FISHERY BULLETIN VOL, 77, NO, 2 



1800- 



<I400- 



;I000 



m 600 



200- 



- n 



nn 



a 



1-5 I0H5 20-25 30-35 40-45 50-55 60-65 



5-10 15-20 25-30 35-40 45-50 55-60 



DEPTH (m) 



FIGI'RE. 3,— The number of theodolite readings of dolphins found 

 in different depths of water lai and the total amount of area 

 available in the study region at different depths of water, at a 

 mean tide height of ,5,0 m above mean low water (bi. Most 

 readings were obtained in shallow water despite the fact that 

 more geographic area was covered by deeper water. The total 

 area used for these calculations is represented by the lined and 

 crosshatched sections of Figure 1 



afternoon, and in deeper water around noon (Fig- 

 ure 4a). When deepwater movement occurred, 

 however, it was brief (16 min average per move- 

 ment in water >10 m deep, SD = 7.1, n = 230) and 

 was interspersed with longer shallow-water 

 travel. As a result, the increase in mean depth 

 around noon of Figure 4a and b was not because 

 animals consistently traveled in deeper water at 

 those times. Instead, they more often moved for 

 brief periods into deep water, and therefore the 

 mean depths increase at those times. When the 

 data were divided into months (Figure 4b, c), the 

 nonsummer months of March, July, October, and 

 November account for the movement into deeper 

 water shown in Figure 3a. This trend was particu- 

 larly strong for July (midwinter in Argentina), 

 with a peak of 23 m depth at 1300 h. On the other 

 hand, no increase in depth of water during midday 

 took place in summer (December and January, 

 Figure 4c). It would appear that there are predict- 

 able seasonal and daily variations in depth of 

 water in which the dolphins move. 



Speed of Movement 



The overall mean speed of the dolphins calcu- 

 402 



lated from the 1.545 theodolite readings made 

 within 30 s of each other was 6.1 km/h. Speed of 

 travel was significantly correlated with depth of 

 water; speed was 5.7 km/h in water <10 m deep, 

 and 13.9 km/h in deeper water (Figure 5). But, was 

 speed directly influenced by depth of water, or was 

 it due to distance from shore, which in general 

 increased with increasing depth? To solve this 

 ambiguity, we took random samples of 10 readings 

 each from 1) >600 m from shore and slO m 

 depth, 2) <600 m from shore and 5^10 m depth, 

 and 3) <600 m from shore and slO m depth, and 

 compared their speeds (Table 1). If speed were 

 influenced by distance from shore, we would ex- 

 pect speeds of li and 2) to be different. Instead, 

 speeds of 2) and 3) were significantly different 

 (P<0.005, Wilcoxin two-.sample test, Sokal and 

 Rohlf 1969), indicating that depth of water, not 

 distance from shore, was probably the prime de- 

 terminant of speed increase. 



Table l ,— Samples of bottlenose dolphin speeds i kilometers per 

 hour! in three different water conditions, selected at random 

 from the data. 



Speed of travel appeared uniform throughout 

 the day (Figure 6a), but a further subdivision into 

 months (Figure 6b, c) shows that there was an 

 increase around noon in nonsummer months, with 

 the average speed over 14 km/h at 1300 h during 

 October. During December and January, no such 

 midday peak was evident, but instead animals 

 traveled more rapidly during late afternoon than 

 at other times of day. 



Movement Patterns 



In water <10 m deep, bottlenose dolphins al- 

 most always moved parallel to the depth lines; 

 that is, they stayed in consistent depth (Figure 71. 

 In deeper water, movement was more random and 

 dolphins at times rapidly crossed into different 

 depths. Nevertheless, a tendency to follow depth 

 contours was still present. 



