OCCURRENCE,DISTRIBUTION, SITE FIDELITY,AND SCHOOL SIZE OF BOTTLENOSE DOLPHINS (TURSIOPS TRUNCATUS) OFF SAN DIEGO,CALIFORNIA

The occurrence, distribution, site fidelity, and school size of bottlenose dolphins (Tursiops truncatus) 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 help to explain the observed intraspecific behavioral variability of this species.     

Sixteen years later: Occurrence,group size,and habitat use of humpback dolphins (Sousa plumbea) in Algoa Bay,South Africa

Coastal dolphins are more prone to cumulative impacts of environmental and anthropogenic changes than pelagic species. However, few studies use historical comparisons to evaluate those impacts across a temporal scale. The study presented here describes the pattern of occurrence, group size, behavior, and habitat use of humpback dolphins in Algoa Bay and compares these to a similar study conducted in the early 1990s. Results indicate a considerable change in the frequency of occurrence. Furthermore, the mean group size has decreased from six to three individuals, mainly due to an increase in occurrence of solitary individuals (36.3% vs. 15.4%). Foraging, which was previously the predominant activity (64%), has now been replaced by traveling (49%). Moreover, dolphins showed a negative reaction towards a variety of watercrafts and swimmers. These observed differences could be due to a change in prey abundance, direct anthropogenic disturbance, and/or population decline. We emphasize the need for long‐term environmental and biological data series and long‐term monitoring of the demographics of this population to accurately assess any changes observed in future.     

Population size estimate of Indo-Pacific bottlenose dolphins in the Algoa Bay region, South Africa

This study estimates the population size of Indo-Pacific bottlenose dolphins ( Tursiops aduncus ) in the Algoa Bay region on the Eastern Cape coast of South Africa. Mark-recapture analyses were performed on photo-identification data collected on 54 occasions during a 3-yr-study period. Using a photographic data set of over 10,000 ID-images, 1,569 individuals were identified, 131 of which were photographed on more than one occasion. Using the POPAN formulation in the software program MARK, a total population of approximately 28,482 individuals (95% CI = 16,220–40,744; CV = 0.220), was estimated (estimate corrected for the proportion of distinctive individuals in the population). This is the largest population estimate to date for this species along the South African coast, suggesting that the bottlenose dolphins inhabiting the Algoa Bay region represent part of a substantially larger population that ranges along a considerable length of the South African coast.     

ABUNDANCE OF INDO-PACIFIC BOTTLENOSE DOLPHINS, TURSIOPS ADUNCUS, OFF POINT LOOKOUT, QUEENSLAND, AUSTRALIA

Managing escalating human impacts on nearshore cetaceans requires information on a species' demography and distribution patterns at appropriate spatial scales. Identification photographs of individuals of a population of inshore Indo-Pacific bottlenose dolphins, Tursiops aduncus , were obtained in the open coastal waters off North Stradbroke Island, Australia and used to estimate population density and residency. Five hundred and eighty-one individuals were identified from 342 school sightings during 1998 and 1999. Mark-recapture analysis using closed population models estimated a population size of between 700 and 1,000 individuals within the study area during winter, with evidence that the population contained resident individuals. This represents an extremely high number of dolphins within a small area, which requires a pre-emptive management strategy to ensure their continued occupancy.     

Association patterns and population dynamics of bottlenose dolphins in the Strait of Sicily (Central Mediterranean Sea): implication for management

An understanding of the population dynamics and social organization of cetaceans is essential to manage the influence of anthropogenic activities. In this study, the population size, site fidelity and social interactions of bottlenose dolphins in the Strait of Sicily (Italy) were investigated to provide recommendations for their conservation. Mark-recapture analysis was based on the encounter histories of 103 marked dolphins from 2004 to 2015. The POPAN formulation of the Jolly–Seber model in MARK software was used to estimate the size of the super-population. Site fidelity and social organization were estimated for individuals re-sighted ≥3 times. The estimated population size was 140 (SE = 15.75; 95% CI = 106–164). Dolphins had low site fidelity, and both adults and sub-adults move outside the study area. Females with calves used the area longer than other individuals. Based on our results, dolphins’ home range likely extended beyond the study area. The mean value of the Half-Weight Association Index was low and the preferred association was by casual acquaintance. However, we found a distinct aggregation of post-parturition females during the final 2 years of the study. Therefore, the pattern of association was apparently a response to an ecological requirement, which was the possibility to breed in high productivity waters. Whether these individuals are part of a larger pelagic population is unknown; however, we can conclude that the management of only coastal waters is insufficient for the conservation of dolphins in the Strait of Sicily.     

Site fidelity and association patterns of Indo-Pacific humpback dolphins off the east coast of Zhanjiang,China

This study examined the population of Indo-Pacific humpback dolphins, Sousa chinensis, inhabiting the waters off the east coast of Zhanjiang, China. A total of 116 dolphins were identified during 147 boat-based surveys, completed between June 2005 and June 2007. Abundance estimates indicated that a small population of 268 dolphins (95% CI = 189–413) inhabited this coastal area. The sighting frequencies of identified dolphins varied between one and five when the 2006 and 2007 data were organized into six occasions. Twenty-three percent (27 individuals) of the cataloged dolphins were identified in more than one calendar year. The two animals (ZJ001, ZJ011) with the highest degree of site fidelity were present in the study area for a period of 23 months. The most frequently (15 times) sighted dolphin (ZJ046) was recorded in nine (56%) of the 16 months surveyed. This evidence indicates the interannual site fidelity of S. chinensis to the area off the eastern coast of Zhanjiang City. Range sizes of eight identified individuals calculated by the minimum convex polygon varied from 2.07 to 331.20 km². Associations between 34 individuals sighted at least three times and more than three times were measured by the half-weight index and SOCPROG program. The majority of these dolphins (80.57%) were not seen together during surveys and the mean association rate between dyads was low at 0.05. Temporal analyses for all the individuals were conducted using a lagged association rate. The results indicated a non-random social structure made by constant companions.     

POPULATION ANALYSES OF INDO-PACIFIC HUMPBACK DOLPHINS SOUSA CHINENSIS IN ALGOA BAY,EASTERN CAPE,SOUTH AFRICA1

Mark-recapture analyses were performed on photo-identification data for Indo-Pacific humpback dolphins (Sousa chinensis) inhabiting Algoa Bay, on the Eastern Cape coast of South Africa. Boat-based photo-ID surveys were undertaken between May 1991 and May 1994. The rate of discovery of newly identified dolphins, distribution of sightings and frequency of resightings of known individuals indicate a high level of seasonal immigration of humpback dolphins into, and emigration from, the Algoa Bay region in summer. Consequently, humpback dolphins from Algoa Bay appear to be part of a substantially larger population that uses a considerable length of the coastal zone. The minimum population size is estimated to be about 466 dolphins.     

Defining bottlenose dolphin (Tursiops truncatus) stocks based on environmental,physical, and behavioral characteristics

The population structure of bottlenose dolphins, Tursiops truncatus, along the U.S. Atlantic coast has recently been redefined from one homogenous population into five coastal stocks. Local studies indicate even finer structure, primarily based on isolation of dolphins inhabiting estuaries. We identified population structuring of non‐estuarine coastal bottlenose dolphins during a study in New Jersey, the northern range along the Atlantic Coast. Using photo‐identification and distribution survey results, an analysis identified two major clusters of individuals significantly separated by five variables (distance from shoreline, group size, occurrence of the barnacle Xenobalanus globicipitis, avoidance behavior, and individual coloration). Sightings assigned to cluster 1 occurred in nearshore shallow waters (0–1.9 km, x?= 3.5 m), and those assigned to cluster 2 occurred further offshore in deeper waters (1.9–6 km, x?= 9.5 m). Only eight of 194 individuals (4%) were identified in both regions. Collectively, this suggests an occurrence of two stocks that are spatially, physically, and behaviorally distinguishable over a small distance. These results indicate that complexity in Tursiops population structure is not limited to latitudinal gradients or barriers created by estuarine habitats, but also by partitioning of habitat as a function of distance from shore and depth over small distances.     

Abundance and site fidelity of Indo‐Pacific Humpback dolphins in the Great Sandy Strait,Queensland, Australia

This study examined the population of Indo‐Pacific humpback dolphins, Sousa chinensis, inhabiting the Great Sandy Strait Marine Park, Queensland, Australia. A total of 106 dolphins were identified during 228 boat‐based surveys, completed between April 2004 and April 2007. Based on the distribution of resighted individuals and the pattern of associations, it was established that this population consists of two largely geographically distinct communities, referred to as the Northern Community (NC) and the Southern Community (SC). The only recorded interaction between the two groups was a single pod composed of one member of the NC and 11 dolphins from the SC. Abundance was estimated for the entire population and by geographical area using open population models. Estimates for the Great Sandy Strait indicate that about 150 dolphins (N_GSS= 148.4, SE = 8.3, 95% CI: 132.5–165.2) used this area during the study. The NC and SC total population sizes was estimated to be 76 (N_NGSS= 75.80, SE = 3.88, 95% CI = 71–86) and 75 (N_SGSS= 74.98, SE = 4.43, 95% CI: 66–83), respectively. Analysis of residence patterns indicates that a majority of the identified dolphins are long‐term residents.     

Mating Group Size in Dusky Dolphins (Lagenorhynchus obscurus): Costs and Benefits of Scramble Competition

In fission–fusion social systems with scramble competition between males, multiple males join mating groups while surrounding an oestrous female. If male decisions to join a mating group have been shaped by natural selection, then there should be an optimal group size resulting from the trade‐offs between the benefits of monopolizing a female in small groups and the energy lost in defending her from rivals in large groups. Male dusky dolphins (Lagenorhynchus obscurus) off Kaikoura, New Zealand, provide a unique opportunity to assess the optimum mating group size because they join transient mating groups not confounded by foraging or predator evasion. Within aggregations of up to 1000 individuals, males search for oestrous females, encountering choices of staying with a large mating group or leaving to find a smaller group. Mating groups typically involve multiple males mating with a single female. We conducted focal follows of mating groups (N = 44) by vessel from November 2011 through January 2012. We used video and a GPS to record group size, behaviour and movement. For each group, we measured potential costs (Swim Speed, Loss of Monopolization Potential) and benefits (Copulation Rate and Duration, Energy Savings). Only Loss of Monopolization Potential was positively correlated with group size, while Energy Savings was negatively correlated. Using these two factors as utility functions, we constructed an optimality model and predicted the optimal mating group size to be seven individuals with a range of 4–11 individuals due to variance. The observed modal mating group size was five dolphins, with a range of 2–15. We compare variation in mating group currencies and sizes to past studies. We discuss potential limitations of applying optimality models to predict mating group size for socially complex and behaviourally plastic species such as dolphins.     

THE SOCIAL STRUCTURE OF BOTTLENOSE DOLPHINS, TURSIOPS TRUNCATUS, IN THE BAHAMAS

The social structure of coastal ecotype bottlenose dolphins, Tursiops truncatus , is largely unknown as they inhabit regions far from shore. This study reports on a community of bottlenose dolphins ≥ 27 km from Grand Bahama Island (May-September, 1993–2002). Resident and non-resident dolphins occurred in the area. Some dolphins traveled over 320 km between communities; others showed long- term site fidelity up to 17 yr. Average group size was 3–5, and was significantly larger with calves present and significantly smaller when traveling. The half-weight index was used to determine coefficients of association (COA) for individuals of known sex annually and for pooled years. Permutation tests revealed non-random associations and presence of preferred/avoided companions in all data sets. Annual COAs were low: female-female χ= 0.31, male-male χ= 0.30, and mixed-sex χ= 0.26. Mother-calf associations showed the highest values. Some males formed strong, long-term bonds. Female COAs fluctuated with reproductive status. Using pooled data, COAs were lower and the same basic trends were evident. However, strong associations seen in the annual data were not evident in pooled data. Bottlenose dolphins that inhabit offshore, shallow water show many of the same social structure characteristics as in well-studied coastal populations.     

Mark‐resight abundance and survival estimation of Indo‐Pacific bottlenose dolphins,Tursiops aduncus,in the Swatch‐of‐No‐Ground,Bangladesh

A mark‐resight analysis under Pollock's robust design was applied to Indo‐Pacific bottlenose dolphins Tursiops aduncus in the Swatch‐of‐No‐Ground (SoNG) submarine canyon, Bangladesh, during the winter seasons of 2005–2009. Information from sightings of photo‐identified individuals (1,144) and unmarked individuals generated abundance estimates of 1,701 (95% confidence interval [CI]= 1,533–1,888), 1,927 (95% CI = 1,851–2,006), 2,150 (95% CI = 1,906–2,425), and 2,239 (95% CI = 1,985–2,524) individuals for seasons 1–4, respectively. This makes the population among the largest assessed of the species. Overall apparent survival was estimated as 0.958 (95% CI = 0.802–0.992). Interseasonal probabilities of transitioning to an unobservable state were estimated as 0.045, 0.363, and 0.300 for years 1–2, 2–3, and 3–4, respectively, and the overall probability of remaining in an unobservable state was 0.688. These probabilities, together with an apparent increase in abundance during the study period, indicate that the identified dolphins are part of a larger superpopulation moving throughout a more extensive geographic area. Of the photo‐identified dolphins, 28.2% exhibited injuries related to entanglements with fishing gear. This implies a strong potential for fatal interactions that could jeopardize the conservation status of the population, which otherwise appears favorable.     

DEMOGRAPHIC PARAMETERS OF INDO-PACIFIC BOTTLENOSE DOLPHINS (TURSIOPS ADUNCUS) AROUND MIKURA ISLAND, JAPAN

Long-term identification surveys were conducted using video records on Indo-Pacific bottlenose dolphin ( Tursiops aduncus ) around Mikura Island, Tokyo, Japan. On 246 surveys, 3,996 min of video were recorded during each summer season from 1994 to 2001. Two hundred and twenty individuals were observed, including juveniles and neonates, that were not cataloged, but could be identified by association with their mothers. Over the eight-year study period, the total number of identified and cataloged individuals was 169. The annual number of newly identified dolphins stabilized at ∼ 10 after 1995. In the middle of each research season, a plateau in the number of reidentified individuals was reached. Most of the dolphins were observed frequently–percentages of reidentified dolphins in adjoining years were greater than 86% and 62 individuals were seen consecutively during the eight years. There were no significant differences in the sex ratio between years. The sex ratio was skewed significantly towards male for subadults and towards female for adults. The survival rate of one-year-old calves was 86.7%, and the mean age at weaning was 3.5 yr. The calving interval averaged 3.4 yr among adult females. The mean annual birth, fecundity, and recruitment rates were 0.071, 0.239, and 0.068, respectively. Mikura Island is located in the pelagic ocean with no protected bay area existing around the island. Dolphins do not regularly inhabit any other locations besides Mikura within the Izu island chain. Thus, this island may be important core habitat of these dolphins.     

Occurrence Patterns,Habitat Associations,and Potential Prey of the River Dolphin,Inia geoffrensis,in the Cinaruco River,Venezuela1

The distribution, habitat association, group size, population structure, and prey availability of river dolphins (Inia geoffrensis) were studied from November 1993-June 1994 in the Cinaruco River, a tributary of the Orinoco River that forms the southern boundary of Venezuela's Santos Luzardo National Park. Dolphins were sampled from a boat using modified strip-width transects, for a total of 418 h. The study area was 1.67 km², and contained 20 km of water courses. Like other rivers of this region, the Cinaruco River undergoes a seasonal flood cycle. Dolphins were seen most often during the period of falling water (41% of total sightings) and least often during the rising water period (24% of total sightings). Dolphins were seen most often in confluence areas (35% of total sightings) and were seldom seen in side channels (13% of total sightings). The presence of rocks or sandbanks was associated with a greater frequency of dolphin sightings, and sightings increased with habitat heterogeneity. Average group size for the 8-mo study was 2.0 (±1.0) and was largest during the rising water period. Calves were first sighted during the end of the dry season and became more common during the early flood season. Six individuals were photo-identified and resighted with one sighted eight times over 186 d. The fish diversity of the study area was high, with 161 species documented in our samples. The stomach of one Inia contained 15 fishes representing at least 4 species.     

Accuracy and precision of dolphin group size estimates

Estimating the number of dolphins in a group is a challenging task. To assess the accuracy and precision of dolphin group size estimates, observer estimates were compared to counts from large‐format vertical aerial photographs. During 11 research cruises, a total of 2,435 size estimates of 434 groups were made by 59 observers. Observer estimates were modeled as a function of the photo count in a hierarchical Bayesian framework. Accuracy varied widely among observers, and somewhat less widely among dolphin species. Most observers tended to underestimate, and the tendency increased with group size. Groups of 25, 50, 100, and 500 were underestimated by <1%, 16%, 27%, and 47%, respectively, on average. Precision of group size estimates was low, and estimates were highly variable among observers for the same group. Predicted true group size, given an observer estimate, was larger than the observer estimate for groups of more than about 25 dolphins. Predicted group size had low precision, with coefficients of variation ranging from 0.7 to 1.9. Studies which depend on group size estimates will be improved if the tendency to underestimate group size and the high uncertainty of group size estimates are included in the analysis.     

Seasonal variations in spatial usage by the estuarine dolphin, Sotalia guianensis (van Bénéden, 1864) (Cetacea; Delphinidae) at its southern limit of distribution.

The use of space by the resident population of estuarine dolphins (Sotalia guianensis) in Norte Bay, southern Brazil was studied by boat surveys between 2000 and 2003. Data such as the geographical position and group size was collected at 5-minute intervals using focal-group sampling. The distribution and seasonal and daily group home-ranges were estimated using a GIS (Geographic Information System) environment. The dolphins did not use the area homogeneously, restricting their activities to a well-defined area located in the western part of the bay, while the use of the eastern part was not observed. A discrepancy between the Environment Protection Area of Anhatomirim, created especially to protect this population and the distribution of the dolphins was observed. Seasonal group home-range varied both qualitatively and quantitatively among the four consecutive seasons analyzed. The dolphins tended to use a larger area in cold seasons, when apparently the abundance of major prey items is lower. Core areas of intense use were identified, and also varied seasonally. These core areas may be regarded as critical habitats and require special legislation and control. The mean daily group home-range did not differ statistically among seasons. The present study corroborates a spatially strict residency pattern of the estuarine dolphin in highly productive coastal ecosystems.     

Male reproductive success increases with alliance size in Indo-Pacific bottlenose dolphins (Tursiops aduncus)

1. Determining the extent of variation in male mating strategies and reproductive success is necessary to understand the fitness benefits of social and cooperative behaviour. 2. This study assesses the reproductive success of male Indo-Pacific bottlenose dolphins in a small embayment population where different behavioural strategies of males have previously been identified. Parentage for 44 sampled calves was examined using 23 microsatellite loci and one mitochondrial DNA marker. Our candidate parent pool of 70 males and 64 females contained individuals sampled from both the embayment and adjacent coastal populations. 3. A moderate level of polygyny was detected in our sample. We assigned paternity of 23 calves to 12 males at the strict 95% confidence level and an additional nine calves to two males at the 80% confidence level. The majority (92%) of successful males were identified as residents to the embayment, and 46% of offspring were located within the same social group or community as their father. 4. Our results suggest that the size of alliances was the best predictor of reproductive success for males in this population, while the strength of association among allied males, alliance stability and male ranging patterns had little influence. In line with predictions for male alliances formed between unrelated individuals, we found that reproductive skew within alliances was not large. 5. Together, our genetic and behavioural analyses demonstrate that alliance formation between male dolphins is a successful strategy to enhance reproductive output.     

Population size, weight distribution and food in a persistent population of the rare medicinal leech, Hirudo medicinalis

1. It is important for species recovery and conservation management projects to know the minimum viable population size for rare and endangered species, such as the medicinal leech, Hirudo medicinalis. Therefore, using a catch‐removal method, this study estimated every two years (1986, 1988, 1990, 1992) the total number of medicinal leeches in a tarn in the English Lake District, and the number of mature adults in the population. 2. Four samples were taken each year in June and July, when water temperatures exceeded 20 °C. Population size was estimated both by maximum likelihood and regression methods. All leeches were weighed alive and size groups were separated by polymodal frequency analysis. A small sample of the blood meal in each leech gut was taken before the leeches were returned to the tarn, and was used to estimate the proportion of mammalian and non‐mammalian blood in the meals. 3. Both methods of estimation produced similar values, increasing confidence in the population estimates. Values for the total population in June and July varied among years from 248 to 288, the maximum value being only 16% higher than the minimum. Values for the number of mature leeches varied from 48 to 58 (19–20% of the total population), and this was an estimate of the effective population size. 4. There were four size groups. The largest mature leeches (live weight >5 g) in group IV formed only 1% of the population, and the smallest (0.02–0.5 g) in group I 14–17%. Most leeches were in two overlapping groups of immature (64–67% of population) and mature (18%) leeches with size ranges of 0.4–3.4 g and 2.5–5 g respectively. The percentage of leeches in each size group was very consistent among years. Blood meals were found in 38–44% of the leeches in group I, 45–50% in group II, 70–75% in group III, and 100% in group IV, but mammalian blood was present only in larger mature leeches (>3.5 g). 5. Medicinal leeches were first detected in the tarn in 1980 and are still present in 2007, so the population has persisted for at least 27 years. Compared with minimum viable population sizes for other species, including many endangered species, values for this medicinal leech population are extremely low, but may be typical of some rare freshwater invertebrates in isolated habitats.     

RANGE CHARACTERISTICS OF PACIFIC COAST BOTTLENOSE DOLPHINS (TURSIOPS TRUNCATUS) IN THE SOUTHERN CALIFORNIA BIGHT

Boat-based photoidentification surveys of bottlenose dolphins (Tursiops truncatus) were conducted from 1982 to 1989 in three discrete coastal study areas within the Southern California Bight: (1) Santa Barbara, California; (2) Orange County, California; (3) Ensenada, Baja California, Mexico. A total of 207 recognizable dolphins were identified in these three “secondary” study areas. These individuals were compared to 404 dolphins identified from 1981 to 1989 in our “primary” study area, San Diego, California, to examine the coastal movement patterns of bottlenose dolphins within the Southern California Bight. A high proportion of dolphins photographed in Santa Barbara (88%), Orange County (92%), and Ensenada (88%) were also photographed in San Diego. Fifty-eight percent (n= 120) of these 207 dolphins exhibited back-and-forth movements between study areas, with no evidence of site fidelity to any particular region. Minimum range estimates were 50 and 470 km. Minimum travel-speed estimates were 11-47 km/d, and all dolphin schools sighted during the study were within 1 km of the shore. These data suggest that bottlenose dolphins within the Southern California Bight are highly mobile within a relatively narrow coastal zone. Home-range dimensions and movement patterns for many vettebrate species are influenced, in part, by variation in food resources. The unique range characteristics documented during this study may reflect the highly dynamic nature of this coastal ecosystem and the associated patchy distribution of food resources available to these bottlenose dolphins.     

ABUNDANCE OF IRRAWADDY DOLPHINS (ORCAELLA BREVIROSTRIS) AND GANGES RIVER DOLPHINS (PLATANISTA GANGETICA GANGETICA) ESTIMATED USING CONCURRENT COUNTS MADE BY INDEPENDENT TEAMS IN WATERWAYS OF THE SUNDARBANS MANGROVE FOREST IN BANGLADESH

Independent observer teams made concurrent counts of Irrawaddy dolphins Orcaella brevirostris and Ganges River dolphins Platanista gangetica gangetica in mangrove channels of the Sundarbans Delta in Bangladesh. These counts were corrected for missed groups using mark-recapture models. For Irrawaddy dolphins, a stratified Lincoln-Petersen model, which incorporated group size and sighting conditions as covariates, and a Huggins conditional likelihood model, which averaged models that individually incorporated group size, sighting conditions, and channel width as covariates, generated abundance estimates of 397 individuals (CV = 10.2%) and 451 individuals (CV = 9.6%), respectively. For Ganges River dolphins, a stratified Lincoln-Petersen model, which incorporated group size as a covariate, and a Huggins conditional likelihood model, which averaged the same models described above, generated abundance estimates of 196 individuals (CV = 12.7%) and 225 individuals (CV = 12.6%), respectively. Although the estimates for both models were relatively close, the analytical advantages of the Huggins models probably outweigh those of the Lincoln-Petersen models. However, the latter should be considered appropriate when simplicity is a priority. This study found that waterways of the Sundarbans support significant numbers of Irrawaddy and Ganges River dolphins, especially compared to other areas where the species have been surveyed.