Photoidentification Research

Photoidentification Studies

The Catalina Project

West end of Santa Catalina Island

Troy DeDecker’s thesis project involves an analysis of the population dynamics of offshore bottlenose dolphins in the waters surrounding Catalina Island. Pacific coast bottlenose dolphins have been studied along the California and Baja Mexico coastline since 1981 by the National Marine Fisheries Service and the Cetacean Behavior Laboratory at San Diego State University (Hansen 1990, Weller 1991, Caldwell 1992, Defran and Weller 1999, Defran et al. 1999). Little is known, however, about the population dynamics of offshore bottlenose dolphins within the Southern California Bight.

Troy DeDecker shoots the photograph of a dorsal fin in the offshore waters of Catalina

While offshore bottlenose dolphins have been documented to range in Catalina waters year-round (Hill and Barlow 1992, Carretta and Forney 1993, Lee 1994, Shane 1994, Carretta et al. 1995), many important details of their occurrence there are yet to be documented. Among the important questions to be answered include how many dolphins utilize the Catalina Island habitat, and whether these dolphins are year-round or occasional residents. This baseline data isnecessary for the management and conservation of these marine mammals and their environment.

Troy’s thesis project will assess the occurrence, site fidelity, distribution, movement patterns, population size, and social organization of offshore bottlenose dolphins in the coastal waters surrounding Catalina Island. Data including school size, number of calves, sighting locations, and related environmental data will be collected during systematic dorsal fin photoidentification surveys around Catalina. This research will allow a test of population overlap by comparing dolphins photographed at Catalina with those photographed along the Southern California Bight coastline.

References

Caldwell, Mj. 1992. A comparison of bottlenose dolphin identified in San Quintín and the Southern California Bight. M.S. thesis, San Diego State University, San Diego, CA. 59 pp.

Carretta, J. V. and K. A. Forney. 1993. Report of the two aerial surveys for marine mammals in California coastal waters utilizing a NOAA DeHavilland Twin Otter aircraft March 9-April 7, 1991 and February 8-April 6, 1992. NOAA Technical Memorandum NMFS, September.

Carretta, J. V., K. A. Forney, and J. Barlow. 1995. Report of 1993-1994 marine mammal aerial surveys conducted within the U.S. Navy Outer Sea test range off Southern California. NOAA Technical Memorandum NMFS, March.

Defran, R. H. and D. W. Weller. 1999. Occurrence, distribution, site fidelity, and school size of bottlenose dolphins (Tursiops truncatus) off San Diego, California. Marine Mammal Science, 15:366-380.

Defran, R. H., D. W. Weller, D. L. Kelly, and M. A. Espinosa. 1999. Range characteristics of Pacific coast bottlenose dolphins (Tursiops truncatus) in the Southern California Bight. Marine Mammal Science, 15:381-393.

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.

Hill, P. S. and J. Barlow. 1992. Report of a marine mammal survey of the California coast aboard the research vessel McArthur July 28-November 5, 1991. NOAA Technical Memorandum NMFS, July.

Lee, T. 1994. Report on cetacean aerial survey data collected between the years of 1974 and 1982. NOAA Technical Memorandum NMFS, January.

Shane, S. H. 1994. Occurrence and habitat use of marine mammals at Santa Catalina Island, California from 1983-1981. Bulletin of Southern California Academic Sciences 93:13-29.

Weller, D. W. 1991. The social ecology of Pacific bottlenose dolphins. M.S. thesis, San Diego State University, San Diego, CA. 93 pp.

Coastal Photoidentification Studies

Boat-based photoidentification studies of Pacific coast bottlenose dolphins have been conducted along the Pacific coast of California and Baja Mexico since 1981 (Hansen 1990, Wells et al. 1990, Caldwell 1992, Feinholz 1996, Defran and Weller 1999, Defran et al. 1999). These studies have been concentrated in San Diego, California with supporting research conducted in Monterey Bay, Santa Barbara, and Orange County, California; and Ensenada and San Quintín, Baja Mexico. Comparison of individual dolphins identified in each area has indicated that a population of bottlenose dolphins inhabits coastal waters ranging throughout the Southern California Bight and as far north as Monterey Bay (Wells et al. 1990, Feinholz 1996, Defran et al. 1999). Within this over 900 km long stretch of coastline, dolphins are highly mobile, displaying little site fidelity to any one area and exhibiting fluid and dynamic social bonds with other members of the population (Weller 1991, Feinholz 1996, Defran and Weller 1999, Defran et al. 1999).

Current thesis projects being carried out by Jennifer Marsh and Aimee Lang plan to build on the extensive photoidentification database already gathered about Pacific coast bottlenose dolphins within the Southern California Bight. Since April 1998 Jennifer and Aimee have been conducting regular photoidentification surveys in the Santa Barbara-Ventura area. In September 1998, they also began surveying San Diego coastal waters as a continuation of the work begun by Kim Dudzik in March 1996. In addition to photographing bottlenose dolphins within each area for future identification of individuals, data is also being collected on size, composition (ratio of adults to calves), and location of all dolphin schools observed.

Aimee’s research will focus on analyzing the occurrence patterns, site fidelity, and distribution of bottlenose dolphins within the San Diego and Santa Barbara-Ventura study areas; comparing these parameters between the two areas; and determining the frequency of movements of individuals between areas. While past research has documented the regular occurrence of bottlenose dolphins in San Diego coastal waters and the movements of individual dolphins within a narrow coastal corridor throughout the Southern California Bight and as far north as Monterey (Wells et al. 1990, Feinholz 1996, Defran and Weller 1999, Defran et al. 1999), a number of important questions remain to be answered. Among the questions Aimee’s research will address are the following: Why are individual sighting frequencies so variable? Do the relative sighting frequencies of individuals remain consistent over time? Do individuals thought to be mothers with calves move between areas as often as other dolphins? Do dolphins use the San Diego and Santa Barbara-Ventura areas in similar ways and with comparable frequencies? How long do individual dolphins remain in or near each study area?

An additional part of Aimee’s project will include collaboration with Gisela Heckel and other researchers at U.A.B.C. in Ensenada, Baja, Mexico and will focus on the photoidentification of coastal bottlenose dolphins in Ensenada and San Quintín, Mexico. Prior work by Caldwell (1992) indicated that a southern range boundary for Southern California Bight bottlenose dolphins existed between Ensenada and San Quintín in 1990. Changes in range limitations as an influence of El Nino have been documented in the past (Wells et al. 1990). Further study in Ensenada and San Quintín will allow assessment of any potential changes in ranging patterns of bottlenose dolphins that may have occurred since 1990, as a result of the 1997-98 El Nino or of other factors. Additional research will also provide the opportunity to gain further understanding of the nature of the range boundary between the two populations. Work in Mexico was begun in July 1999.

Jennifer’s thesis research involves an examination of the social affiliation patterns, as well as school composition and stability of Pacific coast bottlenose dolphins in the Santa Barbara-Ventura and San Diego study areas. Previous studies of the social affiliation patterns of Pacific coast bottlenose dolphins were restricted to data collected in San Diego between 1984 and 1989 (Weller 1991). Prior work in Santa Barbara was limited to 12 boat-based surveys conducted between May and July of 1987 and 1989, and focused on identifying movement patterns of dolphins within the Southern California Bight, rather than on analyzing social affiliation patterns (Defran et al. 1999). Jennifer’s project will incorporate more thorough sampling techniques across all seasons for a 12 month period in both the San Diego and Santa Barbara-Ventura areas. Many of the analyses performed on the 1984-1989 San Diego data set (Weller 1991) will also be performed on the 1998-1999 Santa Barbara-Ventura and San Diego data sets, in order to make comparisons between the two areas and to document any changes in social affiliation patterns that may have occurred over time. Among the parameters to be analyzed are encounter rate, rate of discovery, resighting frequency, school size frequency, overall school size by year and season, percentage of schools with and without calves, level and mean level of association, and mean number of affiliates. Affiliations between probable mothers will also be analyzed in order to better understand the role and function of "nursery groups" and their social bonds.

References:

Caldwell, Mj. 1992. A comparison of bottlenose dolphins identified in San Quintín and the Southern California Bight. M.S. thesis, San Diego State University, San Diego, California. 59 pp.

Feinholz, D. M. 1996. Pacific coast bottlenose dolphins (Tursiops truncatus gilli) in Monterey Bay, California. M. S. thesis, San Jose State University, San Jose, California. 78 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.

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., L. J. Hansen, A. B. Baldridge, T. P. Dohl, D. L. Kelly, and R. H. Defran. 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.

Population Dynamics of Pacific Coast Bottlenose Dolphins

Studying the population dynamics of a particular species is important because it provides baseline information for a sound understanding of shifts and trends in the population size, behavior, and social organization of its individuals. Such biological and behavioral information is important because it will clarify our basic science understanding of coastal bottlenose dolphins and because it can be used as a guideline for management and conservation policies. Population parameters such as births, deaths, abundance, and recruitment are affected by environmental patterns, ecological conditions, and behavioral interactions. The dynamics of a population, therefore, can vary over time. Cetacean Behavior Laboratory researchers, and others, have been collecting photoidentification data on the bottlenose dolphins occupying the nearshore waters of the Southern California Bight since 1981 (Hansen 1990; Weller 1991; Feinholz 1996; Defran and Weller 1999, Defran et al. 1999). With the addition of photoidentification data collected during Kim Dudzik's masters thesis research in the San Diego study area from March 1996 to August 1998, there are sufficient, long term data to estimate shifts and trends in the population dynamics of the Southern California Bight bottlenose dolphins using newly computerized mark-recapture population models.

Mark-recapture through photoidentification is the preferred method of abundance estimation when animals can be individually recognized and resighted (Hammond 1986). Traditional mark-recapture techniques involve "marking" an animal by altering its physical appearance, i.e., leg bands on birds, toe clipping lizards, freeze branding dolphins (Wells et al., 1987; Hammond 1986). Animals are then captured on a later date and all the previously marked animals are counted. Biologists have begun to realize that individual animals can often be identified by their natural marks, thus, avoiding handling the animals which may disrupt behavior or cause harm (Hammond 1986; Würsig and Jefferson 1990). When using natural marks for identification, an animal is "marked" by taking a good quality photo of the distinctive markings, and later "recaptured" by photographing them again at a later date. Several models for estimating population parameters have been developed and improved with the advent of computers. Models are usually classified in one of two categories: closed models and open models. A closed population model assumes that the sampling is conducted during a period of time in which population size is constant throughout the entire study, i.e., no births, deaths, immigration, or emigration takes place during the study period. An open population model assumes that births, immigration, deaths, and emigration do occur during the study period. The Jolly-Seber open population estimator has been chosen for the proposed analysis because the database to be analyzed consists of resightings histories that span several years during which time births, deaths, and possible immigration and emigration have occurred.

A number of analyses of the CBL coastal dolphin population database will be carried out. First, population abundance will be estimated for each year data was collected in order to understand how many dolphins are occupying the nearshore waters of the Southern California Bight, and to understand any shifts in abundance the dolphins may have experienced over time. Second, survival rates will be estimated in order to provide a quantitative estimate of the longevity of adult dolphins. Finally, recruitment rates will be estimated to determine the rate at which new dolphins enter the population through births and immigration.

References

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.

Hammond, P. S. 1986. Estimating the size of naturally marked whale populations using capture-recapture techniques. Reports of the International Whaling Commission (Special Issue 8):253-282.

Hansen, L. J. 1990. California coastal bottlenose dolphins. Pages 403-420 in S. Leatherwood and R. Reeves, eds. The bottlenose dolphin. Academic Press, San Diego, CA.

Weller, D. W. 1991. The social ecology of Pacific bottlenose dolphins. M.S. thesis, San Diego State University, San Diego, CA. 93 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.

Würsig, B. and T. A. Jefferson. 1990. Methods of photo-identification for small cetaceans. Reports of the International Whaling Commission (Special Issue 12): 43-52.

Last updated 11/14/99
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