has been presented by Leatherwood and Reeves 

 (1978). 



MATERIALS AND METHODS 



Dusky dolphins were observed at Golfo San Jose 

 (Figure 1 ) from September 1973 through January 

 1974 and from July 1974 through March 1976. We 

 made observations from shore and from a 4.5 m 

 rubber Zodiac^ boat powered by an 18-hp Evinrude 

 outboard motor. 



Shore observations were made through binocu- 

 lars, and movement patterns of dolphin groups, 

 ranging from six to several hundred individuals, 

 were followed with a Kern Model DKM 1 sur- 

 veyor's theodolite (see Materials and Methods in 

 Wiirsig and Wiirsig 1979). This technique allowed 

 us to describe where and how fast the dolphins 

 moved during different times of day. 



Observations were made from the boat by mov- 

 ing up to a group of dolphins and then stopping the 

 engine. This allowed us to drift near the dolphins 

 while taking notes on their behavior. We believe 

 that the natural behavior of dolphins was at times 

 affected by the presence of the boat, and therefore 

 made an attempt to confirm all behavior seen from 

 the boat by shore-based observations. 



To get some idea of group stability over time, we 

 spaghetti-tagged 24 individuals in conjunction 

 with a radio-tagging study. These tags were 

 color-coded plastic streamers lanced into the thick 

 blubber behind the dorsal fin. For a description of 

 the tags and tagging procedures, as well as radio- 

 track data, see Wiirsig (in press). 



To compare seasonal occurrence data with 

 water temperature, we measured temperature 1 m 

 below the surface 5-10 times per month. For uni- 

 formity, these readings were made 0.5-3 km from 

 shore, and in the afternoon. We used a calibrated 

 laboratory thermometer marked every 0.2° C from 

 5.0° to 30.0° C. 



Underwater sounds made by dolphins were re- 

 corded through U.S. Navy Sonabuoy hydrophones 

 suspended 5-10 m below the boat. They were re- 

 corded on a Sony TC800B reel-to-reel tape record- 

 er. A complete analysis of dolphin vocalizations 

 is not presented in this paper. However, mea- 

 surements were made of approximate distance of 

 travel before attenuation (sounds no longer picked 

 up by the hydrophones) of certain splash sounds 



^Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



872 



FISHERY BULLETIN: VOL, 77, NO. 4 



related to aerial behavior. During such measure- 

 ments, dolphin and boat position were recorded 

 from shore by surveyor's theodolite, thereby pro- 

 viding the distance from the sound source to the 

 hydrophone. 



We used standard statistical techniques to test 

 for differences and similarities of observations. 

 These techniques are from Sokal and Rohlf ( 1969) 

 unless stated otherwise. 



RESULTS 



Seasonal Occurrence Pattern 



On days with winds 20 km/h, it was difficult to 

 see dusky dolphins. Of the 433 days with winds 

 <-20km/h, dolphins were seen on 251 days, or 58%. 

 Dolphins were seen from shore during 19 of 21 

 mo (Figure 2a); June and July 1975 were the only 

 months without sightings. Although the rate of 

 sightings varied from month to month, there was 

 an increase in sightings from late winter ( August) j 

 to summer (February 1975; December 1975), and a ' 

 decrease from fall to midwinter (March through 

 June 1975). During both years, dolphins were pres- 

 ent on over 507c of days during which observa- 

 tions were made from August through February, 

 with the one exception of -50% on the days in 

 January 1976. 



Could this cycle of dolphin occurrence be related 

 to water temperature? Figure 2b shows average 

 surface temperature per month within 3 km of 

 shore during the same 21-mo period. Although 

 upon superficial examination it appears that dol- 

 phins were less often present during the coldest 

 months, this is not strictly true. Thus, although 

 August was the coldest month in both years, dol- 

 phins in August were present over 70% of sighting 

 days. The rise in temperature in spring-summer 

 1975-76, however, occurred earlier than in 1974- 

 75, and temperatures from September to February 

 were 1°-2°C higher per month than in the preced- 

 ing year. Dolphins were more abundant earlier in 

 1975-76 than in 1974-75. While "the peak" of dol- 

 phin presence occurred in January 1975, it oc- 

 curred in October 1975 in the next season, with a 

 sharp drop-off to January 1976. 



Where were the dolphins during the period from 

 March through July, when they were rarely 

 sighted in the study area? During 19 of 24 (79%) 

 boat trips made throughout the bay in these 

 months, we found them in the western part of 

 Golfo San Jose, closer to the mouth of the bay and 



