DISTRIBUTION AND ABUNDANCE OF COMMON DOLPHIN, 



DELPHINUS DELPHIS, IN THE SOUTHERN CALIFORNIA BIGHT: 



A QUANTITATIVE ASSESSMENT BASED UPON AERIAL TRANSECT DATA 



Thomas P. Dohl, Michael L. Bonnell, and R. Glenn Ford 1 



ABSTRACT 



On 35 aerial transect surveys of the Southern California Bight, 157 sightings of common dolphin, 

 Delphinus delphis, schools were observed and mapped for distributional analysis. Sightings were pooled 

 into 30' of latitude by 30' of longitude sampling quadrats, and density estimates were obtained by fitting 

 a Fourier series to a frequency distribution of perpendicular sighting distances. Two distinct seasonal 

 distributions are represented by density contour maps: a winter-spring distribution when schools were 

 confined to the easternmost and warmest waters of the area, and a summer-autumn distribution when 

 schools were widespread. Mean seasonal population estimates were 15,448 for winter-spring and 57,270 

 for summer-autumn (cv of 0.36 and 0.17, respectively). During the warmer water months, the common 

 dolphin population expands its use of the Southern California Bight. They enter from the south, apparently 

 following the major undersea ridges and escarpments, and flow through the Southern California Bight 

 in a generalized counterclockwise fashion. Observational evidence suggests that there is mixing of both 

 the nearshore and pelagic forms of this species in the offshore waters over the Santa Rosa-Cortes Ridge 

 and Patton Escarpment. 



The common dolphin, Delphinus delphis, is the most 

 abundant cetacean in the waters of the Southern 

 California Bight (SCB). On an annual basis the num- 

 bers of common dolphins exceed, on average, the 

 combined total of all other cetaceans in this area by 

 2.75 times (Dohl et al. 1980). 



Common dolphins inhabit subtropical waters of 

 Mexico and the SCB throughout the year (Norris 

 and Prescott 1961). Density estimates for this 

 species and other dolphins (Stenella sp.) in waters 

 offshore of Mexico and Central America were 

 calculated by the National Marine Fisheries Service 

 in 1974 (Smith 1981). The distribution of common 

 dolphins in the eastern parts of the Southern Califor- 

 nia Bight was described by Evans (1975). 



In order to understand the role of the common 

 dolphin in the ecology of the SCB and to understand 

 when and where this population is mostly vulnerable 

 to human activities, we have constructed a spatial- 

 seasonal distributional model with two aims: 1) to 

 generate population estimates for the entire area 

 and 2) to describe the general features of seasonal 

 distribution patterns. This is the first study to ex- 

 amine the spatial heterogeneity of common dolphin 

 distribution in the SCB and to generate confidence 

 limits for density and seasonal mean population size 

 estimates. 



'Institute for Marine Sciences, University of California, Santa 

 Cruz, CA 95064. 



Manuscript accepted July 1985. 



FISHERY BULLETIN: VOL. 84, NO. 2, 1986. 



From April 1975 through March 1978, nearly 

 110,000 nmi (200,000 km) of combined aerial and 

 ship surveys were conducted within the SCB for the 

 Department of the Interior, Bureau of Land Man- 

 agement (now the Minerals Management Service). 

 During this marine mammal and seabird study, a 

 total of 505 schools of 134,675 Delphinus delphis 

 were recorded. 



This paper is primarily concerned with one subset 

 of the 3 yr, common dolphin sighting data base. To 

 avoid the statistical pitfalls of pooling data obtained 

 from a variety of platforms performing their mis- 

 sions at different speeds, at different altitudes, and 

 over varying portions of the study area, we re- 

 stricted these analyses to 35 monthly flights flown 

 at 1,000 ft above sea level (ASL). Each of these 

 surveys required about 15 overwater flight hours 

 and covered about 1,350 nmi (2,500 km) of track- 

 line. All species of cetaceans encountered were 

 recorded as to location, number, behavior, direction 

 of movement, and number of juveniles. Common 

 dolphins were encountered 157 times in this flight 

 series, for a total of 46,153 animals or 69% of all 

 cetaceans observed. 



The results of the distributional study and accom- 

 panying figures were derived from the 1,000 ft ASL 

 aerial survey data defined above. However, material 

 in the Discussion section draws upon observations 

 made from all survey platforms used during this 

 study. 



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