FISHERY BULLETIN: VOL. 86, NO. 2 



been carefully edited for positional errors. 

 Cruises from 1979 were excluded either because 

 of insufficient positional data or because the ves- 

 sel made port stops during the middle of the 

 cruise. Inclusion of the time spent searching on 

 the way in and out of port could distort the results 

 on searching and encounter rates between clus- 

 ters. Preliminary analyses suggested that there is 

 little difference in the results between vessels 

 that went into port and those that did not. 

 However, to avoid further complicating the 

 analyses, these cruises have not been included 

 (see Polacheck 1983 for more detail). 



RESULTS 



Is Effort Concentrated? 



The percentage of sets or chases for which the 

 nearest one was not the preceding or next set or 

 chase ranged from 12 to 77 among cruises (Fig. 3). 

 The mean percentage was 41 (SE = 1.9, n = 35). 

 The average ratio of the physical distances be- 

 tween the next or preceding set or chase com- 

 pared to the distance to the nearest set or chase 

 within a cruise was 1.45 (SE = 0.044, n = 35) and 

 ranged from 1.00 to 2.24. Consideration of the 

 same statistics for the distances between sight- 

 ings of spotted dolphin indicates an even more 

 concentrated pattern. The mean percentage of 

 sightings for which their nearest neighbor was 

 not either the next or preceding one equalled 80 

 (SE = 2.8, n = 34, note one cruise recorded no 

 sightings of spotted dolphins) and ranged from 25 

 to 100 (Fig. 4). The average ratio of the distance 

 between the next or preceding sighting compared 

 with the nearest sighting was 4.05 (SE = 0.261, 

 n = 34) and ranged from 1.14 to 7.82. For these 

 sighting statistics, low percentages and ratios 

 near 1.00 are found in vessels with few sightings 

 (Fig. 4). These results suggest that in general ves- 

 sels return to the area of a previous sighting and/ 

 or set and search in that area at least 41% of the 

 time. 



There appears to be large differences among 

 vessels in their ability and success at locating 

 potential fishing targets. Thus, the average dis- 

 tance searched between sets or chases varies by 

 about a factor of 4 among cruises, while the aver- 

 age physical distance between sets or chases 

 varies by about a factor of 6 (Fig. 5). There is little 

 relationship between the average distance 

 searched between sets and chases and the aver- 

 age distance to the next one. However, vessels 



15 



> 

 u 



z 



U4 

 3 

 O 

 m 

 a 

 u. 



10 



I 1 1 1 1 1 1 1 



10 20 30 40 50 60 70 80 



PERCENT OF THE SETS AND CHASES 



IN WHICH THE NEAREST ONE WAS 



NEITHER THE PRECEDING OR NEXT ONE 



Figure 3. — The frequency distribution for the percentage of the 

 sets and chases within a cruise for which the nearest other set 

 or chase was neither the preceding or next one. 



15 



10 



UJ 



O 



111 



cc 

 u. 



5 - 



— I — 

 60 



100 



30 40 50 60 70 80 90 



PERCENT OF SIGHTINGS IN WHICH 



THE NEAREST ONE WAS NEITHER 



THE PRECEDING OR NEXT ONE 



Figure 4. — The frequency distribution for the jjercentage of 

 sightings of schools of spwtted dolphins, Stennella attenuata , 

 within a cruise for which the nearest other sighting was neither 

 the preceding or next one. Shaded portions represent cruises 

 with less than 15 sightings (note one cruise had no sightings of 

 spotted dolphins). 



356 



