River Dolphin Research Articles

Editorial

Editorial

Spatio-temporal distribution and community structure of zooplankton in the Gangetic Dolphin Sanctuary, 2009

The Vikramshila Gangetic Dolphin Sanctuary (VGDS) covers a 50 km stretch of the main channel of the Ganga River and is protected for the endangered freshwater dolphin (Platanista gangetica). We studied spatio-temporal patterns of zooplankton community structure during January through December 2009, at three sites. A total of 35 species including rotifers, cladocerans and copepods were identified. Integrating all samples, the top five dominant zooplankton included copepodids, copepod nauplii, B. caudatus, Ceriodaphnia rigaudi and Mesocyclops leuckarti. The zooplankton community was represented by diverse feeding groups including herbivores, carnivores, detritivores and bacterivores. The hierarchical dendrogram revealed distinct spatiotemporal variations. The Sultanganj community was separated at first hierarchical level, whereas species assemblage of August–October at Kahalgaon was distinguished at second hierarchical level. Top three indicative species at site-I (Sultanganj) were C. reticulata, Daphnia pulex and B. quadridentatus throughout the year. During monsoon the top three indicator species were C. rigaudi, M. leuckarti and B. falcatus at Bhagalpur and copepod developmental stages, B. caudatus and Keratella tropica at Kahalgaon. Adult Mesocyclops sp., copepod developmental stages and D. carinata were dominant in winter, whereas, Moina sp., C. rigaudi and M. leuckarti were the top three indicator species in summer at Bhagalpur and Kahalgaon. Of all the sites studied, Kahalgaon was recorded as having higher species richness.

A multimodal detection model of dolphins to estimate abundance validated by field experiments

Abundance estimation of marine mammals requires matching of detection of an animal or a group of animal by two independent means. A multimodal detection model using visual and acoustic cues (surfacing and phonation) that enables abundance estimation of dolphins is proposed. The method does not require a specific time window to match the cues of both means for applying mark-recapture method. The proposed model was evaluated using data obtained in field observations of Ganges River dolphins and Irrawaddy dolphins, as examples of dispersed and condensed distributions of animals, respectively. The acoustic detection probability was approximately 80%, 20% higher than that of visual detection for both species, regardless of the distribution of the animals in present study sites. The abundance estimates of Ganges River dolphins and Irrawaddy dolphins fairly agreed with the numbers reported in previous monitoring studies. The single animal detection probability was smaller than that of larger cluster size, as predicted by the model and confirmed by field data. However, dense groups of Irrawaddy dolphins showed difference in cluster sizes observed by visual and acoustic methods. Lower detection probability of single clusters of this species seemed to be caused by the clumped distribution of this species.

The Status of the Ganges River Dolphin Platanista gangetica gangetica in the River Barak, Assam, India

The Ganges River Dolphin Platanista gangetica gangetica , once common in the River Barak in southern Assam, is now rarely sighted. Currently, the dolphins in winter (December-February) congregate in a 14 km stretch of the Barak centring on two sites. During the rainy season (mainly June-August), they frequent tributaries and flooded paddy fields. The population in the 135 km stretch of the Barak, before its bifurcation near the Indo-Bangladesh border, was estimated at 14, 12, 10, 9, 9, 8, and 8 in 1999, 2001, 2003, 2005, 2006, 2007, and 2008, respectively. About 60%, 31%, and 9% of the population was made up of adults, subadults and calves respectively. Information on 29 dolphin mortalities in the Barak comprised 28% adults, 34% subadults and 38% calves. Fishing activities were responsible for 90% of the mortalities; 37% of deaths of calves were due to entanglement in gill-nets. Direct and indirect hunting, uses of dolphin oil as fish attractant and medicine, habitat loss and degradation, depletion of prey fish, water development projects, lack of people's awareness, and poor law enforcement threaten the dolphins. Among others, notification of Barak Dolphin Sanctuary and strict enforcement of laws are required to conserve the dolphins in the Barak.

River Dolphin (Inia geoffrensis, Sotalia fluviatilis) Mortality Events Attributed to Artisanal Fisheries in the Western Brazilian Amazon

In the Western Brazilian Amazon, interactions of boto (Inia geoffrensis) and tucuxi (Sotalia fluviatilis) dolphins with fishing activities are common, but the prevalence of incidental/intentional catches is not known. This article describes incidental mortality events and intentional killing of I. geoffrensis and S. fluviatilis entangled in artisanal fishing gear and the opportunistic use of carcasses as bait. Between October 2010 and November 2011, surveys were conducted in waters of the lower Japura River, between the Mamiraua and Amana sustainable development reserves. In order to obtain information on interactions and to try to establish a stranding/entanglement response program (SERP), informal conversations were exchanged with local inhabitants (n = 174). Intense carcasssearch surveys (n = 171) along the river in the four hydrological seasons (e.g., low, rising, high, and falling waters) were conducted, comprising a total of 1,197 h of sampling effort. Twenty-five dolphin– fishing interaction events were recorded (11 I. geoffrensis and 14 S. fluviatilis), 19 in 2011 and six in 2012 (through SERP). A total of 11 necropsies (three I. geoffrensis and eight S. fluviatilis) were performed. Four individuals (two I. geoffrensis and two S. fluviatilis) exhibited evidence of physical violence before death, and two (one I. geoffrensis and one S. fluviatilis) died in abandoned gillnets. Two intentional killing events of I. geoffrensis incidentally entangled for bait use in the piracatinga (Calophysus macropterus) fishery were reported by fishermen, while three carcasses (two I. geoffrensis and one S. fluviatilis) with gillnet marks were also used in that activity. At least six of the S. fluviatilis entanglement events occurred in fishing gear used for tambaqui (Colossoma macropomum) and pirapitinga (Piaractus brachypomus) (90/100-mm mesh-size gillnet), two of the most important commercial fish species in the Amazon Basin. As seasonal fishing constitutes the main income for riverine human populations, the negative reactions that cetacean presence causes to people could have a catalyst effect for the transition from “inciden- tal capture” to “intentional capture and competi- tor removal.” Law enforcement and precautionary measures through good fishing practices inside dolphin critical foraging areas should be taken together with fisheries’ managers and fishermen to start to develop multiple-species management and ensure sustainable fishing practices.

A Miocene relative of the Ganges River dolphin (Odontoceti, Platanistidae) from the Amazonian Basin

Click to increase image sizeClick to decrease image size ACKNOWLEDGMENTS P. Loubry (MNHN) took the photographs. We are much indebted to P. Baby, M. Benammi, S. Brusset, D. De Franceschi, and N. Espurt for their participation in field work. This research project was undertaken thanks to the IRD-Perupetro S.A. research agreement, and supported by IRD (UR 154), Devanlay Peru S.A., and the French INSU-CNRS programs DyETI and ECLIPSE II, headed by P. Baby and P.-O. Antoine, respectively. Constructive comments of E. Fitzgerald, an anonymous reviewer, and the editor J. Geisler considerably improved a previous version of this paper. The work of O. Lambert at the IRSNB is funded by a Return Grant of the Belgian Federal Science Policy Office. Handling editor: Jonathan Geisler.

Clicking in Shallow Rivers: Short-Range Echolocation of Irrawaddy and Ganges River Dolphins in a Shallow, Acoustically Complex Habitat

Toothed whales (Cetacea, odontoceti) use biosonar to navigate their environment and to find and catch prey. All studied toothed whale species have evolved highly directional, high-amplitude ultrasonic clicks suited for long-range echolocation of prey in open water. Little is known about the biosonar signals of toothed whale species inhabiting freshwater habitats such as endangered river dolphins. To address the evolutionary pressures shaping the echolocation signal parameters of non-marine toothed whales, we investigated the biosonar source parameters of Ganges river dolphins (Platanista gangetica gangetica) and Irrawaddy dolphins (Orcaella brevirostris) within the river systems of the Sundarban mangrove forest. Both Ganges and Irrawaddy dolphins produced echolocation clicks with a high repetition rate and low source level compared to marine species. Irrawaddy dolphins, inhabiting coastal and riverine habitats, produced a mean source level of 195 dB (max 203 dB) re 1 µPapp whereas Ganges river dolphins, living exclusively upriver, produced a mean source level of 184 dB (max 191) re 1 µPapp. These source levels are 1–2 orders of magnitude lower than those of similar sized marine delphinids and may reflect an adaptation to a shallow, acoustically complex freshwater habitat with high reverberation and acoustic clutter. The centroid frequency of Ganges river dolphin clicks are an octave lower than predicted from scaling, but with an estimated beamwidth comparable to that of porpoises. The unique bony maxillary crests found in the Platanista forehead may help achieve a higher directionality than expected using clicks nearly an octave lower than similar sized odontocetes.

Time Dependent Activity Pattern of Ganges River DolphinPlatanista gangetica gangeticaand its Response to Human Presence in Upper Ganges River, India

Information on activity profile of the Ganges river dolphin (Platanista gangetica gangetica) and its responses to human interference were lacking. A study following focal animal sampling protocol with individual-follow method and assessment of human interference on dolphin occurrence and behavior was hence conducted. Based on logical reasoning and prior assumptions, dolphin activities were broadly categorized into four types viz., surfacing, movement, chasing/feeding and underwater. Percentage frequencies of occurrence of these activities in dolphin activity budget were 50%, 26%, 6% and 18% with corresponding time allocations of 1%, 60%, 4% and 35%, respectively; without any significant difference between male and female activity patterns (χ2 = 0.832, df = 3, P > 0.05). Five different modes of surfacing were observed with dive-times ranging from 4 to 504 sec and average prey chase-time of 38 (SD = ±29) sec. Human interference had a negative relationship with dolphin presence (β = -0.6398, z = -3.816, P < 0.001). All recorded dolphin activities showed a positive relationship with human presence except underwater activity (β = -0.1115, z = -3.76, P < 0.001). Detailed behavioral study and proper check on increasing anthropogenic influence is necessary to ensure the species safe survival.

An assessment of the impact of Koshi floods to birds and mammals

Flood impact assessment with a focus on wetland dependent birds and mammals was carried out in 2009. Data compared to 2008, showed decline in the total number of birds. The results showed that while some bird benefitted from replenished fish stock, many others suffered. South Asian River Dolphin Platanista gangetica increased in number within the Koshi Tappu Wildlife Reserve after the flooding. Poverty and illiteracy contributing to lack of awareness amongst local people were hampering conservation works in the Reserve.DOI: http://dx.doi.org/10.3126/njbs.v2i0.7482 Nepalese Journal of Biosciences 2 : 1-4 (2012)

PRESUMED NORMAL ULTRASONOGRAPHIC FINDINGS OF THE TESTIS AND EPIDIDYMIS OF BOTOS (INIA GEOFFRENSIS)

Fifteen live adult male botos, or Amazon river dolphins (Inia geoffrensis), were examined using ultrasonography during the yearly capture expedition, between October and November 2005, at the Mamirauá Sustainable Development Reserve, within the Brazilian Amazon (3 degrees S, 65 degrees W). All examinations were performed with a Sonosite 180 plus ultrasound unit in conjunction with a 2- to 5-MHz multifrequency transducer convex array 180 Plus/Elite-C60. Age and maturity estimates were determined considering the body length, weight, and external characteristics. In all examinations, the testes were discerned by the presence of a hyperechoic central line, called the mediastinum testis, a landmark for their identification during ultrasonography. No significant differences in echogenicity were detected on the ultrasonographic appearance of the testes among the studied animals. On adult male botos, apparent parenchymal nodulation of the testis was observed on scanning in most of the animals and probably constituted evidence of reproductive maturity. Using the color Doppler technique, blood flow was detected along the mediastinum testis that progressively decreased toward the periphery of this organ. Little blood flow could be identified by color Doppler. Power Doppler allowed better accuracy to identify testicular vessels, their topography, and their differentiation from adjacent structures. Ultrasonographic examination provides useful data for morphologic characterization of the boto's testes. Examination using Doppler techniques was considered a valuable tool to evidence blood flow through the testicular parenchyma.

Effect of illegal harvest on apparent survival of Amazon River dolphins (Inia geoffrensis)

The Amazon River dolphin (Inia geoffrensis), or boto, is illegally harvested for use as bait in fisheries for the catfish Calophysus macropterus. To determine the effect of this harvest, we estimated apparent survival for a boto population in the central Brazilian Amazon where direct harvest is known to have occurred since 2000. For our analysis, we used capture and recapture/resighting data of 528 marked botos over a 17-year period (1994–2011). Time-dependent models estimated that apparent survival after the first reports of harvest (ϕ=0.899; SE=0.007) was significantly lower than in years prior to harvest (ϕ=0.968; SE=0.009). The decline in apparent survival suggests that current harvest rates exceed conservation limits and may be unsustainable. This issue requires the attention of natural resource managers from all countries of the Amazon basin, as the harvest is widespread and decline in survival could be mirrored in numerous locales.

Ultrastructure of enamel and dentine in extant dolphins (Cetacea: Delphinoidea and Inioidea)

Longitudinal and cross sections of teeth from 17 species of the Recent dolphins (Delphinoidea and Inioidea) were examined under scanning electron microscope to study the arrangement and ultrastructure of dental tissues with reference to phylogenetic and functional constraints. For most species, enamel had a simple bi-layered structure of radial enamel and an outer layer of prismless enamel. The outer prismless layer varied from 5 to 30 % of enamel thickness. The enamel of Burmeister’s porpoise (Phocoena spinipinnis) was entirely prismless. The prisms had an open sheath; tubules and tuft-like structures were common at the enamel-dentine junction. Cetacean dentine was characterized by irregularly distributed dentinal tubules in a relatively homogenous dentinal matrix. Radial enamel was observed in all Delphinoidea and in the franciscana (Pontoporia blainvillei), whereas the Amazon river dolphin (Inia geoffrensis) had prisms organized in Hunter–Schreger bands. HSB in enamel are regarded as a device for resisting propagation of cracks. These may occur due to increased functional demands, possibly related to the hardness of the species diet. Simplification in tooth shape and reduced biomechanical demands plausibly explain the primitive radial organization among delphinoids and Pontoporia. The HSB structure in the Amazon river dolphin, similar to those of extinct archaeocetes, seems to have secondary functional implications. However, the distribution of HSB in more-basal odontocetes is too poorly known to judge whether the HSB of Inia are a retained plesiomorphic feature or convergence.

Likely future extirpation of another Asian river dolphin: The critically endangered population of the Irrawaddy dolphin in the Mekong River is small and declining

AbstractThe population of Irrawaddy dolphins that occupies the Mekong River in southern Lao People's Democratic Republic and Cambodia is classified as Critically Endangered by the IUCN. Based on capture‐recapture of photo‐identified individuals, we estimated that the total population numbered 93 ± SE 3.90 individuals (95% CI 86–101), as of April 2007. The combined photo‐identification and carcass recovery program undertaken from 2001 to 2007 established that the Irrawaddy dolphin population inhabiting the Mekong River has reached a critical point with regards to its continued survival, where immediate research and management actions are required to greatly reduce adult mortality, and establish the cause of newborn mortality. In addition, community consultation is required to initiate, and evaluate, urgently required conservation measures. An ongoing well‐designed combined program of abundance estimation (i.e., photo‐identification) and carcass recovery is required to monitor total population size and mortality rates, to inform and evaluate management initiatives. The conclusions of this paper are likely generic to river dolphin populations, particularly where photo‐identification is possible.

Conflicts between river dolphins (Cetacea: Odontoceti) and fisheries in the Central Amazon: a path toward tragedy?

Dolphin interactions with fishermen have increased significantly and pose potential risks to the boto, Inia geoffrensis (Blainville, 1817), and the tucuxi, Sotalia fluviatilis (Gervais & Deville, 1853). The main objective of the present paper was to describe the existing conflicts between river dolphins and fishermen in the municipality of Manacapuru region. Sixteen fishermen were interviewed in Manacapuru, state of Amazonas, Brazil who described a situation of ongoing conflict that may be unsustainable. Two merchants from Manacapuru made unconfirmed reports on a boto carcass trade. Data collection for this study occurred between April 20th and April 25th, 2009, but the first author had been conducting research on river dolphins and fisheries in Manacapuru and nearby cities since the beginning of 2008, in order to gain the trust of the fishermen interviewed. The hunting and deliberate killing of the species is probably more threatening to botos than their incidental capture in fishing gears in the Manacapuru region. This practice may result from the fact that dolphins are prone to damaging fishing equipment, and stealing (and possibly damaging) fish from the nets. They are portrayed negatively in numerous myths and superstitions of traditional Amazonian folklore, making them extremely undesired or even hated, seen as pests, and used in the piracatinga, Calophysus macropterus (Lichtenstein, 1819) fishery as bait. For tucuxis, incidental capture still represents the major threat to their conservation in the region evaluated here.

Diel changes in the movement patterns of Ganges River dolphins monitored using stationed stereo acoustic data loggers

AbstractWe monitored the underwater movements of Ganges River dolphins using stationed stereo acoustic data loggers. We estimated these movements using changes in the relative angle of the sound source direction (trajectory). Of the total acoustic recordings (66 h), 26.2% contained trajectories of dolphins, and 78.6% of these trajectories involved single animals, suggesting that dolphins tended to swim alone and were localized near the monitoring station. The observed trajectories were categorized as follows: staying type characterized by small changes in the sound source direction, moving type A (moving in the same direction), and moving type B (moving up and down the stream during recording). The average interpulse intervals of sounds in moving types A and B were significantly shorter than that of the staying type, suggesting that dolphins produce the former types of trajectories to echolocate across shorter distances during movement. The frequency of occurrence of moving type A increased during the night, whereas that of type B increased in the late afternoon and that of the staying type increased during the daytime. These results indicate that dolphins moving at night tended to use short‐range echolocation, whereas during the day, they remained in relatively small areas and used long‐range sonar.

Molecular cytogenetic characterization of the Amazon River dolphin Inia geoffrensis

Classical and molecular cytogenetic (18S rDNA, telomeric sequence, and LINE-1 retrotransposon probes) studies were carried out to contribute to an understanding of the organization of repeated DNA elements in the Amazon River dolphin (boto, Inia geoffrensis). Twenty-seven specimens were examined, each presenting 2n = 44 chromosomes, the karyotype formula 12m + 14sm + 6st + 10t + XX/XY, and fundamental number (FN) = 74. C-positive heterochromatin was observed in terminal and interstitial positions, with the occurrence of polymorphism. Interstitial telomeric sequences were not observed. The nucleolar organizer region (NOR) was located at a single site on a smallest autosomal pair. LINE-1 was preferentially distributed in the euchromatin regions, with the greatest accumulation on the X chromosome. Although the karyotype structure in cetaceans is considered to be conserved, the boto karyotype demonstrated significant variations in its formula, heterochromatin distribution, and the location of the NOR compared to other cetacean species. These results contribute to knowledge of the chromosome organization in boto and to a better understanding of karyoevolution in cetaceans.

Robust abundance estimate for endangered river dolphin subspecies in South Asia

Robust estimates of absolute abundance are vital for management of threatened spe- cies, but these have rarely been generated for endangered South Asian river dolphins due to methodological challenges. An estimate of abundance for the Indus River dolphin in 2006 was gen- erated by conducting tandem vessel-based direct counts; conditional likelihood capture-recapture models were then used to correct for missed animals. Group size and sighting conditions were in- cluded as covariates, and abundances of the 3 largest subpopulations were estimated as 101 (coef- ficient of variation, CV = 44.1%) between Chashma and Taunsa barrages, 52 (CV = 14.9%) be- tween Taunsa barrage and Ghazi Ghat, and 1289 (CV = 33.4%) between Guddu and Sukkur barrages. A total of 75.3% of groups were seen by both independent survey teams, and single ani- mals were almost 5 times more likely to be missed than groups of 3 or more. Providing groups can be matched with minimal error, this survey method shows good potential for abundance estimation of dolphins in confined habitat and the shallow rivers of South Asia. Dolphin encounter rates within the Guddu-Sukkur subpopulation (10.35 km −1 ) are the highest reported for any river dolphin. Direct counts conducted over a 35 yr period, suggest that this subpopulation may have been in- creasing in abundance, probably due to the cessation of hunting and possible immigration from other subpopulations. The future of South Asian river dolphins is intimately tied to water security in the region, and escalating and competing demands for freshwater mean that the long-term future of South Asia's river dolphins is uncertain.

River Dolphins Can Act as Population Trend Indicators in Degraded Freshwater Systems

Conservation attention on charismatic large vertebrates such as dolphins is often supported by the suggestion that these species represent surrogates for wider biodiversity, or act as indicators of ecosystem health. However, their capacity to act as indicators of patterns or trends in regional biodiversity has rarely been tested. An extensive new dataset of >300 last-sighting records for the Yangtze River dolphin or baiji and two formerly economically important fishes, the Yangtze paddlefish and Reeves’ shad, all of which are probably now extinct in the Yangtze, was collected during an interview survey of fishing communities across the middle-lower Yangtze drainage. Untransformed last-sighting date frequency distributions for these species show similar decline curves over time, and the linear gradients of transformed last-sighting date series are not significantly different from each other, demonstrating that these species experienced correlated population declines in both timing and rate of decline. Whereas species may be expected to respond differently at the population level even in highly degraded ecosystems, highly vulnerable (e.g. migratory) species can therefore display very similar responses to extrinsic threats, even if they represent otherwise very different taxonomic, biological and ecological groupings. Monitoring the status of river dolphins or other megafauna therefore has the potential to provide wider information on the status of other threatened components of sympatric freshwater biotas, and so represents a potentially important monitoring tool for conservation management. We also show that interview surveys can provide robust quantitative data on relative population dynamics of different species.

Mercury Status of the Amazon Continental Shelf: Guiana Dolphins (Sotalia guianensis, Van Benédén 1864) as a Bioindicator

Total mercury (Hg) was analyzed in muscle tissue of 27 accidentally captured Guiana dolphins (Sotalia guianensis) in order to evaluate Hg contamination levels present in the Amazon Continental Shelf, in Amapá state, North Brazil. The samples showed a mean concentration of 0.4 ± 0.16 μg/g wet weight (ww), ranging from 0.07 to 0.79 μg/g ww. As observed in several other cetacean species, Hg concentrations presented positive correlations to body length, related to the capacity to bioaccumulate this element throughout life. Hg concentrations were not significantly different between males (mean = 0.38 μg/g ww; n = 15) and females (mean = 0.42 μg/g ww; n = 12). Concentrations were low when compared to results of studies carried out with small cetaceans in the Northern Hemisphere, and with some previous studies in the south-eastern region of Brazil. In contrast with high Hg concentrations normally detected in river dolphin samples from Amazon River tributaries, our results suggest that the Amazon coast contains low levels of Hg in bioavailable form.

Habitat use by a freshwater dolphin in the low‐water season

ABSTRACT Many river dolphin populations are most vulnerable during the low‐water season when habitat is limited. Indus River dolphin habitat selection in the dry season was investigated using Generalized Linear Models of dolphin distribution and abundance in relation to physical features of river geomorphology and channel geometry in cross‐section. Dolphins selected locations in the river with significantly greater mean depth, maximum depth, cross‐sectional area, and hydraulic radius, and significantly narrower river width and a lower degree of braiding than areas where dolphins were absent. They were also recorded with higher frequency at river constrictions and at confluences. Channel cross‐sectional area was the most important factor affecting dolphin presence and abundance, with the area of water below 1 m in depth exerting the greatest influence. Indus dolphins avoided channels with small cross‐sectional area (&lt;700 m2), presumably owing to the risk of entrapment and reduced foraging opportunities. Channel geometry had a greater ability to explain dolphin distribution than river geomorphology; however, both analyses indicated similar types of habitat selection. The dolphin–habitat relationships identified in the river geomorphology analysis were scale‐dependent, indicating that dolphin distribution is driven by the occurrence of discrete small‐scale features, such as confluences and constrictions, as well as by broader‐scale habitat complexes. There are numerous plans to impound or extract more water from the Indus River system. If low‐water season flows are allowed to decrease further, the amount of deeper habitat will decline, there may be insufficient patches of suitable habitat to support the dolphin population through the low‐water season, and dolphins may become isolated within deeper river sections, unable or unwilling to traverse through shallows between favourable patches of habitat. Copyright © 2012 John Wiley &amp; Sons, Ltd.