HEWITT DOLPHINS" REACTION TO A SURVEY VESSEL 



be sufficient so as not to elicit a response from the 

 animals. The dolphins appeared to be swimming 

 calmly throughout the tracking; similar experi- 

 ence was reported by Au and Ferryman (1982). It 

 also placed the helicopter above the shipboard ob- 

 servers' vertical field of vision and therefore did 

 not prematurely cue them on a school. Two oil 

 drums were released and tracked at the beginning 

 of the cruise to test the procedure: The resolution 

 of radar measurements was 1-2° in bearing and 0.1 

 nmi in range; at 1,200 ft (370 m) altitude we were 

 able to maintain visual contact with aim object; 

 and the shipboard observers were not aware of the 

 helicopter until it was within 1 nmi of the ship, 

 where the noise signaled its presence. Shipboard 

 observers were questioned periodically through- 

 out the experiment as to their cognizance of the 

 helicopter; answers were always in the negative 

 except when the binoculars were purposefully di- 

 rected above the searching field. Observers were 

 aware that looking for the helicopter would com- 

 promise the experiment and did not do so. 



At the finish of a track, the helicopter descended 

 to a lower altitude for additional photography and 

 to estimate school size and species composition. 

 The ship approached a limited number of schools 

 to enable close-range shipboard estimates of the 

 same school parameters. After school size and 

 species composition were determined, normal sur- 

 vey operation resumed, with the helicopter search- 

 ing ahead of the vessel and the shipboard observers 

 actively scanning and recording search effort. 



Relative motion radar plots were maintained. 

 Apparent change in the relative direction of dol- 

 phin school movement was used as an indication of 



avoidance; field notes of aerial observations of be- 

 havior supplemented this information. The 

 criteria defining reaction was a change of 30° or 

 more in the direction of relative motion that was 

 sustained over 2 or more subsequent fixes (Fig. 2). 

 The experimental design was opportunistic and 

 only specifically designed to compare between a 

 steam-powered survey vessel (NOAA Ship Sur- 

 veyor) and a diesel-powered survey vessel (NOAA 

 Ship David Starr Jordan). The experiment was 

 conducted within a 100 square nmi area to the 

 north and east of Clipperton Island (lat. 10°N, 

 long. 110°W) during March and April 1983.^ Ob- 

 servations were conducted with the Surveyor from 

 10 March through 17 March; the ship then ported 

 at Manzanillo, Mexico, to take on fuel and sub- 

 sequently met the David Starr Jordan, which had 

 just completed a marine mammal survey^ on 26 

 March at Clipperton Island. Observations were 

 conducted in the same area with the David Starr 

 Jordan until 7 April. 



RESULTS 



Avoidance 



Tracks were started on a total of 26 dolphin 

 schools, 5 in front of the Surveyor and 21 in front of 



^Cruise Report NOAA Ship Surveyor Cruise RP-12-SU-83 

 dated May 24, 1983, on file at the Southwest Fisheries Center, 

 National Marine Fisheries Service, NOAA, P.O. Box 271. La 

 Jolla, CA 92038. 



^Cruise Report NOAA Ship David Starr Jordan Cruise DS- 

 83-01 dated May 6, 1983, on file at the Southwest Fisheries 

 Center, National Marine Fisheries Service, NOAA, RO. Box 271, 

 La Jolla, CA 92038. 



300 



270' 



SCHOOL 8 

 Visual cue (birds) at 1306 

 Dolphins sighted at 1306 



300 



090° 270° 



SCHOOL 23 

 Visual cue (birds) at 0926 

 Dolphins sighted at 0954 



-090° 



Figure 2. — Relative motion plots of dolphin school #8 and school #23. School #8 appeared to react to the approach of the ship; the 

 sighting cue was reported after the dolphins' initial reaction. School #23 did not appear to react to the survey vessel. 



189 



