FISHERY BULLETIN: VOL. 77, NO. 4 



OCCURRENCE 



-1 — 1 — I — I — I I r 



iJASOND|J FMAMJJASONDiJFM, 

 ii74 1975 19^6 



18- 



5 16 



I I I I I 

 A S N D, 



rr 



-TT 



I I I I I I I 



A S N D| J F 



1974 



1975 



1976 



Figure 2.— Fraction of possible days per month on which dusky 

 dolphins were sighted, and were seen surface feeding (a). The 

 y-axis represents the ratio of number of days on which dolphins 

 were sighted or were seen feeding divided by the number of days 

 each month with winds <20 km/h (sightable days). During all 

 sightable days, observations were made from dawn to dusk 

 Numbers above points represent the number of sightable days 

 per corresponding month. Average surface temperatures within 

 3 km of shore during the same 21-mo period as in Figure 2a(b). 



near the open ocean (Figure la). A large oceanic 

 mass of water changes temperature less rapidly 

 than nearshore shallow water, and this may have 

 influenced the dolphin's movement, perhaps by a 

 shift in prey location. Dolphins were found near 

 the mouth of the bay from March through July, 

 when temperatures in the study area dropped 

 from 17° to 11° C (Figure 2), and it is likely that 

 near-mouth temperatures decreased more slowly 

 due to the influence of the open ocean water. 



Although dolphins were present at the study 

 site most of the year, and were found in Golfo San 

 Jose the entire year, we did not know whether the 

 animals were part of the same population or herd 

 during all seasons. However, four spaghetti tags 

 inserted in December and January were resighted 

 in August, November, December, and January of 

 subsequent years. This indicated that at least 

 some of the animals were present in different sea- 

 sons, and thus did not appear to migrate. 



Seasonal and 

 Daily Surface Feeding Cycles 



Surface feeding of dusky dolphins was often 

 highly visible, with birds flocking above the feed- 

 ing site, allowing us to estimate from a distance 

 when and where the dolphins were feeding on 

 schooling fish (Figure 2a). Regardless of season, 

 whenever dolphins were seen they were often feed- 

 ing. However, in August and September 1974 and 

 1975, dolphins were present much of the time but 

 little surface feeding appeared to take place. Little 

 or no surface feeding took place in low-dolphin 

 months of June and July and in high-dolphin 

 months of August and September. This low in sur- 

 face feeding corresponded with the lowest temper- 

 ature period (about 12= C and below) of the year, 

 possibly because fewer food fish were in the area. 



When surface feeding bouts occurred, they were 

 observed throughout the day. However, the length 

 of feeding bouts increased as the day advanced. 

 Feeding bouts were longest at 1500 h, then de- 

 clined as evening approached (Figure 3). 



Although feeding lasted longer during the af- 

 ternoon (to 1500 h), there were nevertheless some 

 long feeding bouts in the morning (Figure 4), with 

 a significant increase in long bouts in the after- 

 noon. 



Depth of Water and Speed of Movement 



Are dusky dolphins found at certain water 

 depths and does their swimming speed vary with 

 water depth? To answer these and similar ques- 

 tions, we tracked group movements by surveyor's 

 theodolite. Figure 5a shows that they were most 

 often tracked while in water 5-10 m deep. This 

 peak is probably somewhat biased because obser- 

 vations were possible more often within about 1 

 km from shore, where depths of 0-30 m were found. 

 Nevertheless, since both 0-5 m and 10-30 m depth 

 areas approximated the area at 5-10 m, dolphins 

 appeared to have a clear preference for traveling 

 in water 5- 10 m deep while near shore. A small but 

 significant secondary peak also occurred at 35-45 

 m. Although dolphins traveled in water >65 m, 

 this has not been represented in Figure 5a, since 

 no water within sight was >65 m. For radio 

 tracked movement out of sight of land see Wiirsig 

 (in press). 



The overall average speed was 7.7 km/h. There 

 was a shift in speed depending upon depth of water 

 in which the animals were traveling (Figure 6). 



874 



