WURSIG and WURSIG: BEHAVIOR AND ECOLOGY OF THE DUSKY DOLPHIN 



50 — 



40- 



30- 



20- 



10 — 



10 12 14 



HOUR OF DAY 



18 



10 12 14 



HOUR OF DAY 



18 



Figure 3. — The number (a) and mean lengths (b) of dusky dol- 

 phin feeding bouts throughout the day, summed for 21 mo from 

 June 1974 to March 1976. Bars above and below mean feeding 

 lengths enclose 95^^ confidence intervals for means. 



Groups moved at about 5 km/h in water 1-10 m, 

 and faster in deeper water (average speed in water 

 55-60 m was 16 km/h). Furthermore, there was a 

 general movement from shallow to deeper water 

 as the day advanced (Figure 7a), and dolphins 

 moved more rapidly in the afternoon than in the 

 morning (Figure 7b). 



Because water depth and dolphin speed were 

 related (Figure 6), it is not surprising that dol- 

 phins, on the average, moved faster in those 

 months in which they were in deeper water (com- 

 pare Figure 8a with b). At the same time there was 

 a strong correlation between depth and speed dur- 

 ing different months and the amount of feeding 



n 



6 7 8 9 10 II 12 13 14 15 16 17 

 HOUR OF DAY 



Figure 4. — The number of >50-min surface feeding periods of 

 dusky dolphins during different times of day. Significantly 

 longer surface feeding periods occurred in the afternoon (0600- 

 1200h = 9or28'7r; 1200-1700h = 23 or 72%; testing equality of 

 percentages, arc sine transformation ofr statistic (5 1. 



iSOOi 



8 



z 



S 1400- 



< 



!^ 



u. 1000- 

 o 



□: 



S 600- 

 S 



2 



200- 



10- 

 8- 

 6- 

 4 



2- 

 



l^tTTTii^ 



10 20 30 40 50 60 

 DEPTH (m) 



FIGURE 5. — Number of theodolite readings of dusky dolphins 

 over depths from 2 to 65 m (a). Although most readings were at 

 5-10 m, a smaller peak occurred at 35-45 m which appeared to 

 correlate with feeding activity at that depth (see text). Amount of 

 area available in the study region as a function of water depth, at 

 a mean tide height of 5.0 m above mean low water (h). 



activity during those months (compare Figure 8a 

 with Figure 2a; correlation = 0.77, P = 0.003, 

 Kendall coefficient of rank correlation). 



Dolphin groups moved into deeper water in the 

 afternoon in each of the 7 mo for which adequate 

 depth versus time of day data exist (Figure 9a). In 

 August and September, when little surface feed- 

 ing occurred, and when water temperatures were 

 lower than in summer, dolphins stayed in rela- 



875 



