Dudzik And Defran 1998

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Population estimates of Pacific Coast bottlenose dolphins (Tursiops truncatus) photographed off San Diego, California Kimberly J. Dudzik, R. H. Defran Contact e-mail: [email protected] Cetacean Behavior Laboratory Department of Psychology San Diego State University San Diego, CA 92182-4611

ABSTRACT A 29 month photo-identification study was conducted from 1996 to 1998 in the San Diego study area to obtain new mark- recapture based population estimates for the Southern California Bight population of coastal bottlenose dolphins (Tursiops truncatus). Chao’s closed model Mth was applied to the current and earlier photographic data collected in this area from 1984 to 1989. These earlier data were partitioned into 2.5 year sample periods (84/86, 87/89) comparable in duration to the 96/98 sample period. A photo quality rating system was applied to photographic data from all three sample periods to minimize heterogeneous capture probabilities. Abundance estimates for the three sample periods were: 28984/86 (95% CI = 230 - 398), 35487/89 (95% CI = 330 - 390) and 35696/98 (95% CI = 306 - 437). The 1984-1986 sample period encountered fewer schools and dolphins and had fewer resights and thus may have been negatively biased relative to other sample periods. Abundance estimates derived for all sampling periods are probably lower than the true population size because they exclude individuals among the 37% of this population that lack identifying marks, and because the sighting frequencies vary among members of this population. When considering the expected negative bias of the 84/86 estimate and the similarity of the 87/89 and 96/98 estimates, the size of population seems to have remained relatively stable across a 15 yr time span. Occurrence patterns and school size characteristics from the 1996-1998 and the earlier 1984-1989 data sets were similar, providing further evidence of stability over time. No obvious impacts from the 1997/1998 El Niño were detected in occurrence, school size, or population estimates.

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Keywords: ABUNDANCE ESTIMATE; PACIFIC OCEAN; INDEX OF ABUNDANCE; COMMON BOTTLENOSE DOLPHIN; MARK-RECAPTURE; PHOTO-ID

Submitted to Journal of Cetacean Research and Management – Please do not cite or circulate without authors’ permission.

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INTRODUCTION Photo-identification research on Pacific coast bottlenose dolphins (Tursiops truncatus) since 1981 has shown that members of this population inhabit a narrow coastal corridor of over 900 km from Ensenada, Baja California Norte, Mexico to Monterey Bay, California (Feinholz, 1996; Defran et al., 1999). Within this range, members of this population are highly nomadic and make frequent movements between coastal areas. A number of mark-recapture estimates of this population have been derived using both photo-identification (Hansen, 1990; Defran and Weller, 1999) and aerial survey (Carretta et al. 1998) sampling techniques. Hansen (1990) photographed coastal bottlenose dolphins occurring off northern San Diego County from September 1981 to January 1983 and used Tanaka’s closed model to derive an estimate of 240 individuals (95% CI = 120- 477). The broad confidence intervals associated with this estimate diminishes its utility and are probably the result of low survey effort, the short interval between a number of the surveys and the small size of the photographic data set (Cf. Hansen, 1990; Defran and Weller, 1999). Defran and Weller (1999) carried out photo-identification surveys in north San Diego County from 1984 to 1989 and estimated population size using the Jolly-Seber technique. Population size was estimated for each year possible between 1984 and 1989, and yielded estimates ranging from 234 (95% CI = 205-263) to 285 (95% CI = 265-306). Population size estimated by the Jolly-Seber model, however, is likely to be biased downward due to violations of equal catchability that assumes that every animal in the population has the same capture probability (Pollock et al., 1990). When capture probabilities are not equal among all individuals, this estimate will be negatively biased (Hammond, 1986; Pollock et al., 1990). Defran and Weller (1999) found that sighting frequencies among individuals during the 84/89 study varied greatly from a single sighting to 24 sightings. Such heterogeneous sighting frequencies could be due to individual movement patterns, individual distinctiveness, sampling effort and photographic quality or some combination of these factors. 3

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In the current study we carried out new photo-identification surveys in the San Diego study area in order to provide a more contemporary estimate for this population. Further, we attempted to diminish negative bias associated with equal catchability violations by correcting for the effects of lower photographic quality and by selecting a closed model estimator less affected by variations in catchability. Closed population estimators operate under three assumptions that must be met in order to obtain precise population estimates: 1) the population is closed to births, deaths, immigration, and emigration; 2) each individual has the same probability of being captured; 3) marks remain stable over time (Pollock et al., 1990). Each of these assumptions is examined in detail below. Closure The first assumption requires that no births, deaths, immigration or emigration occur during the study period. Bottlenose dolphins tend to be long lived animals with low mortality rates and low birth rates (Wells and Scott, 1990). Choosing shorter sample periods, as in this study, minimizes violations of this assumption since births and deaths occurring during the shorter periods would have little effect on the estimate (Calambokidis et al., 1989; Calambokidis et al., 1993; Wilson et al., 1999). Accordingly prior data collected in San Diego from 1984 to 1989, and in the current study from 1996 to 1998 were partitioned into 2.5 y sample periods. The second closed model assumption is that each individual in the population has the same probability of being captured. When capture probabilities are heterogeneous among individuals in a population, the resulting abundance estimates are negatively biased (Hammond 1986). Two sources of heterogeneous capture probabilities in the current study were photographic quality and the extensive geographic range of dolphins in this population. Photographic Quality Differences in photographic quality such as focus and exposure may have an effect on whether some dolphins can be resighted and lead to heterogeneous capture probabilities (Hammond, 1986). We minimized heterogeneity due to photographic quality by developing and applying a rating

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system that allowed us to eliminate low quality (difficult to match) photographs. (Arnbom, 1987; Cerchio, 1998; Friday et al., 2000; Urban et al., 1999; Whitehead et al., 1997; Wilson et al., 1999). Geographic range Variability in the probability that a dolphin will visit some portion of the population range, such as a more geographically limited study area, may diminish equal catchability and thus induce a negative bias on the population estimate. Defran et al. (1999) and Feinholz (1996) reported that bottlenose dolphins photographed along the central California and Southern California Bight coastline utilize 905km range from Ensenada, Baja California Norte, Mexico to Monterey Bay, CA. However, photographic sampling in Defran and Weller (1999) and in the current research only occurred within a very small portion (32km) of the entire range. Between 88% and 94% of the dolphins photographed off Santa Barbara, Orange County, and Ensenada from 1981 to 1989 were also photographically captured in San Diego (Defran et al., 1999; Fig. 1). Similarly, 81% of dolphins photographed in Monterey Bay between 1992 and 1994 were previously identified in San Diego1. Furthermore, low resighting rates and regular movement patterns between these areas showed limited site-fidelity to any particular area (Defran et al., 1999). Thus, the San Diego study area, centrally located within this population’s range and visited by many members of this population, offered an opportunity to sample individuals from across this region. However, the high sighting variability of San Diego dolphins combined with their less than 100% photographic overlap with dolphins from other Southern California Bight (SCB) study areas must be considered. It seems likely, therefore, that some individuals may utilize the San Diego area less frequently than others, or not at all, and that geographic bias may contribute to lower population estimates. Mark recognition The last closed model assumption requires that marks remain stable over time and individuals are not mismatched. Bottlenose dolphins obtain notches along the trailing edge of the dorsal fin that tend to remain stable over time, and a photographic record of unique notch patterns over time

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Based on our reanalysis of the Feinholz (1996) photographic data. 5

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allows the investigator to see subtle changes (Wursig and Jefferson, 1990). The San Diego photographic catalog extends from 1981 to 1998 and has numerous examples of bottlenose dolphins showing only subtle fin changes across 10 or more years. Both open and closed population models share the equal catchability assumption, which is difficult to meet in most population studies (Hammond, 1986; Pollock et al., 1990). Several closed models have been developed, however, in order to allow for certain varying capture probabilities to occur (Pollock et al., 1990). For the current estimates we selected Chao’s model Mth which assumes that capture probabilities can change from one sampling period to the next (time variation), and that each member of the population has a unique capture probability due to sex, age, home range, etc., that is independent of all other members of the population (heterogeneity) (Chao and Jeng, 1992; Otis et al., 1978). METHODS Study area The San Diego study area is a 32km strip of coastline that extends from Scripps Pier, La Jolla (320 52’N) to South Carlsbad State Beach (330 08’N; Fig. 1). The shoreline is characterized by steep cliffs with intermittent sandy beaches. Dolphins in the area utilize a variable habitat that consists of submerged reefs, rock outcrop, sloping sand and estuary mouths. Although much of the study area is exposed to the full force of the open ocean, there are also areas bordered by dense patches of kelp located about .5km offshore. Photographic surveys Survey procedures closely followed those of Defran and Weller (1999) and are briefly summarized. Surveys were conducted aboard a 5.7m Chaparral boat equipped with a 115hp outboard motor. The survey vessel was motored along the coastline, 90- 180m offshore of the surf zone. At least three observers scanned the water’s surface from the beach to about 2km offshore. Once dolphins were sighted, the boat proceeded approximately 1km past the school to ensure that all members of the school were located. Information on location, time, sighting number, dolphin 6

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behavior, and field estimates of the number of adults and calves present were recorded before dorsal fin photography began. School size and calf definitions were the same as in Defran and Weller (1999). The research vessel was positioned to within 3- 12m of the school in order to photograph individual dorsal fins. All dorsal fin photographs were taken with a Canon AE-1, 35 mm camera equipped with a Canon 400mm lens and high-speed motor drive (5 exposures/sec) and loaded with Kodak Tri-X, 400 ASA black and white film. An attempt was made to photograph all dolphins without regard to the apparent distinctiveness of their dorsal fin notch patterns. Once photographic effort was considered complete, the boat was motored offshore where all film shot was labelled with date, location, school number, and roll number. The research vessel then resumed travel along the coastline to locate additional dolphin schools. Photo-identification Procedures for identifying and matching individual dorsal fins closely followed the methodology of Defran et al. (1990) and are briefly summarized. A clear photograph of each distinctive dorsal fin was chosen as a “type specimen” to which all other photographs were compared. Only unambiguous matches to the “type specimen” were considered resightings. Further, all photographs were rated by three experienced photo analysts on four criteria of photographic quality: focus (three levels), exposure (three levels), proportion of the fin above the waterline (three levels), and size of the fin within the frame (four levels). Photographs that had the lowest possible rating on any of these four criteria were labelled ‘poor quality’ and were excluded from population analyses, while the remaining photographs were labelled as ‘good quality’ (See Dudzik, 1999 for additional details of the photo quality analysis). Data analysis Student t-tests were used to evaluate occurrence and school size differences. The program CAPTURE (Rexstad and Burnham, 1991) was used for closed model analyses. CAPTURE features several closed model estimators and Chao’s model Mth was determined to be the most appropriate

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model for the current study. CAPTURE includes estimates of abundance, point estimate averages, standard errors and confidence intervals. Power analysis was used to determine if the population estimates had enough power to detect a trend over the 15 year time span. Power was calculated using Gerrodette’s (1987) general model: r2n3>12cv2(z α/2 + zβ)2 where r is the rate of change over time, n is the number of estimates of abundance, cv is the coefficient of variation of estimated total population size, z α/2 is the probability of making a Type I error and zβ is the probability of making a Type II error.

Terminology ‘Photographic catalog’ refers to all individuals identified in coastal areas of the Southern California Bight from 1981 to 1989 (cf. Defran and Weller, 1999), central California from 1990 to 1993 (Feinholz, 1996) and San Diego from 1996 to 1998 (current study). Two San Diego study area photographic datasets are distinguished: ‘84/89’ – dolphins photographed by Defran and Weller (1999) from 1984 to 1989; ‘96/98’ – dolphins photographed in the current study from 1996 to 1998. Finally, photographic data from the San Diego study area were also partitioned into three sample periods: ‘84/86’ and ‘87/89 from Defran and Weller (1999); ‘96/98’ from the current study. RESULTS Survey effort, occurrence, school size Sixty-six photographic surveys were conducted between March 1996 and August 1998 in the San Diego study area. Dolphins were encountered on 79% (n = 52) of all surveys, and 1,497 dolphins (field estimate) were observed in 84 schools. Eleven percent of all dolphins encountered were classified as calves. Although some surveys were cancelled due to unfavorable weather conditions and equipment failures, survey effort was partitioned more or less equally across each year with a small increase in survey effort in 1997 (1996 surveys = 19; 1997 surveys = 27; 1998 surveys = 20). Even though more surveys were conducted in 1997, considerably fewer dolphins

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were encountered (1996: n = 736; 1997: n = 239; 1998: n = 522). Comparisons of encounter rates, number of schools and resighting rates between current and past research are summarized in Table 1. In the current study overall mean school size was 17.8 (SD = 23.42) with yearly means of 25.4 in 1996 (SD = 33.31), 11.4 in 1997 (SD = 13.15) and 15.4 in 1998 (SD = 15.77). School sizes were variable and ranged from a single dolphin to 140 dolphins. Comparisons of occurrence patterns were made between the 96/98 data and the 84/89 data (Defran and Weller, 1999). No significant difference was found between the percentage of surveys on which dolphins were encountered, average school size, number of dolphins encountered per complete survey, and percentage of calves encountered (Table 2). Photographic dataset A total of 233 dolphins were photographically identified in the current study. Sighting frequencies were variable and ranged from 1 to 14 ( x = 2.6). Thirty-seven percent of identified individuals were sighted only once and 56% (n = 131) of the individuals identified from 1996 to 1998 were resightings from 1984 to 1989. In order to analyze the rate at which individuals were first identified, rate of discovery was plotted as the cumulative number of individuals identified over blocks of five consecutive surveys in which at least one dolphin was identified (Fig. 2). The rate of discovery curve shows a continuous increase throughout the study period with one large increase between blocks two and three. By the end of the study period, between 11% and 17% of all individuals were newly identified. Photographic quality and Population estimates For each dataset, only those photographs meeting “good quality” rating criteria and those having distinctive fins were included in the analysis of population size. After applying the photo quality rating criteria to the 84/89 data, 73 poor quality photographs were removed from the catalog. This removal resulted in a reduction of the number of individuals in the 84/89 data from 373 to 366 9

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individuals. The number of individuals that qualified as ‘good quality’ from the total in each sample period were: 84/86: n = 160 out of 165 total individuals; 87/89: n = 284 out of 309; 96/98: n = 225 out of 233. Abundance estimates derived by Chao’s closed model Mth were as follows: 84/86 - 289 (95% CI = 230 - 398), 87/89 - 354 (95% CI = 330-390), 96/98 - 356 (95% CI = 306 437). Power analysis indicated that the estimates had only moderate power to detect a trend (power = .61 at α = .05). DISCUSSION Closed model population estimates based on the 96/98 photo-identification data collected in this study, as well as those based on our selected subset of the 84/89 photographic data, were higher than earlier estimates reported by Defran and Weller (1999) or Hansen (1990). These new estimates, along with the recently collected occurrence and school size data, provided an opportunity to examine trends in these parameters spanning a 15 y period. In addition, possible El Niño effects on these population estimates were evaluated. We reduced negative bias in our estimates by using Chao’s model Mth which allows for variations in heterogeneity and time (see Introduction) and through our application of a photographic quality rating system. The resulting estimates for our most recent sample periods were the highest obtained and were quite similar (87/89est = 354, 96/98est = 356), while the estimate for the earliest sample period (84/86est = 289) was somewhat lower. In our interpretation of these values and in comparisons between them we reviewed possible sources of bias associated with the assumption of equal catchability. Some of the bias we identified applies to all sample periods while other sources relate to a specific sample period. Although long-term photographic effort reveals that a high proportion of dolphins utilize the San Diego study area as a part of their range (81% - 94% over 9yrs) choosing the short sampling periods necessary for closed models probably introduced a degree of geographic bias. Some dolphins in the population utilize the San Diego study area more often than others as evidenced by variable sighting frequencies reported by Defran and Weller (1999) during their 84/89 study and reported in the 10

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current research. An additional negative bias in our population estimates occurs because not all dolphins have distinctively marked dorsal fins. Approximately 37% of the dolphins encountered in the San Diego study area, including the 11% calves, are not distinctly marked and are thus excluded from the population estimate (Defran and Weller, 1999).

Although negatively biased, the point estimates we derived reveal a possible stable trend in the population size (est. = 354-356) between the 87/89 and 96/98 sample periods. The 84/86 sampling period was further biased due to fewer resightings as well as fewer schools and dolphins (Table 1). When capture probabilities are low (low resighting rate), the estimate is biased downward (Chao and Jeng, 1992; Hammond, 1986). Considering the low resighting rate for the 84/86 sampling period, the estimate may actually have been more similar to the 87/89 and 96/98 estimates. If so, this population of coastal bottlenose dolphins may have remained relatively stable over a 15 y period. Although power analysis indicates only moderate power to detect a trend, this decrease in power is most likely due to a small sample size of only two estimates (Gerrodette, 1987). Stability in the population is also evidenced in the occurrence patterns compared over time (Table 2). One would expect that if a major shift in population size had occurred, then a similar shift might be reflected in school size, encounter rates, percentage of calves, or other parameters. El Niño effects The 1997/1998 El Niño event occurred during our 96/98 sample period. Negative impacts associated with El Niño have affected several populations of marine organisms. For example, California sea lion (Zalophus californianus) pup production decreased by 64% in 1998 due to the 97/98 El Niño (Forney, et al., 2000). Pinniped pups generally succumb to nutritional stress due to strong storms separating mothers from pups, and reduced food availability that forces mothers to remain at sea for longer periods instead of returning to feed their pups (Riedman, 1990). Pacific coast bottlenose dolphins within the SCB have also been affected by El Niño in the past. Following the major 1982/1983 El Niño event there was a northward range extension of SCB bottlenose dolphins from Los Angeles to Monterey Bay (Wells et al., 1990). The 1997/98 El Niño 11

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event offered an opportunity to examine possible effects on population size as well as occurrence patterns and school size. Statistical comparisons of these parameters (Table 2) between the 84/89 and 96/98 datasets showed comparable values and no significant differences. It appears, therefore, that coastal bottlenose dolphins in the SCB escaped obvious negative effects of the 1997 El Niño. We are cautious about these interpretations, however, as it is possible that some 1997/1998 El Niño effects may have occurred too late to appear in the 96/98 sample period, or that they affected parameters we did not evaluate. In conclusion, the mark-recapture techniques and model used in this research helped to decrease bias in population estimates in order to better understand the population characteristics of SCB coastal dolphins. Variable sighting frequencies of individuals photographed in the San Diego study area suggest that significant distributional shifts occur within this population and contribute to geographic bias still evident in the estimates. A continued photographic effort in the secondary study areas would help to further eliminate bias due to geographic distribution and, therefore, strengthen population estimates. The population characteristics reported in this research, however, provide a long-term view of trends within the SCB coastal population and the most current abundance estimate. ACKNOWLEDGMENTS

The authors would like to thank A. Lang, J. Marsh, T. Dedecker, E. Howarth, and J. Oswald for their valuable assistance in the field. We are grateful to A. Lang, J. Marsh, and the many Cetacean Behavior Laboratory interns who provided their time and assistance in the laboratory. Appreciation goes out to K. Forney, J. Carretta, and J. Barlow of the National Marine Fisheries Service, Southwest Fisheries Science Center, and to J. Calambokidis of the Cascadia Research Collective who provided initial insights and comments about our research design and analysis. Appreciation also goes to A. Acevedo-Gutierrez of the San Francisco California Academy of Sciences for his assistance with the power analysis. We acknowledge the valuable and constructive

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comments offered by J. Barlow, D. Weller, and two anonymous reviewers on earlier drafts of the manuscript.

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REFERENCES Arnbom, T. 1987. Individual identification of sperm whales. Rep. int. Whal. Commn 37:201-204. Calambokidis, J., Steiger, G.H., Cubbage, J.C., Balcomb, K.C. and. Bloedel, P. 1989. Biology of humpback whales in the Gulf of the Farallones. Gulf of the Farallones National Marine Sanctuary NOAA Report CX 8000-6-0003. 86 pp. Calambokidis, J., Steiger, G.H. and Evenson, J.R. 1993. Photographic identification and abundance estimates of humpback and blue whales off California in 1991-92. Southwest Fisheries Center Administrative Report 50ABNF100137. 67 pp. Carretta, J.V., Forney, K.A. and Laake, J.L. 1998. Abundance of Southern California coastal bottlenose dolphins estimated from tandem aerial surveys. Mar. Mamm. Sci. 14: 665-675. Cerchio, S. 1998. Estimates of humpback whale abundance off Kauai, 1989 to 1993: evaluating biases associated with sampling the Hawaiian Islands breeding assemblage. Marine Ecol. Prog. Ser. 175: 23-34. Chao, A., Lee, S.M. and Jeng, S.L. 1992. Estimating population size for capture-recapture data when capture probabilities vary by time and individual animal. Biometrics 48: 201-216. Defran, R. H., G. M. Shultz, and D. W. Weller. 1990. A technique for the photo identification and catalog of dorsal fins of the bottlenose dolphin (Tursiops truncatus). Rep. int. Whal. Commn (Special Issue 12): 53-55. Defran, R.H. and Weller, D.W. 1999. Occurrence, distribution, site fidelity, and school size of bottlenose dolphins (Tursiops truncatus) off San Diego, California. Mar. Mamm. Sci. 15:366-380. Defran, R.H., Weller, D.W., Kelly, D. and Espinosa, M.A. 1999. Range characteristics of Pacific coast bottlenose dolphins (Tursiops truncatus) in the Southern California Bight. Mar. Mamm. Sci. 13:381-393. Dudzik, K.J. 1999. Population dynamics of the Pacific Coast bottlenose dolphin (Tursiops truncates). M.S. thesis, San Diego State University, San Diego, CA. 63pp. Feinholz, D.M. 1996. Pacific coast bottlenose dolphins (Tursiops truncatus gilli) in Monterey Bay, California. M.S. thesis, San Jose State University, San Jose, CA. 78 pp. Forney, K.A., Barlow, J., Muto, M.M., Lowry, M., Baker, J., Cameron, G., Mobley, J., Stinchcomb, C., and Carretta, J.V. 2000. U.S. Pacific Marine Mammal Stock Assessments: 2000. U.S. Department of Commerce. 276 pp. Friday, N., Smith, T.D., Stevick, P.T. and Allen, A. 2000. Measurement of photographic quality and distinctiveness for the photographic identification of humpback whales, (Megaptera noveangliae). Mar. Mamm. Sci. 16:355-374. Gerrodette, T. 1987. A power analysis for detecting trends. Ecology 68:1364-1372. Hammond, P.S. 1986. Estimating the size of naturally marked whale populations using capturerecapture techniques. Rep. int. Whal. Commn (Special Issue 8):253-282. 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. Norris, K.S. and Prescott, J.H. 1961. Observations on Pacific cetaceans of Californian and Mexican waters. University of California Publications in Zoology 63:291-402. Otis, D.L., Burnham, K.P., White, G.C., and Anderson, D.R. 1978. Statistical inference from capture data on closed animal populations. Wildl. Monogr. 62:1-135. 14

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Pollock, K.H., Nichols, J.D., Brownie, C. and Hines, J.E. 1990. Statistical inference for capturerecapture experiments. Wildl. Monogr. 107:1-97. Rexstad, E. and Burnham, K. 1991. User’s guide for interactive program CAPTURE. Colorado Cooperative Fish and Wildlife Research Unit, Colorado State University, Fort Collins, Colorado 80523 USA. Riedman, M. 1990. The Pinnipeds: Seals, Sea lions, and Walruses. University of California Press, Berkeley, CA. Urban R., J., Alvarez F., C., Salinas Z., M., Jacobsen, J., Balcomb, K.C., Jaramillo L., A., Ladron de Guevara P., P. and Aguayo L., A. 1999. Population size of humpback whale, (Megaptera novaeangliae) in waters off the Pacific coast of Mexico. Fish. Bull. 97:10171024. Wells, R.S. and Scott, M.D. 1990. Estimating bottlenose dolphin population parameters from individual identification and capture-release techniques. Rep. int. Whal. Commn (Special Issue 12): 407-415. Wells, R.S., Hansen, L.J., Baldridge, A., Dohl, T.P., Kelly, D.L. and Defran, R.H. 1990. Northward extension of the range of bottlenose dolphins along the California coast. Pages 421-431 in S. Leatherwood and R. R. Reeves, eds. The Bottlenose Dolphin. Academic Press, San Diego, CA. Whitehead, H., Gowans, S., Faucher, A. and McCarrey, S.W. 1997. Population analysis of northern bottlenose whales in the Gully, Nova Scotia. Mar. Mamm. Sci. 13:173-185. Wilson, B., Hammond, P.S. and Thompson, P.M. 1999. Estimating size and assessing trends in a coastal bottlenose dolphin population. Ecol. Appl. 9: 288-300. Wursig, B. and Jefferson, T.A. 1990. Methods of photo-identification for small cetaceans. Rep. int. Whal. Commn (Special Issue 12): 43-52.

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Table 1. Summary of encounter, school, and resight statistics for each sample period. “Number resighted” reflects the number of individuals resighted within each study period.

Dolphins

Number

Number

Sample Period

Encountered

Schools

Resighted

84/86

975

61

68

87/89

1894

85

250

96/98

1497

84

146

Table 2. Summary of encounter, calf and school size statistics for 84/89 and 96/98 datasets. Mean # of Dolphins % Surveys

Encountered

Mean

Dolphins

Per Complete

School

Dataset

Encountered

Survey

% Calves

Size

1984/1989

79%

26.8

11%

19.8

1996/1998

79%

21.1

11%

17.8

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Figure 1. San Diego study area. Inset shows location of other photoidentification study areas along the California and Baja California Norte coastline mentioned in the text.

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Cumulative Number of Identified Dolphins

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250 225 200 175 150 125 100 75 50 25 0 1

2

3

4

5

6

7

8

9

10

Blocks of Five Surveys

Figure 2. Rate of discovery for dolphins photographed in the San Diego study area from 1996 to 1998. Only surveys on which at least one identifiable dolphin was photographed were used. Block 10 represents the last four surveys.

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