Defran And Weller 1999

MARINE MAMMAL SCIENCE, 15(2):366-380 (April 1999) 0 1999 by the Society for Marine Mammalogy

OCCURRENCE, DISTRIBUTION, SITE FIDELITY, AND SCHOOL SIZE OF BOTTLENOSE DOLPHINS (TURSIOPS T R U N C A T U S ) OFF SAN DIEGO, CALIFORNIA R. H. DEFRAN DAVID W. WELLER~ Cetacean Behavior Laboratory, Department of Psychology, San Diego State University, San Diego, California 92182-4611, U S A . E-mail: [email protected]

ABSTRACT The occurrence, distribution, site fidelity, and school size of bottlenose dolphins (Ttlrsiops trancattls) in the coastal waters of north San Diego County, California were assessed during a six-year boat-based photoidentification study. A total of 146 photographic surveys were conducted between January 1984 and December 1989. Dolphin schools were encountered on 79% of all surveys, and 2,869 individuals were observed in 145 separate schools. Three-hundred seventy-three dolphins were individually identified. All schools were sighted within 1 km of shore, and more than two thirds of the schools were encountered in the southern half of the 32-km long study area. School size (mean = 19.8, SD = 18.40) and the number of dolphins encountered per survey (mean = 26.8, SD = 22.30) were highly variable. Low resighting rates of known individuals provided little evidence for longterm site fidelity. When our six-year photoidentification database was combined with previous data, 404 dolphins were identified in the study area from September 1981 to December 1989. Jolly-Seber population estimates during the 1984-1989 study period varied between 234 and 285. The combination of regular dolphin occurrence, low site fidelity by known individuals, and the continuous increase in the rate at which new dolphins were identified indicates that numerous different individuals were visiting the study area across and within years. The open California coastline differs in habitat structure and prey distribution from more protected study areas where bottlenose dolphins display site fidelity. These habitat differences may Current Address: Marine Mammal Research Program, Texas A&M University, 4700 Avenue U, Building 303, Galveston, Texas 77551-5923, U.S.A.

366

DEFRAN A N D WELLER: BOTTLENOSE DOLPHINS

367

help to explain the observed intraspecific behavioral variability of this species. Key words: bottlenose dolphins, Tursiops truncatus, California, photoidentification, occurrence patterns, distribution, site fidelity, school size.

Investigations of geographically distinct populations of bottlenose dolphins (Tursiops truncatus) have clarified striking and subtle intraspecific behavioral differences commonly interpreted as specific adaptations to local ecological conditions (e.g., Wiirsig and Wursig 1977, 1979; Wiirsig 1978; Wells 1986; Ballance 1990, 1992; Shane 1990; Bearzi e t al. 1997). The reported variation in patterns of occurrence, distribution, school size, and site fidelity of bottlenose dolphins has been suggested to reflect differing habitat characteristics (Ballance 1992). This apparent interplay between environment and behavior is particularly evident when bottlenose dolphins occurring in protected marine environments are compared to communities living in more open habitats (Shane et a f . 1986, Wells et a f . 1987, Ballance 1992). Protected habitats have been found to foster relatively small school sizes, some degree of regional site fidelity, and limited movement patterns (Wells et af. 1987, Shane 1980). In contrast, semi-open habitats often sustain larger school sizes, diminished levels of site fidelity, and more expansive home ranges (Wursig 1978, Ballance 1992, Bearzi et al. 1997). The research presented here provides new information on the biology of bottlenose dolphins occurring in the open and fully exposed nearshore habitat of southern California. This coastline is quite dissimilar from regions where most other long-term studies on this species have been conducted and provides the opportunity to further evaluate the influence of habitat on bottlenose dolphin behavior.

METHODS Study Area The San Diego study area extended along 32 km of coastline from Scripps Pier, La Jolla (32"52'N) north to South Carlsbad State Beach (33'08") (Fig. 1). This coastline is located near the center of the Southern California Bight, a 732-km stretch of coastline extending from Point Conception in the north to Punta Colnett in the south. The sharp eastern break in the California coastline south of Point Conception marks a conspicuous change in the coastal climate and marine fauna due to the interruption of the California Current system (Southern California Coastal Water Research Project (SCCWRP) 1973, Dailey et al. 1993, Hickey 1993). The departure of the California Current from the coast results in a warming of the water within the Bight and creates the Southern California Countercurrent, The most significant effect of this nearshore countercurrent is the creation of a region in which northern, southern, western, and upwelling bottom waters converge to form a variable ocean-

368

MARINE MAMMAL SCIENCE, VOL. 15, NO. 2 , 1999

Figure 1. Map of San Diego study area. Inset shows relative location along California coastline.

DEFRAN AND WELLER: BOTTLENOSE DOLPHINS

369

ographic and biological coastal ecosystem (SCCWRP 1973, Dailey et al. 1993, Hickey 1993). Beaches in the San Diego study area offer little natural protection from oceanic conditions and are variable in composition, including gently sloping sand, steeply inclined cobblestone, estuary mouths, and rocky outcrops. Nearshore underwater topography ranges from submerged reefs, sea grass flats, and dense kelp canopies to relatively barren, sandy expanses. Vessel traffic within the study area is minimal and consists primarily of lobster and gillnet fishermen and occasional recreational anglers. Photoident$cation Surveys

Photographic surveys involved travel in a 4.3- or 5.2-m boat moving parallel to the coast, 90-180 m offshore of the surf line. All surveys were conducted in Beaufort sea state 1 3 and under visibility conditions adequate for finding and photographing dolphins. Two to four observers searched the area from the shore to 2 km offshore until a school of dolphins was sighted. To ensure that all members of a spatially separated school were located, the research vessel progressed approximately 1 km past these dolphins and then motored to a vantage point approximately 60-90 m from the school. Initial estimates of school size, as well as information on time, location, environmental conditions, and behavior were then noted. During most surveys, dolphins alternated between coalescing into a larger school and separating into smaller subgroups. These smaller subgroups usually remained within 500 m of each other and were considered to be part of the larger school from which they originated. Thus, a school was defined as any dolphins observed in close proximity to one another and usually moving in the same direction and engaged in similar behavior. School-size estimates were based on a consensus among all trained observers on board the research vessel and represented the maximum estimate during an observation period. Following initial estimates of school size, the vessel maneuvered to within 3-12 m of the school, and individual dorsal-fin notch patterns were photographed with 35-mm motor driven cameras, a 400-mm f4.0 telephoto lens and Kodak Tri-X film. Attempts were made to photograph every dolphin within a school. Initial estimates of school size were revised as necessary, and contact with the school was maintained until photographic effort was completed. The research vessel then motored offshore, where all film was labeled with the date, location, and school number. Identical procedures were repeated as the research vessel resumed travel on the predetermined survey route and as additional dolphin schools were encountered. Upon reaching the northern terminus of the study area, the research vessel motored offshore back to Scripps Pier. These offshore return trips varied from 1 to 4 km from shore and permitted us to opportunistically sample for bottlenose dolphins occurring outside the nearshore coastal study area. Two survey types were conducted during the study: complete and partial. Complete surveys covered the entire 32-km study area. Partial surveys, short-

370

MARINE MAMMAL SCIENCE, VOL. 15, NO. 2, 1999

ened by inclement weather or equipment failure, covered only a portion of the study area and always encountered at least one dolphin school.

Photoidentijication Our method for analyzing dorsal-fin photographs has been detailed elsewhere (Defran et al. 1990). Briefly summarized, only clear photographs of distinctive dorsal fins were used to establish a “type specimen” to which all other photographs were compared. Subsequently, only unambiguous matches with the “type specimen” were accepted as resightings. Two modifications were made in our application of this procedure: (1) the criteria used to categorize a fin as distinctive became more conservative as the size of the photoidentification catalog grew; (2) all photographs were examined by two, and more commonly, three people. As a consequence of these methodological refinements, minor changes in the summary statistics from earlier stages of our work (Defran et al. 1986, Hansen and Defran 1990) have resulted. Important population parameters and the site-fidelity characteristics described in these earlier summaries, however, remained unchanged. Our initial photographic catalog consisted of dorsal-fin photographs of 114 individuals which met our criteria for distinctiveness. One hundred and three of these dolphins were selected from those photographed by Hansen (1990) between September 1981 and January 1983, and 11 were selected from those photographed by Hansen and his associates between August 1983 and November 1983 at the National Marine Fisheries Service Southwest Fisheries Science Center (NMFS). Unless otherwise noted, all photographs taken between September 1981 and November 1983 are referred to as the NMFS sample, while photographs taken between January 1984 and December 1989 are referred to as the Cetacean Behavior Laboratory (CBL) sample. Results from the NMFS sample have been summarized by Hansen (1990). Thus, results presented here are mostly from the six-year CBL sample. For several analyses, however, the NMFS and CBL data sets were combined to form a nine-year sample.

RESULTS Survey Eflort One hundred and forty-six boat-based photoidentification surveys were conducted in the San Diego study area from January 1984 to December 1989. The mean interval between surveys was 15 d and the mean interval between surveys on which at least one dolphin was photographed was 22 d. Approximately 200 h were spent in direct observation and photography of 2,869 dolphins in 145 separate schools. Observations on individual schools averaged 83 min. Complete surveys accounted for 75% (n = 109) of all surveys conducted. Sixteen of these extended to Oceanside (33’12’ N), 7.4 km beyond the north-

DEFRAN AND WELLER: BOTTLENOSE DOLPHINS

60

371

-

r

gg s'L1 E

8'9

-U

0 2

5040-

30-

ZP, E a

20-

w

10-

If

0

I

ern terminus of our study area (Fig. 1). Partial surveys constituted 25% (n = 37) of all surveys. Twenty-five partial surveys were terminated in the southern half of the study area (<33"00'N), while 12 surveys extended by varying distances into the northern portion of the study area. Most partial surveys (84%) were carried out between 1984-1986. Only one of the 29 complete surveys between 1984-1986 encountered more than one dolphin school, suggesting that the single schools encountered on partial surveys were likely to have been the only school in the study area. Occurrence

Dolphin schools were encountered on 79% ( n = 116) of all surveys. Encounter percentages varied annually and ranged from a low of 60% in 1987 to a high of 95% in 1989. Of the 7 9 complete surveys on which dolphin schools were sighted, 75% ( n = 59) encountered only one school. Multiple schools (range = 2-5) were sighted on 25% ( n = 20) of these 7 9 surveys. Seventy percent ( n = 14) encountered two schools, 20% ( n = 4) encountered three schools, and 10% (n = 2 ) included either four or five schools. The mean number of dolphins encountered on a complete survey varied considerably both within and across years (Fig. 2). SeusonaZity

Survey effort, expressed as the number of surveys, was similar in all four seasons: winter (December-February; n = 32), spring (March-May; n = 38), summer (June-August; n = 41), fall (September-November; n = 35). Dolphins photographed 10 or more times between January 1984 and December 1989 ( n = 38) were used to test for evidence of seasonal occurrence. We restricted our analysis to this subset of frequently sighted individuals to reduce the possibility of creating spuriously high seasonal trends by inclusion of dol-

372

MARINE MAMMAL SCIENCE, VOL. 15, NO. 2 , 1999

-

15 10

1984

1985

1986

1987

1988

1989

Year

Figure 3. Mean school size by year. Error bars represent standard deviations.

phins sighted infrequently and over short periods of time. No evidence of seasonal occurrence patterns was apparent from this sample. Eighty-seven percent ( n = 33) of these 38 frequently sighted dolphins were photographed in all four seasons, while the remaining 13% ( n = 5) were photographed in at least three different seasons. Distribution

Sightings of bottlenose dolphins were distributed throughout the study area. These dolphins were the only odontocetes encountered during coastal surveys. All dolphin schools were sighted within 1 km of shore and most often were within 250 m of the shoreline. No bottlenose dolphins were encountered on our offshore return route. The majority of dolphins (70%) and dolphin schools (67%) were sighted in the southern portion of the study area between Torrey Pines State Park (32'52' N) and Solana Beach (33"OO' N) (Fig. 1). Similarly, when we analyzed only complete surveys, 60% of sightings occurred in the southern portion of the study area, indicating that this trend was not an artifact of our south-tonorth survey strategy. School Size, Site Fidelity, and Population Size

Mean school size ranged from 12.7 in 1986 to 28.8 in 1988 with an overall mean of 19.8 individuals (range = 2-90, SD = 18.40) (Fig. 3). School sizes were highly variable. Many schools (22%) contained between two and five dolphins (n = 32), and 75% ( n = 109) were composed of 25 or fewer dolphins (Fig. 4). On no occasion was a solitary dolphin observed. A total of 373 individuals were photographically identified in the study area from 1984 to 1989. When the study period was extended to include the 27 mo of NMFS data, the number of identified individuals increased to 404 (Fig. 5). The rate at which individual dolphins were first identified (rate of

373

DEFRAN AND WELLER: BOTTLENOSE DOLPHINS

35

School Size

Figwe

4. Frequency distribution of school size.

discovery) was analyzed by partitioning the combined NMFS and CBL photographic data sets (1981-1989) into blocks of 10 consecutive surveys in which at least one dolphin was identified (n = 125 surveys, Fig. 5). Rate of discovery was plotted as both the percent of new sightings per 10 surveys, and the cumulative number of new individuals identified. The cumulative rate of discovery curve showed a continued increase in the number of dolphins identified from survey blocks 1-12 (Fig. 5). One of the greatest increases in the number of new dolphins identified occurred late in the study period between blocks 9 and 10. By the end of 1989 (block 12), however, this function appeared to be asymptotic and only 4% of the dolphins photographed were "new. Sighting frequencies for the 373 dolphins identified during the study "

"

I

2

3

4

5 6 7 8 9 Blocks of Ten Surveys

1011

12

Figure 5. Percentage of new sightings and rate of discovery curve for dolphins photographed in San Diego from 1981 to 1989. National Marine Fisheries Service (NMFS) (1981-1983) and.CBL (1984-1989) data sets plotted in blocks of 10 surveys. Only surveys on which at least one identifiable dolphin was photographed were used. Blocks 1 and 2 contain data from 26 Hansen and NMFS surveys. Block 12 contains data from only nine surveys.

374

MARINE MAMMAL SCIENCE, VOL. 1 5 , NO. 2 , 1999 100,

90

2

B

E

t

-

80 70

P

60

3

40

50 c

ki 0

5

30 20 10

-

0

1

2

3

4

5

6

7

8

9 10 11 12 13 15 16 17 18 24

Number of Times Photographed

Figure 6.

Sighting frequencies for dolphins identified from 1984-1989.

ranged between 1 and 24 (mean = 4.6) (Fig. 6). The mean interval between resightings was 203 d, and many dolphins (24%) were sighted only once. Sixty-six percent (n = 245) of dolphins identified were resighted fewer than six times, an average of less than one sighting per year. Sightings-per-opportunityratios, which reflect sighting frequency as a funcsix tion of effort, were calculated for dolphins “frequently” photographed (I times). Ratios were derived by dividing the number of sightings for an individual dolphin by the number of surveys (opportunities) on which at least one dolphin was photographed (n = 99) over the six-year study period. The mean number of sightings per opportunity for these “frequently” sighted dolphins was 0.09 (SD = 0.030). This low mean value would have been further reduced had the calculation included surveys on which no dolphin schools were sighted. Hansen (1990) reported that 25% of the dolphins he identified were photographed only once. We examined the CBL photographic data set for the presence of the 103 dolphins we retained from the Hansen (1990) data set. Thirty (29%) of the dolphins first identified by Hansen in 1981-1983 were not photographed during the 1984-1989 study period. Among the subset of dolphins Hansen (1990) most frequently sighted (5-9 sightings), four were never photographed by us and six were sighted less than once per year. One of Hansen’s most frequently sighted dolphins, however, was also our most frequently sighted dolphin. Multiple mark-recapture estimates were calculated using the Jolly-Seber population estimator golly 1965; Caughley 1977; Seber 1982; Krebs 1985, 1989).This estimator, which is appropriate for open populations, was selected because of the low resighting frequencies (Fig. 6) and low sightings-per-opportunity ratios for dolphins in this study. Additional findings on coastal movement patterns (Defran e t al. 1999) indicate that dolphins photographed in San Diego are part of a larger open population. Dolphins identified in San Diego in 1984 (n = 98) were used as the marked individuals, and those identified each year between 1985 and 1988 were used as the recaptures.

DEFRAN AND WELLER: BOTTLENOSE DOLPHINS

375

Results from these calculations yielded the following annual population estimates and associated 95% confidence intervals: 1985 = 237 (CI 214-262), 1986 = 234 (CI 205-263), 1987 = 285 (CI 265-306), 1988 = 284 (CI 274-294).

DISCUSSION The behavioral ecology of most animals is thought to reflect specific adaptive strategies shaped, in part, by the unique ecological conditions under which a species or population exists. Bottlenose dolphins have now been studied in a variety of marine environments, with the most detailed and long-term information resulting from the work of Wells (1986) and his colleagues in Sarasota Bay, Florida (Wells et a/. 1987). The West Florida barrier-island habitat differs significantly from the open coastal environment of San Diego. Differences in patterns of site fidelity, school size, and population size between Florida and San Diego may be related to these habitat differences. While information on bottlenose dolphins in other habitats similar to San Diego has been collected (Wursig and Wursig 1977, 1979; Wursig 1978; Ballance 1990, 1992), the relatively short-term nature of these studies limits longitudinal comparison with the present research.

School Size School sizes for coastal bottlenose dolphins range from one to over 100 individuals but are most commonly 2-15 dolphins (Shane et al. 1986). The mean school size for bottlenose dolphins in San Diego (19.8) was larger than for most other coastal bottlenose dolphin populations. School sizes reported for this species are variable, due in part to differing definitions of a “school” (Shane et al. 1986). Coastal bottlenose dolphins in Argentina (Wursig 1978), the Gulf of California (Ballance 1990), and Portugal (dos Santos and Lacerda 1987) assemble in schools similar in size to those observed in San Diego. In contrast, considerably smaller mean school sizes (range = 3-7) have been reported for study areas in Texas (Shane 1980, Gruber 1981, Henningsen 1991, Brager 1992, Fertl 1994), the northern Adriatic Sea (Bearzi et a/. 1997), and Florida (Wells 1986, Shane 1990). Weller (1991) examined the social ecology of bottlenose dolphins in San Diego and concluded that the relatively large school sizes for dolphins in this area may be a response to a generally patchy distribution of primary prey species within the Southern California Bight (Dailey et a/. 1993). Variations in school size were further hypothesized to serve as a sociobiological adaptation to permit exploitation of variable and unstable food resources. Rate of Discovery

The rate at which previously unidentified dolphins were discovered remained relatively constant across the duration of the study, only approaching

376

MARINE MAMMAL SCIENCE, VOL. 1 5 , NO. 2 , 1999

a possible asymptote by 1989 (Fig. 5, blocks 10-12). Without further photographic data it is impossible to determine if this trend would have persisted. The continuous increase in the slope of the discovery curve may indicate that dolphins in the San Diego study area belong to a population that is larger than previously estimated (Hansen 1983). Hansen (1983) calculated a closed population estimate of 173-240 bottlenose dolphins in San Diego. The growth in the number of recognizable dolphins over time, however, along with considerations raised in the following sections, suggests that this population is more likely to be open (size of population may change during study period) than closed (see Jolly 1965; Seber 1982; Krebs 1985, 1989 for a review of distinctions between open and closed populations). Results of studies on bottlenose dolphins from other regions suggest that rate of discovery curves for new individuals reach an asymptote over shorter periods of time. The shape of such acquisition curves is, of course, related to the size and distribution of the population under study, the geographic scope of surveys, and the number of days in the field. Wells (1986) reported that after the initial 27 d of field work in Sarasota, Florida, a majority of the dolphins identified during the entire study had been photographed. Ballance (1990) reported similar findings for bottlenose dolphins in the Gulf of California, with most individuals identified approximately one month into the study. Finally, Shane (1987) reported that the rate of discovery for bottlenose dolphins at Sanibel Island, Florida, began to reach an asymptote by the end of her one-year study. It is important to consider, however, that the studies of Ballance (1990) and Shane (1987) were of limited duration and may not be directly comparable to the long-term work of Wells (1986) or the research presented here. Population Size Bottlenose dolphins in San Diego appear to belong to an open population estimated to range in size from 234 to 284 animals. Our population estimates closely match bottlenose dolphin abundance estimates recently derived from tandem aerial surveys in southern California (Carretta et al. 1998). Both of these estimates, however, may be slightly conservative. Aerial survey work (Carretta et al. 1998) covered only a portion of the known range for California coastal bottlenose dolphins (Defran et al. 1999). Photoidentification studies determined that approximately 65% of all dolphins observed had distinctively marked fins (Hansen and Defran 1990). Therefore, if the occurrence and distribution patterns of unmarked dolphins are similar to those of marked dolphins, the total number of identified dolphins may underestimate the population size by at least 35%. Site Fidelity and Distribution

Bottlenose dolphins occur throughout the year in the nearshore waters of San Diego but are not year-round or seasonal residents. Numerous lines of

DEFRAN AND WELLER: BOTTLENOSE DOLPHINS

377

evidence suggest that dolphins are essentially transient within the San Diego study area. Individual sighting frequencies (Fig. 6) are low across the six-year study period, as are sightings-per-opportunityratios. Also, the high variability in the number of dolphins encountered during a survey, both within and across years (Fig. 2), indicates that many dolphins present on some occasions were not present on others. Similarly, on numerous surveys there were no dolphins observed in the study area, especially during those years with low daily encounter probabilities (e.g,, 1984, 1985, 1987). Hansen (1990) concluded that the San Diego study area served as part of a permanent year-round home range for these dolphins and as part of a seasonal home range for others. Most of the dolphins Hansen identified, however, were sighted only once or twice during our study. Furthermore, the 21 dolphins Hansen identified as showing site fidelity to the San Diego study area were sighted on less than half of his surveys. In fact, many of these dolphins accumulated a high number of resightings over short periods of time (<1 mo) suggesting a pattern of short rather than long-term fidelity to the area. All schools encountered during photoidentification surveys were within 1 km of shore, and no dolphin schools were ever encountered 1-4 km offshore during our return surveys. Similarly, Hansen (1990) always found dolphin schools within 1 km of shore during boat-based and aerial surveys. Carretta et al. (1998) reported that 87% of all dolphin groups sighted during aerial surveys were within 300 m of shore, and Hanson and Defran (1993) reported that 99% of all dolphin sightings during a 12-mo cliff-based behavioral study were within 1 km of shore. This nearshore distribution pattern coincides with water depths ranging between 10 and 30 m. Wiirsig and Wiirsig (1979) found that bottlenose dolphins off Argentina showed a preference for water less than 10 m deep, and Ross and Cockcroft (1990) reported similar preferences for coastal bottlenose dolphins off southern Africa, where most sightings occurred in water depths of 15-30 m. When our distributional data collected nearshore are combined with findings of Barlow (1993) and Shane (1994) the probable dimensions of the preferred habitat for California coastal bottlenose dolphins are further refined. Barlow (1993) conducted systematic boat-based surveys of California waters up to 555 km offshore and found bottlenose dolphin schools to be concentrated near and among the offshore Channel Islands. Shane (1994) reported no matches when photographs of recognizable bottlenose dolphins from Catalina Island (-42 km offshore) were compared to identified individuals from San Diego. We propose, therefore, that a potential distributional gap exists between inshore bottlenose dolphins and those found near the offshore islands within the Southern California Bight. Kenney (1990) reported similar evidence for a distributional gap between inshore and offshore bottlenose dolphins off the northeastern United States. The preferred habitat of bottlenose dolphins off San Diego is a narrow “coastal corridor” which extends at least the 32-km length of our study area, with an offshore boundary no farther than 1 km from shore. Movement pattern data presented in Defran et al. (1999) suggest that this corridor also extends well to the north and south of San Diego.

378

MARINE MAMMAL SCIENCE. VOL. 1 5 , NO. 2 . 1999

Bottlenose dolphins in the nearshore waters of San Diego differ in their distribution, site fidelity, and school size from other coastal populations of this species. Differences in habitat structure and prey distribution between the open California coastline and the more protected study areas where bottlenose dolphins display some site fidelity and smaller school sizes undoubtedly contribute to these reported intraspecific differences. A recent extension of the research presented here has been the incorporation of photoidentification studies of coastal bottlenose dolphins at other locations within the Southern California Bight (Defran et al. 1999). Most of the bottlenose dolphins identified in these study areas have also been photographed in San Diego, suggesting an extensive coastal home range for this population.

ACKNOWLEDGMENTS This research was carried out under the authorization of National Marine Fisheries Service Permit 387 and 619. The authors wish to acknowledge the important field work and laboratory contributions of a number of individuals. During the early years of this work, A. Weaver was a valued partner in our field work, as was G. Shultz who also refined and managed our photoidentification activities. In more recent times, J. Scott, M. Caldwell, and A. Kesaris assisted in the photoidentification process, L. Quigley and E. Tepper verified many of our calculations, and A. Acevedo, L. Ballance, and one anonymous reviewer gave us thoughtful and thorough comments on earlier drafts of the manuscript. Our gratitude is also extended to J. Barlow who advised us on population estimates, to B. Wiirsig for laboratory support, and the National Marine Fisheries Service, Southwest Fisheries Science Center, La Jolla, for camera equipment indispensable to our research. Special appreciation is extended to L. Hansen who patiently introduced us to this work, gave us access to his photographic data set, and provided important feedback on our evolving interpretation of the data. Finally, we acknowledge the loving contributions of Barbara J. Weller (1938-1992) to the authors, and we dedicate this article to her.

LITERATURE CITED BALLANCE, L. T. 1990. Residence patterns, group organization, and surfacing associations of bottlenose dolphins in Kin0 Bay, Gulf of California, Mexico. Pages 267283 in S . Leatherwood and R. R. Reeves, eds. The bottlenose dolphin. Academic Press, San Diego, CA. BALLANCE, L. T. 1992. Habitat use patterns and ranges of the bottlenose dolphin in the Gulf of California, Mexico. Marine Mammal Science 8:262-274. BARLOW, J. 1993. The abundance of cetaceans in California waters estimated from ship surveys in summerlfall 1991. National Marine Fisheries Service, Southwest Fisheries Science Center Administrative Report No. LJ-93-09. 39 pp. Available from Southwest Fisheries Science Center, P. 0. Box 271, La Jolla, CA. 92038. AND E. POLITI. 1997. Social ecology of botBEARZI,G., G. NOTARBARTOLO-DI-SCIARA tlenose dolphins in KvarneriC (northern Adriatic Sea). Marine Mammal Science 13:650-668. S. 1992. Untersuchungen zur Orstreue und zum Vergesellschaftungsmuster BRAGER. des Grossen Tummlers, Ttlrsiops trzlncattls (Montagu, 1821). Diploma Thesis, Christian-Albrechts-Universitat,Kiel, Germany. 97 pp. J. V., K. A. FORNEY AND J. L. LAAKE. 1998. Abundance of southern CaliCARRETTA,

DEFRAN AND WELLER: BOTTLENOSE DOLPHINS

379

fornia coastal bottlenose dolphins estimated from tandem aerial surveys. Marine Mammal Science 14:655-675. CAUGHLEY, G. 1977. Analysis of vertebrate populations. Wiley, New York, W. DAILEY, M. D., D. J. REISHA N D J. W. ANDERSON. 1993. Ecology of the Southern California Bight: A synthesis and interpretation. University of California Press, Berkeley and Los Angeles, CA. DEFRAN, R. H., D. L. KELLY,G. M. SHULTZ, A. CH. WEAVER AND M. A. ESPINOSA. 1986. The occurrence and movements of the bottlenose dolphin in the Southern California Bight. NOAA Administrative Report No. LJ-86-36C. 26 pp. Available from Southwest Fisheries Science Center, P. 0. Box 27 1, La Jolla, CA. 92038. DEFRAN, R. H., G. M. SHULTZ AND D. W. WELLER. 1990. A technique for the photographic identification and cataloging of dorsal fins of the bottlenose dolphin (Tursiops truncatus). Report of the International Whaling Commission (Special Issue 12):53-55. DEFRAN, R. H., D. W. WELLER, D. L. KELLY AND M. A. ESPINOSA. 1999. Range characteristics of Pacific coast bottlenose dolphins in the Southern California Bight. Marine Mammal Science 15:381-393. FERTL,D. C. 1994. Occurrence, movements, and behavior of bottlenose dolphins (Tursiops truncatus) in association with the shrimp fishery in Galveston Bay, Texas. M.S. thesis. Texas A&M University, College Station, TX. 117 pp. J. A. 1981. Ecology of the Atlantic bottlenosed dolphin (TursiopJ truncatus) GRUBER, in the Pass Cavallo area of Madagorda Bay, Texas. M.S. thesis, Texas A&M University, College Station, TX. 182 pp. HANSEN, L. J. 1983. Population biology of the coastal bottlenose dolphin (Tursiops truncatus) of southern California. M.S. thesis, Sacramento State University, Sacramento, CA. 104 pp. HANSEN, L. J. 1990. California coastal bottlenose dolphins. Pages 403-420 in S. Leatherwood and R. R. Reeves, eds. The bottlenose dolphin. Academic Press, San Diego, CA. HANSEN, L. J., AND R. H. DEFRAN. 1990. A comparison of photoidentification studies of California coastal bottlenose dolphins. Report of the International Whaling Commission (Special Issue 12):lOl-104. HANSON, M. T., AND R. H. DEFRAN. 1993. The behaviour and feeding ecology of the Pacific Coast bottlenose dolphin, Tursiops truncatus. Aquatic Mammals 19:127-

142. HENNINGSEN, T. 1991. Zur Verbreitung und Okologie des grossen Tummlers (Tursiops truncatus) in Galveston, Texas. Diploma Thesis, Christian-Albrechts-Universitat, Kiel, Germany. 80 pp. HICKEY, B. M. 1993. Physical oceanography. Pages 19-70 in M. D. Dailey, D. J. Reish and J. W. Anderson, eds. Ecology of the Southern California Bight: A synthesis and interpretation. University of California Press, Berkeley and Los Angeles, CA. JOLLY, G. M. 1965. Explicit estimates from capture-recapture data with both death and immigration-stochastic model. Biometrika 5:225-247. KENNEY, R. D. 1990. Bottlenose dolphins off the Northeastern United States. Pages 369-386 in S. Leatherwood and R. R. Reeves, eds. The bottlenose dolphin. Academic Press, San Diego, CA. KREBS,C. J. 1985. Ecology: The experimental analysis of distribution and abundance. Harper & Row, Publishers, New York, NY. KREBS, C. J. 1989. Ecological methodology. Harper Collins Publishers, New York, NY. Ross, G. J. B., AND V. G. COCKCROFT. 1990. Comments on Australian bottlenose dolphins and the taxonomic status of Tursiops aduncus (Ehrenberg, 1832). Pages 101-128 in S. Leatherwood and R. R. Reeves, eds. The bottlenose dolphin. Academic Press, San Diego, CA. DOS SANTOS, M. E., AND M. LACERDA. 1987. Preliminary observations of the bottlenose

380

MARINE MAMMAL SCIENCE, VOL. 1 5 , NO. 2 , 1999

dolphin (Tursiopstruncatus) in the Sado estuary (Portugal). Aquatic Mammals 13: 65-80. SEBER, G. A. F. 1982. The estimation of animal abundance. MacMillan Publishing Co., New York, Ny. SHANE, S. H. 1980. Occurrence, movements, and distribution of bottlenose dolphin, Tursiops truncatus, in southern Texas. Fishery Bulletin, U.S. 78:593-601. S. H. 1987. The behavioral ecology of the bottlenose dolphin. Ph.D. disserSHANE, tation. University of California, Santa Cruz, CA. 97 pp. S. H. 1990. Behavior and ecology of the bottlenose dolphin at Sanibel Island, SHANE, Florida. Pages 245-265 in S. Leatherwood and R. R. Reeves, eds. The bottlenose dolphin. Academic Press, San Diego, CA. SHANE, S. H. 1994. Occurrence and habitat use of marine mammals at Santa Catalina Island, California from 1983 to 1991. Bulletin of Southern California Academy of Sciences 93:13-29. S. H., R. S. WELLSAND B. WURSIG.1986. Ecology, behavior and social orgaSHANE, nization of the bottlenose dolphin: A review. Marine Mammal Science 234-63. SOUTHERN CALIFORNIA COASTAL WATERRESEARCH PROJECT. 1973. The ecology of the Southern California Bight: Implications for water quality management. Southern California Coastal Water Research Project, TR104. Available from Southern California Coastal Water Research Project, 7171 Fenwick Lane, Westminster, CA 92683. WELLER, D. W . 1991. The social ecology of Pacific coast bottlenose dolphins. M.S. thesis, San Diego State University, San Diego. CA. 93 pp. WELLS,R. S. 1986. Structural aspects of dolphin societies. Ph.D. dissertation. University of California, Santa Cruz, CA. 234 pp. WELLS, R. S., M. D. SCOTT AND A. B. IRVINE. 1987. The social structure of free-ranging bottlenose dolphins. Pages 247-305 in H. H. Genoways, ed. Current mammalogy. Plenum Press, New York, NY. WURSIG,B. 1978. Occurrence and group organization of Atlantic bottlenose porpoises (Tursiops trtmcatus) in an Argentine Bay. Biological Bulletin 1 5 4 3 4 8 4 5 9 . WURSIG,B., AND M. WURSIG.1977. The photographic determination of group size, composition, and stability of coastal porpoises (Tursiopstruncatus). Science 198: 7 5 5-7 56. . Behavior and ecology of bottlenose porpoises, WURSIG,B., AND M. W ~ R S I G1979. Tursiops truncatus, in the South Atlantic. Fishery Bulletin, U.S. 77:399-442. Received: 12 June 1995 Accepted: 27 July 1998