POLACHECK: DISTRIBUTION OF SEARCHING EFFORT 



However, some running can still be expected 

 within clusters because vessels mark favorable 

 logs with radio transmitter in order to return to 

 them at a latter time. It should be noted that 

 three or more sets or chases rarely occur on the 

 same day (Fig. 8). Therefore, the proportionately 

 smaller amount of running that occurs within 

 clusters is not an artifact resulting from clusters 

 that do not span more than a single day. 



Are Clusters Areas of 

 High Spotted Dolphin Densities? 



Encounter rates {total number of sightings of 

 spotted dolphins divided by the total distance 

 searched) tend to be much greater within clusters 

 dominated by sets in association with dolphins 

 than either within clusters dominated by non- 

 dolphin sets or while searching between clusters 

 (Table 5). The clustering algorithm would tend to 

 ensure that such differences are likely to occur, 

 but the magnitude of the differences is large and 



900 



800 



700 



600 



> 



o 



500 



LLI 



o 



lu 400 

 a. 



300 

 200 

 100 



° 1 2 3 4 5 



NUMBER OF SETS OR CHASES 



Figure 8. — The frequency distribution of the number of sets 

 or chases during a day for all days on which there was at 

 least one chase or set. 



Table 5. — Comparison of encounter rates (number of schools of spotted dolphins, Stenella 

 attenuata, per 100 miles searched) while searching within clusters classified according to the 

 percentage of sets involving dolphins, and while searching between clusters. Unweighted 

 means are the average value of the encounter within each cruise. Weighted means are based 

 on the encounter rate weighted by the distance searched by a cruise and are equivalent to the 

 total number of sightings divided by the total distance searched pooled across all vessels. 



361 



