

 attenuata, and spinner dolphin, S. longirostris, 

 reacted at a distance of 0.5 to 2.5 nmi and were 

 able to maintain a separation of 0.5 to 2.0 nmi from 

 the ship; one school of striped dolphins, S. 

 coeruleoalba. was successfully tracked and these 

 animals stayed on a collision course with the ship 

 until they were only a few hundred meters away. 

 In all cases but one (school 8), the schools were 

 detected by shipboard observers at distances far 

 greater than the reaction distance. 



None of the four dolphin schools successfully 

 tracked in front of the Surveyor appeared to react 

 to the approach of the ship. Five out of 15 schools 

 appeared to react to the approach of the David 

 Starr Jordan. 



Estimates of School Size and 

 Species Composition 



Six schools were approached at close range by 

 the David Starr Jordan so that shipboard observ- 

 ers could make estimates of school size and species 

 composition using the same techniques that were 

 used on previous abundance surveys. Estimates of 

 school size and species composition were made in- 

 dependently by four to six shipboard observers and 

 averaged, giving each an equal weight. These es- 

 timates compared favorably with estimates made 

 by a single aerial observer stationed in the heli- 

 copter (Table 2 ). Shipboard estimates of school size 

 ranged from 65 to 134% of the aerial estimates and 

 averaged 101% (mean difference = 1.167; Pr = 

 0.713, paired t test of mean difference = 0); ship- 

 board and aerial observers agreed on the species 

 composition for all six schools compared, although 

 there was some variation in the proportion as- 

 signed to each species. 



DISCUSSION 



The density estimator used in line-transect ap- 

 plications, formally derived by Burnham and An- 

 derson (1976), and used to estimate the density of 

 dolphin schools by Smith (1981) and Holt and Pow- 

 ers (1982), is: 



D 



Nf(0) 

 2L 



where D is the estimated density of dolphin 

 schools in the survey area based on the number of 

 schools observed, A^, over transect length L. The 

 function fix) is a probability density function fit to 

 the observed perpendicular sighting distances and 

 estimating its value at zero distance, /"(Oi, is the 

 critical concern in the application of line-transect 

 methods (Burnham et al. 1980). 



The frequency distribution of observed perpen- 

 dicular sighting distances reflects both the detec- 

 tion abilities of the observer and the reactions of 

 the observed (Burnham et al. 1980). Dolphin 

 schools are more difficult to see with distance from 

 the track line and avoidance, prior to detection, 

 may cause fewer schools to be seen close to the 

 track line and more schools to be seen further from 

 the track line. The school that did move away from 

 the transect line before shipboard detection (#8) 

 would have been sighted at 0.1 nmi off the transect 

 line if it had not altered the direction of its move- 

 ment. Instead it was detected at 1.0 nmi off the 

 transect line. If the sample size was larger, such 

 information could be used to dissect the frequency 

 distribution of perpendicular sighting distances 

 into that component which is the result of decreas- 

 ing visibility with distance from the transect line 

 and that component which is the result of dolphin 

 schools adjusting their natural spatial disposition 



Table 2. — Comparison of shipboard and aerial estimates of dolphin school size and species composition. 



'Not detected by shipboard observers while In survey mode; ship was directed to school by aerial observer. 



191 



