FISHERY BULLETIN: VOL, 77, NO. 4 



Table l .—Average speeds of dusky dolphins not associated with feeding, and associated with feeding activity. The difference in speed 



between no feeding activity seen (row 1 1 and speed around feeding activity (rows 2, 3. and 4) is significant (P<0.001, r-test of equality 



of means when variances are assumed to be heteroscedastic). 



Category 



No feeding activity seen 



Dolphins not associated with feeding activity in the area 



Movement towards feeding activity 



Movement out of feeding activity 



Relationships of Group Sizes, 

 Feeding, and Aerial Behavior 



For the purposes of this paper, we defined a 

 group as a number of animals that are swimming 

 together and moving as a unit (but not necessarily 

 all pointed in the same direction). Individuals of a 

 group were usually within visual range and cer- 

 tainly within acoustic range of at least some con- 

 specifics. Group sizes varied from 6 to about 300 

 individuals. There was a seasonal shift in group 

 sizes. From May through September, groups with 

 <20 animals were more common than at other 

 times of the year (Figure 11). As stated earlier, a 

 low in feeding bouts occurred in the southeast part 

 of Golfo San Jose from March to September (Fig- 

 ure 2a), and we gained the impression from boat 

 trips to the middle and western section of the bay 

 that surface feeding there occurred with high fre- 

 quency in March and April , but did not often occur 

 anywhere in the bay from May to September. As a 

 result, it appears that smaller groups were most 

 abundant during the nonsurface feeding months 

 of May to September. 



There was a direct relationship between size of 

 dolphin group and surface feeding frequency. 

 Thus, groups with 20 individuals were found in 

 feeding bouts only 19% of the times they were 

 spotted, while groups with >20 animals were seen 

 feeding more of the time (Table 2). Because a sur- 

 face feeding-speed relationship was noted, it is not 

 surprising that speed of group travel increased 

 with increasing group size. While small groups 



<20 



>20 



Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 

 MONTHS 



Figure 1 1 . — Percentage of dusky dolphin groups with < 20 indi- 

 viduals compared with those with s20 individuals, by month. 

 Dashed lines connect one or both points with < 10 groups sighted 

 that month. October-April percentages are significantly diifer- 

 ent from those of May-September (P- 0.001, equality of percent- 

 age test with arc sine transformation). 



occurred most often in the morning (and were not 

 surface feeding), larger groups were most often 

 associated with feeding bouts in the afternoon 

 (Table 2). 



The larger the group size, the longer the feeding 

 activity lasted (Table 3). The number of birds also 

 increased with dolphin group size, and with length 

 of feeding (Table 3). Species of birds, in approxi- 

 mate order of decreasing numbers, were the 

 black-headed gull, Larus dominicanus; cormor- 

 ants, Phalacrocorax brasilianus and P. magel- 

 lanicus;tems, Sterna spp.;diff"erent species of Pro- 



TaBLE 2.— Average speed and time of day related to group size estimates, and percentage of times dusky dolphins of 

 three different group sizes were associated with feeding activity. Groups with <20 individuals were seen feeding less 

 frequently than larger groups (P- 0.001. testing quality of percentages, arc sine transformation of (-statistic). 



Dolphin group size (estimate) 



Item 



Number of theodolite readings used for speed data 



Average speed (l<m/h) 



Standard deviation 



Average time of day. Argentine local time (h mm) 



standard deviation expressed in hours and minutes 



Number of times seen feeding (a) 



Number of times seen while not feeding (b) 



Percentage of times seen feeding during total sightings (lOOaja 



b)l 



878 



