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 1 Current address: Wildlife Conservation Society, Asian Freshwater and Coastal Cetacean Program, 27/16 Soi Naya, Moo 1, Muang, Rawaii, Phuket 83130 Thailand. 527 528 MARINE MAMMAL SCIENCE, VOL. 22, NO. 3, 2006 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.

ABSTRACT1. Generalized additive models of sighting data for cetaceans collected during two surveys of waterways in the Sundarbans mangrove forest of Bangladesh indicated that Ganges River dolphin Platanista gangetica gangetica distribution was conditionally dependent (P50.05) on low salinity, high turbidity, and moderate depth during both low and high freshwater flow; and Irrawaddy dolphin Orcaella brevirostris distribution was conditionally dependent (P50.05) on low salinity during high freshwater flow, high and moderate depths during low and high freshwater flow, respectively; low and high-low extremes of turbidity during low and high freshwater flow, respectively; and high temperature and increasing numbers of large-small channel confluences during low freshwater flow.2. According to sighting data collected over a 3-year period by the captains of three nature tourism vessels, there were significant differences between the actual and expected frequencies of Ganges River dolphin sightings and individuals according to various channel types (chi-square=64.22, P50.0001 and chi-square=134.14, P50.0001, respectively, df=6) and of Irrawaddy dolphin sightings and individuals (chi-square=15.28, P=0.0182, and chi-square=29.42, P50.0001, respectively, df=6), with shared preferences for wide sinuous channels with at least two small confluences or one large confluence.3. The dependency exhibited by both species for environmental characteristics associated with abundant freshwater flow, including low salinity and the availability of confluences, make them particularly vulnerable to habitat loss due to upstream water abstraction and sea-level rise.4. Although the results of this study may not affect plans for construction in India of large-scale, inter-basin water transfer projects that will result in further declines in freshwater flows, or decisions within the international community about CO 2 emissions affecting global sea levels, they can be used to prioritize locations where protective measures could be employed to benefit the long-term conservation of both species.

Detection of animals during visual surveys is rarely perfect or constant, and failure to account for imperfect detectability affects the accuracy of abundance estimates. Freshwater cetaceans are among the most threatened group of mammals, and visual surveys are a commonly employed method for estimating population size despite concerns over imperfect and unquantified detectability. We used a combined visual-acoustic survey to estimate detectability of Ganges River dolphins (Platanista gangetica gangetica) in four waterways of southern Bangladesh. The combined visual-acoustic survey resulted in consistently higher detectability than a single observer-team visual survey, thereby improving power to detect trends. Visual detectability was particularly low for dolphins close to meanders where these habitat features temporarily block the view of the preceding river surface. This systematic bias in detectability during visual-only surveys may lead researchers to underestimate the importance of heavily meandering river reaches. Although the benefits of acoustic surveys are increasingly recognised for marine cetaceans, they have not been widely used for monitoring abundance of freshwater cetaceans due to perceived costs and technical skill requirements. We show that acoustic surveys are in fact a relatively cost-effective approach for surveying freshwater cetaceans, once it is acknowledged that methods that do not account for imperfect detectability are of limited value for monitoring.

Surveys for shushuks Platanista gangetica were conducted during January to April 1999 in Kaptai Lake and the southern rivers of Bangladesh. A population of at least 125 dolphins was recorded in the Karnaphuli and Sangu rivers and connecting canal. The overall encounter rate was 0.76 dolphins per km. Density was highest in the lower reaches of the Sangu, where we recorded 1.36 dolphins per km. These rates are fairly high when compared with other areas of shushuk distribution. Dolphin movements in the Sikal-baha±Chandkhali Canal were consistent with it being used as a corridor for migration and dispersal between the Karnaphuli and Sangu. Shushuks were also sighted in marine waters of the Karnaphuli and Sangu river mouths, adding credibility to the hypothesis that dolphins move along the coast between the Ganges± Brahmaputra±Meghna and Karnaphuli±Sangu systems, perhaps during the monsoon when freshwater plumes extend into the Bay of Bengal. No dolphins were observed in Kaptai Lake, a dam-created reservoir of the upper Karnaphuli, despite reports of occasional sightings by local ®shermen. No shushuks were observed in the Bagkhali and Matamuhuri rivers, possibly because of seasonal-closure dams present near the mouths of both rivers. The main threats to dolphin survival in the Karnaphuli±Sangu system are probably accidental entanglement in mono®lament gillnets, bioaccumulation of persistent contaminants and possibly collisions with motorized vessels and a decline in prey as a result of over ®shing. The most signi®cant conservation measure that could be taken would be to establish a protected area for dolphins in the Sangu River below the Dohazari Bridge.

Surveys for shushuks Platanista gangetica were conducted during January to April 1999 in Kaptai Lake and the southern rivers of Bangladesh. A population of at least 125 dolphins was recorded in the Karnaphuli and Sangu rivers and connecting canal. The overall encounter rate was 0.76 dolphins per km. Density was highest in the lower reaches of the Sangu, where we recorded 1.36 dolphins per km. These rates are fairly high when compared with other areas of shushuk distribution. Dolphin movements in the Sikalbaha–Chandkhali Canal were consistent with it being used as a corridor for migration and dispersal between the Karnaphuli and Sangu. Shushuks were also sighted in marine waters of the Karnaphuli and Sangu river mouths, adding credibility to the hypothesis that dolphins move along the coast between the Ganges–Brahmaputra–Meghna and Karnaphuli–Sangu systems, perhaps during the monsoon when freshwater plumes extend into the Bay of Bengal. No dolphins were observed in Kaptai Lake, a dam‐created reservoir of the upper Karnaphuli, despite reports of occasional sightings by local fishermen. No shushuks were observed in the Bagkhali and Matamuhuri rivers, possibly because of seasonal‐closure dams present near the mouths of both rivers. The main threats to dolphin survival in the Karnaphuli–Sangu system are probably accidental entanglement in monofilament gillnets, bioaccumulation of persistent contaminants and possibly collisions with motorized vessels and a decline in prey as a result of over fishing. The most significant conservation measure that could be taken would be to establish a protected area for dolphins in the Sangu River below the Dohazari Bridge.