Inia geoffrensis (de
English: Amazon river dolphin, Boto, Inia
French: Dauphin de l'Amazone
Inia geoffrensis © Würtz-Artescienza (see "links")
The boto is the largest of the river dolphins. Males reach a maximum
body length of 255 cm and a mass of 185 kg, the smaller females
reach 215 cm and 150 kg. The body is corpulent and heavy but extremely
flexible: the head can be moved in all directions. The flukes are
broad and triangular, the dorsal fin is low, keel-shaped long, extending
from the midbody to the caudal peduncle. The flippers are large,
broad and paddle-like. Whereas swimming speed is not very high,
botos are capable of manoeuvring very well between trees in the
flooded forest. The rostrum and mandible are long and robust and
the melon is small and flaccid. Its shape can be muscularly controlled.
Whereas young animals are dark grey, older botos are completely
pink or blotched pink and may have a darker back (da Silva, 2002).
The boto has a very wide distribution and can be found almost everywhere
it can physically reach without venturing into marine waters (da
Silva, 2002). There are three morphologically distinguishable populations,
which are best recognised at the subspecific level (Rice, 1998):
I. g. humboldtiana (Pilleri and Gihr, 1978): ranges in the
Orinoco River system, including the Apure and Meta rivers, upstream
as far as the rapids at Puerto Ayacucho (Rice, 1998). Contact between
this race and the next is restricted, at least during low water,
by waterfalls on the upper Rio Negro, by the rapids on the Orinoco
river between Samariapo and Puerto Ayacucho, and by the Casiquiare
Canal itself (da Silva and Martin, 2000).
General distribution of Inia geoffrensis
in the Amazon-Orinoco river systems (Reeves et al. 2008;
© IUCN; click
here for large map).
I. g. geoffrensis: found throughout most of the Amazon River
and its tributary rivers (below an elevation of about 100 m), including
the Tocantins, the Araguaia, the lower Xingu up to the rapids at
Altamira, the lower Tapajós up to the rapids at Sao Luis,
the Madeira as far as the rapids at Porto Velho, the Purús,
the Juruá, the Ica, the Japura, the Branco, and up the Negro
through the Canal Casiquiare into the headwaters of the Orinoco,
from whence in ranges as far downstream as San Fernando de Atabapo,
including its tributary the Guaviare (Rice, 1998).
I. g. boliviensis (d'Orbigny, 1834): occurs in the upper
Rio Madeira drainage in Bolivia, where it is confined to the Rio
Mamoré and its main branch the Rio Iténez (= Rio Guaporé),
including lower reaches of their larger tributaries (at an elevation
of 100-300m). There are no credible reports from the Rio Beni or
any of its tributaries above Riberalta. This subspecies appears
to be isolated from the previous one by 400km of rapids from Porto
Velho on the Rio Madeira in Brazil upstream to Riberalta on the
Rio Beni in Bolivia. However, Inias of undetermined subspecies
live in the Rio Abuna and its tributary the Rio Negro, which enters
the Madeira/Beni an the border between Brazil and Bolivia (Rice,
1998 and references therein). Botos in the Beni system may, in fact,
constitute a separate species (da Silva 1994).
Although, at present, a single species is recognised, Banguera-Hinestroza
et al. (2002) compared samples from specimens in the Orinoco basin
(four rivers), the Putumayo River, a tributary of the Colombian
Amazon and the Mamoré, and the Tijamuchy and Ipurupuru rivers
in the Bolivian Amazon. From mitochondrial DNA and mitochondrial
cytochrome b gene analysis, a subdivision of the Inia genus
was proposed into at least two evolutionarily signifcant units:
one connected to the Bolivian river basin and the other widely distributed
across the Amazon and Orinoco basins. However, the IWC sub-committee
(IWC, 2000) and more recently, da Silva (2009) recognised that this
was still an unresolved issue.
3. Population size
The boto is the most common river dolphin and population densities
appear to be relatively high throughout much of its range (IWC,
2000). Its current distribution and abundance apparently do not
differ from the past, although relative abundance and density are
highly seasonal and appear to vary among rivers (da Silva, 2002,
2009). Overall population size, however, is unknown and precise
data on trends are insufficient for any of the three subspecies.
Differences in density exist between different river systems. Pilleri
and Gihr (1977) report an average of one dolphin per 4 km over 130
km on Rio Ichilo, one per 0.9 km on Rio Ipurupuru, and one per 1.0
km on Rio Ibare. On the Amazon River bordering Colombia, Peru, and
Brazil Vidal et al. (1997) found that Inia density was highest in
tributaries with 4.8 dolphin/km, followed by areas around islands
2.7 dolphin/km and along main banks 2.0 dolphin/km.
Aliaga-Rossel et al. (2006) investigated encounter-rates in the
Mamore River of the Bolivian Amazon and four of its tributaries
during the low water season. Inia encounter rates were in the range
1.6-5.8 km-1 and are the highest recorded anywhere in its broad
geographic range. Mean group size was greatest in the Tijamuchi
River (3.3 +- 2.96) and smallest in theYacuma River (1.8 +- 0.75).
In absolute numbers, Aliaga-Rossel (2002) counted 208 bufeos in
the Tijamuchi River of Bolivia. Surveys in a 1,200 km section of
the Amazon River between Manaus and Santo Antonio de Ica yielded
estimates averaging 332 dolphins (Best and da Silva, 1989). Martin
and da Silva (2004a) found the boto population of the central Amazon,
to be structured on the basis of floodplain lake systems, with extensive
animal movement between systems. They estimate that 13,000 botos
occur in the 11,240 km² Mamiraua Sustainable Development Reserve,
which covers an estimated 11%- 18% of varzea habitat in Brazil.
4. Biology and Behaviour
Habitat: The Amazon river dolphin is an exclusively fresh-water
species. In the Orinoco and Amazon basins, the species is found
in a variety of riverine habitat types, including rivers, small
channels and lakes, excepting the estuaries and strong rapids and
waterfalls. Concentrations occur mainly at the mouth of rivers,
below rapids and smaller channels running parallel to the main river.
During the high-water season dolphins may utilize both the flooded
forest and grasslands, throughout most of Amazon River and its tributary
rivers (Reyes, 1991).
Martin and daSilva (2004b) investigated habitat use in and around
the Mamiraua Reserve, Brazil. Largely forested with numerous channels
and lakes, Mamiraua comprises a variety of seasonal floodplain habitats
known collectively as varzea. The annual cycle of flooding in this
region (amplitude 11-15 m) dominates all life. Profound seasonal
differences in dolphin density between habitats were consistent
with known fish movements, in turn dictated by changes in water
level and dissolved oxygen. An exodus of botos from floodplains
to river at low water prevents dolphins being trapped in areas that
become entirely dry. Densities of botos in floodplain channels were
seasonally higher (up to 18 per km²) than reported for any
cetacean worldwide. Adults were largely segregated by sex except
at low water. Some 80% of botos occurring on rivers were within
150 m of the margins. The reliance of adult females and calves on
varzea in a region with exceptional dolphin densities demonstrates
the importance of floodplain habitats for the boto, and may be the
key determinant of this species' distribution.
In Peru's Pacaya-Samiria National Reserve McGuire (2002) found
that boto encounter rates were highest in confluences, intermediate
in lakes, and lowest in rivers. Encounter rates for botos in rivers
and lakes did not differ among seasons. During low water, boto persisted
longer in the confluences throughout the sampling day, and occurred
in higher densities than in any other season; the reverse pattern
was observed during high water.
Schooling: Although rarely seen in groups of four or more,
Inia is most often observed as a solitary individual. Loose
aggregations have been observed at feeding areas. Most groups of
two are apparently mothers and calves. In the survey done by Magnusson
et al. (1980), from Manaus to Tefé 81% of the sightings were
of a single individual and only 3% of sightings were of four or
more animals. Of 407 sightings made from Manaus to Tabatinga, 69%
were of one animal and 3% were of four or more. In surveys from
Leticia, 58% of sightings were of one animal while 14% were of four
or more (Best and da Silva, 1989). In the Tijamuchi River, Beni,
Bolivia 42% of observations were of solitary dolphins, 32% were
of pairs, and maximum group size was 19. Calves were seen most often
during falling and low waters (Aliaga-Rossel , 2002). In Peru's
Pacaya-Samiria National Reserve, botos were seen most often as single
animals and seasonal differences in group size were not detected
Although more often a solitary feeder, Inia sometimes form
loose groups that fish in a coordinated fashion to herd and attack
shoals. These groups may also include the tucuxi (Sotalia
fluviatilis) and the giant otter (Pteronura brasiliensis).
Similar group relationships can develop with man in his fishing
canoe. Fishermen, on their part, use dolphins to localise shoals
of fish and the dolphins use the human fishing operation as a means
of disrupting the shoal to their advantage (Best and da Silva, 1989).
Food: Inias may frequent shallow waters primarily
for feeding (Best and da Silva, l989). About 50 species of fish
have been reported as the food of Amazon river dolphins in the central
Amazon. Sciaenids, cichlids and characins are the preferred prey;
some of them are of commercial value (Best and da Silva, l989).
Reproduction: Reproduction of Amazon River Dolphins, based
on observations of live dolphins from the Orinoco, Amazon, and Mamore
river basins (in Venezuela, Peru and Bolivia, respectively) indicate
that reproduction in Inia often occurs year-round, with seasonal
peaks varying according to geographic location. Inia neonates in
Peru and Bolivia were seen in all seasons, and were observed most
often in falling water (season was defined by relative water level).
Conversely, neonates in Venezuela were seen at the end of low water
and in rising water, yet were never observed during falling water.
Inia mating behavior in Peru was observed in all seasons, while
mating was observed only during falling and low water in Bolivia
(McGuire and Aliaga-Rossel 2007). The authors suggest variation
in reproductive seasonality, with year-round reproduction in some
areas. Seasonality of peaks in births varies according to study
area, and may be more closely associated with local environmental
and prey conditions than with taxonomic relatedness, relative seasonal
differences in water levels, or broad geographic distribution. Gestation
lasts 10-11 months (Best and da Silva, 1989).
McGuire and Henningsen (2007) used photo-identification to recognize
Inia from scars, cuts, nicks, pigmentation patterns, and abnormal
beaks in Peru's Pacaya-Samiria Reserve. 72 Inia were identified,
and 25 were resighted between 1991 and 2000. Sighting histories
ranged from 1 d to 7.6 y. Maximum range of movement was 220 km,
with a mean range of 60.8 km. The greatest rates of movement observed
were 120 km/d for Inia , with a mean rate of movement of
14.5 km/d. Identified dolphins were always observed within the same
tributary system. 90% of all Inia resighted in one river
system were seen in the same lake at least once, and 33% of dolphins
resighted in the lake were never seen outside of the lake. This
confirms earlier work by Da Silva and Martin (2000) in the central
Amazon of Brazil, where most animals moved only a few tens of kilometres
between high and low water seasons. Of more than 160 marked animals,
however three had been resighted more than 100 km from the tag site.
Seasonal migrations seem to represent slight extensions of more
or less stable home ranges. Some of these migrations, mostly during
flood seasons, are known to cross international boundaries: in the
Casiquiare Canal and Upper Rio Negro (Venezuela, Colombia and Brazil);
in the Rio Madeira-Guapore system (Brazil and Bolivia); in the Takatu
River (Brazil and Guyana) and at Leticia (Peru, Colombia and Brazil)
(Best and da Silva, 1989).
Direct Catch: Parts of stranded or incidentally caught dolphins
may be sold as love charms. In the Beni district, Bolivia, hunting
with rifles and nets was previously reported (Pilleri, 1969; Pilleri
and Gihr, 1977). Da Silva and Best (1996) conducted interviews with
fishermen in boats, in the fishmarket and in the shops supposedly
selling dolphin products in an attempt to quantify the overall incidental
kill attributed to commercial fisheries operations. The results
showed that in the Central Amazon, dolphin catches are incidental
and only a very small number of these carcasses are used for commercial
purposes. In the Colombian Amazon some fishermen have killed Inia
(including harpooning, shooting and deliberate poisoning) to deter
gear interactions. In the Orinoco system and Peruvian Amazon there
are also reports of some deliberate killings apparently due to interactions
with fisheries (IWC, 2000).
Incidental catches: The main causes of man-made mortality
of dolphins in Bolivia were identified as collisions with outboard
motors and entanglement in fishing nets (Aliaga-Rossel, 2002). In
Peru's Pacaya-Samiria National Reserve major threats are related
to human fishing activity, and include entanglement in fishing gear
and possibly poisoning to reduce net damage and predation on fish.
Potential threats include boat strikes, oil spills, water and noise
pollution, and overfishing of prey (McGuire, 2002).
By-catch is also reported in the Amazon and Orinoco Rivers, but
there are no estimates of the magnitude of these catches. However,
fish landings have increased several fold in some areas, representing
an increase in fishing effort. A major reason for this increase
was the introduction of nylon gillnets in the 1960s. Lampara seine
nets, fixed and drift gillnets are responsible for the majority
of dolphin deaths. A yet unknown number of dolphins are killed by
explosions during illegal fishing operations (Best and da Silva,
Martin et al. (2004) found that Inia's most preferred habitat type
was where a channel of sediment-rich white water meets one carrying
acidic black water; the resultant mixing producing particularly
productive, and obviously attractive, conditions for dolphins. These
areas are also known to be favored for gill net deployment by local
fishermen, and may explain why entanglement is apparently a common
cause of mortality.
In general, incidental mortalities of this species appear to be
seasonal and patchily distributed throughout the range. There are
no estimates of total incidental mortality, and all accounts are
anecdotal. The IWC Scientific Committee (2000) agreed that, in the
absence of any information on total numbers taken or total population
size, it was impossible to assess the significance of this source
of mortality. The sub-committee recognised that it would be extremely
difficult to obtain reliable estimates of incidental mortality because
of the small-scale nature of the fisheries involved. A more sensible
approach to the issue might be, in the first instance, to try to
determine the scale of incidental mortalities in different types
of fishing gear in different regions (IWC, 2000).
Deliberate killing: Amazon river dolphins have learned to
take advantage of some fishing activities. They may tear fish from
nets (in particular from lampara seine nets) causing considerable
loss of fish catch and damage to fishing gear. Also, these dolphins
congregate to eat fish stunned by dynamite used illegally by some
fishermen. In both instances, fishermen may decide to kill the dolphins.
Best and da Silva (1989) mention that at least two reports of harpooned
dolphins exist, probably due to this interference with fishing operations.
Overfishing: According to da Silva and Best (1996) the use
of nylon gill nets in the Amazon fishery is widely spread throughout
the whole region, and with increasing fisheries pressure the potential
for dolphin/fisheries interactions is much greater. Competition
between man and dolphin for commercial fish, however, is still minimal
in the Central Amazon. Dietary analysis has shown that only 43%
of 53 identified prey species are of commercial value and that the
dolphins generally prey on size-classes of fish below those of commercial
Habitat degradation: Human populations are expanding rapidly
in many areas of the boto´s range, especially in Colombia
and Brazil. Such population increases result in increased agriculture,
deforestation, cattle ranching and the establishment of plantations
(IWC, 2000). Deforestation in flood plains for agriculture and the
timber industry affects the hydrological cycle and the riverine
ecosystem as a whole. One of the major effects of deforestation
is the reduction of fish productivity, and hence reduction of food
supply for river dolphins and other aquatic animals. Hydroelectric
development is at present not a great threat, but several dams are
projected for the next few years in the river systems of both Brazil
and Venezuela (Best and da Silva, 1989, IWC, 2000). Dams may prevent
migrations, breaking the populations into very small units with
insufficient genetic variability, and reduce food supply (Ralls,
1989, in Reyes, 1991). Strandings in the Formosa River have been
reported as resulting from changes in the water level produced by
the deviation of waters for irrigation (Best and da Silva, 1989).
Furthermore, the water areas behind dams provide an impoverished
environment for Inia, with lower oxygen concentrations, lower
pH levels and fewer fish (IWC, 2000).
Recently (IWC, 2000) oil exploration and production were also identified
as a potential threat to Inia. In Colombia there had been
many oil spills in recent years as a result of the ongoing guerrilla
war in the upland regions. Some of these had been very extensive,
and represented a potential threat that has not yet been quantified.
Anecdotal accounts of a decline in numbers were reported in Ecuador.
These reported declines were linked to oil spills in the region,
though the subcommittee noted that fluctuations in numbers would
also be expected due to water level fluctuations.
Pollution: According to Reyes (1991), large quantities of
pesticides are being used increasingly in agriculture in the Amazon
and Orinoco Basins. Pollution by heavy metals in the Amazon comes
from gold mining and associated indiscriminate use of mercury. Effluents
from pulp mills are also a potential source of pollution (Best and
da Silva, 1989). However, Rosas and Lethi (1996) report that the
mercury concentration (176 ng/ml) found in the milk of a lactating
Inia caught in the Amazon River near Manaus, Brazil was very
close to the minimum level of methylmercury toxicity for non-pregnant
human adults. This suggests that at least in this part of the river
system, contamination is low..
Range states (Reeves et al. 2008):
Bolivia; Brazil; Colombia; Ecuador; French Guiana; Peru; Venezuela
Inia geoffrensis is categorised as "Data defficient"
by the IUCN (Reeves et al. 2008). The species was previously listed
as "Vulnerable" but is now considered Data Deficient due
to the limited amount of current information available on threats,
ecology, and population numbers and trends. However, the IWC (2008)
has expressed concern over the capture of boto for bait in the central
Brazilian Amazonas, which is considered to be an emergent, but already
large-scale, problem (Reeves et al. 2008).
The species is listed in appendix II of CITES and it is also listed
in Appendix II of CMS.
According to an evaluation by the Scientific Committee of the IWC
(2000), populations of the boto appear to be large and, at present,
there is little or no evidence of any decrease in numbers or range.
The sub-committee noted the increasing human pressures on the region,
and recognised that future anthropogenic effects are to be expected,
with declines in range and population fragmentation the most likely
consequences. The Asian river dolphins provide a model for the possible
effects of increased human populations and dam construction. The
subcommittee therefore agreed that there is a need for appropriate
monitoring schemes and formulated its recommendations accordingly.
The IWC sub-committee (IWC, 2000) recommended:
- that work on stock structure of Inia be conducted and existing
studies should be brought to publication as soon as possible,
- that a registry of the distribution of this species should be
established, recording in which waterways botos are present, and
that the locations of all existing and proposed dams and other large-scale
engineering works should be included. Information on other potential
threats, such as the scale of fishing operations and the locations
of oil pipelines might also usefully be included where practicable,
- that for each population, research should be directed towards
detecting trends in abundance or any diminution of range, and identifying
causes of any declines. Trends in abundance should be documented
by making repeatable, statistically rigorous estimates of density
in a range of regions and habitats.
The most significant anthropogenic impact on this species at present
appears to be mortalities in fishing operations. These are either
entirely incidental (entanglement) or to a greater or lesser extent
deliberate, as fishermen are reportedly poisoning botos with baited
fish, to limit net depredation, and also shooting and otherwise
killing animals found in or near to nets. The sub-committee recommends
that information should be collected to allow evaluation of the
relative levels of mortality, both indirect and direct, associated
with different fishing methods (IWC, 2000).
The management of renewable natural resources in developing countries
has been hampered by a mix of socioeconomic and political difficulties
that in turn have resulted in insufficient scientific knowledge,
limited environmental awareness and education, and limited commitment
to conservation (Vidal, 1993). Aquatic mammals provide good examples.
Because many aquatic mammal populations are shared by several Latin
American countries, international co-operation is critical to ensuring
their long-term conservation.
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© Illustrations by Maurizio Würtz, Artescienza.
© Maps by IUCN.