River Cetaceans and Habitat Change: Generalist Resilience or Specialist Vulnerability?

Table 1

Details on the conservation status, behavior, and ecology of the obligate freshwater dolphins (i.e., “true” river dolphins). Information on Ganges and Indus dolphins from [16–20]; Amazon dolphin or boto from [13, 21–26]; Yangtze dolphin or baiji from [4, 6, 27–29]. EN: endangered, DD: data deficient, CR: critically endangered, and PE: possibly extinct.

Species	IUCN Red List status and justification*	Aggregation size^+	Seasonal movements	Habitat preferences	Observations of sociality	Prey
Ganges and Indus dolphins	EN—Reduction in population size of more than 50% in three generations (60 years) inferred given that dam construction associated with the range declines of both subspecies occurred after 1944.	Alone or in clusters of 2-3, but occasionally as many as 25 present in the same area.	Range expansion into smaller tributaries and sometimes into irrigation channels as water levels rise.	Countercurrents and deep pools located downstream of channel confluences and sharp meanders. Generally in the deepest river channel and less common in secondary channels.	Few observations reported except for mother-young associations, and observations of mating.	At least 36 teleost fishes and four crustaceans.
Amazon dolphin or boto	DD—Previously listed as VU but it is now considered DD due to limited information on threats, ecology, and population numbers and trends.	Variable from solitary to small groups.	Some resident to specific areas year-round whereas others move several tens to hundreds of km.	Prefers river edges, confluences, and seasonally flooded lakes and channels off the main rivers. At high water, many animals utilise fish resources of the flooded forest.	Sometimes form loose groups that fish in a coordinated fashion. Interspecific play in the wild and food sharing in captivity. Male-on-male aggression common.	Wide variety of prey including at least 43 fishes, generally bottom feeders and including armoured catfish, as well as small turtles.
Yangtze dolphin or baiji	CR (PE)—Total number of mature individuals <250, a continuing decline is projected and all individuals belong to a single population. Recent survey found no animals.	1–4 but occasionally as many as 10, although broken up into several subgroups.	Range apparently expanded upstream when water levels were high and then contracted downstream when water levels were low.	Generally found in eddy countercurrents below meanders and channel convergences. Also, historically in lakes appended to the Yangtze mainstem.	Little information available but side-by-side swimming has been reported, with a group of four individuals splitting apart when disturbed but rejoining later.	Various fishes, including surface and bottom feeders. Only limitation appears to be size (<6.5 cm in width and <250 gm in weight).

*See IUCN (2001) for details on Red List categories. ⁺The term “aggregation” includes both social groups and clusters that form due to the common use of habitat. In practice, these are often difficult to tell apart on the basis of surfacing behavior and the context of “group size estimates” is often unclear in the literature.