The Amazon is classified as a “whitewater” river, i.e., it is turbid, yellowish-brown, and very limited in transparency because of the large load of suspended sediment. The tributaries, channels, and lakes are classified as “blackwater,” i.e., they are more transparent due to a lack of suspended sediment but are of a dark color due to high concentrations of dissolved humic and fulvic acids.
The abundance of the Amazon River dolphins (Inia geoffrensis, also known as the boto, bufeo, or pink river dolphin) and the tucuxi (Sotalia fluviatilis) along with ca. 120 km of the Amazon River bordering Colombia, Brazil, and Peru,. Dolphins in riverine environments include some of the most endangered of the world’s cetaceans.
The principal threats are incidental mortality in fisheries, habitat loss and degradation, directed killing, death in construction, and collision with boats. The current population is decreasing day by day. The baiji (Lipotes vexillifer) is considered the most endangered cetacean. It is likely that this species will become extinct within the next decade.
The situation is no more promising for many of the other dolphins of the superfamily Platanistoidea. The bhulan (Platanista minor) and the baiji are listed as “endangered,” and the susu (P gangetica) and the Amazon river dolphin (also known in Spanish as bufeo colorado, and in the Portuguese language it is called boto) (Inia geoffrensis, hereafter referred to as Inia) as vulnerable.
The status of the tucuxi (also known in Spanish as bufeo negro) (Sotalia fluviatilis, hereafter referred to as Sotalia), a delphinid, is unknown. The biology and conservation of platanistoids hope for the improvement of techniques to estimate the population sizes of these dolphins and to determine trends in their abundance. The fresh-water cetacean populations are a challenge.
It is not easy to photograph species that spend most of their lives in dark and turbid waters, and, when at the surface, tend to be inconspicuous, shy, and unpredictable. Moreover, given that all cetacean populations that inhabit freshwater ecosystems live in the watersheds of developing countries, the funding and technology available to conduct research are limited.
The Inia and Sotalia are distributed in the Amazon and Orinoco basins, the largest river systems in the world. Most of what is known about these dolphins in the wild is from work conducted since the early 1980s near Manaus, Brazil. Pigmentation patterns on the dorsal ridge of Inia are the most prevalent mark type and are often used as one of the principal marks for photo-identification of the species.
The pigmentation patterns are likely the result of discoloration of the skin, parasites, or abrasions caused by rubbing against objects or by injuries from their conspecifics. Pigmentation patterns of many species remain unchanged across multiple years, such as in bottlenose whales.
Thus, pigmentation patterns on the flanks are considered supplementary. Despite this, they were one of the main features used to describe body coloration. Pigmentation patterns were first described for Inia in the same Amazon location in Trujillo, but it was not known whether these would be reliable marks across multiple years.
The supplementary mark types (scratches, scrapes, black marks, white marks, and pigmentation patterns on the flank, neck, and head) are, overall, not reliable over the long term but are useful for identifying individuals. Scrapes and scratches formed from tooth rakes of conspecifics, that can be single or parallel lines that may be produced by inanimate objects such as flooded vegetation.
These marks cannot be used to identify Inia dolphins for a period longer than one week and one month, respectively. Therefore, the scratches are highly prevalent in Inia dolphins; however, as has also been observed in other species, they have high gain and loss rates and, thus, limited persistence. For instance, scratches are like the linear marks and tooth rakes described in long-finned pilot whales (Globicephala melas), which are also not persistent in the population.
Further, the Znia is the most geographically prevalent of the platanistoid dolphins, being found in several parts of Bolivia, Colombia, Ecuador, Peru, Guyana, and Venezuela. Also, the Sotalia is normally found in the Amazon-Orinoco River system but also inhabits the sea and can be found in the Caribbean off the coast of Panama and along South America’s north and northeastern shores from Colombia to southern Brazil.
The observations of the abundance of Znia or Sotalia, were most incidental to other activities and limited to a few sightings in small areas. Only a few of these previous works produced quantitative estimates of density or abundance.
The water level of the Amazon River reaches a maximum in May, coinciding with the peaks of the rainy seasons in the Peruvian and Ecuadorean headwaters, and reaches a minimum during July-August. The Amazon River ranged between approximately 0.5 and 2 km in width, as maximum widths of the tributaries ranged between 60 and 200 m.
The Amazon River Dolphins or boto (Inia geoffrensis) is found in the Amazon and Orinoco river basins. It inhabits slow and fast-flowing rivers, side channels, lakes, and flooded forests and grasslands. The principal limits to its distribution seem to be impassable rapids and cold waters in small tributaries at the headwaters of the Amazon basin in the Andes.
Although the Amazon river dolphin is the most widespread freshwater dolphin in the world, its distribution is limited compared to that of most marine odontocetes, and it is therefore very likely to become a threatened species. For the management of Amazon river dolphins in the wild, information is needed about population size, age composition, sex ratio, seasonal distribution, diet, energy requirements relative to seasonal prey distribution and density, and competition with other animals and with fisheries.
This information could facilitate prey management to allow for a certain number of Amazon river dolphins in their distribution area. The Amazon river dolphin is a generalist feeder, whose diet is known to include over 50 fish species.
Fish types are taken by Amazon river dolphins (mainly sciaenids, cichlids, and characins in order of importance) and their proportions in the diet. However, very little is known about the energetic requirements of individual odontocetes of various ages and sizes. It is, at present, impossible to measure their energetic requirements in the wild. The Amazon river dolphin is usually seen in one or two pods but may also appear in pods that seldom contain more than eight individuals.
Further to the image-identification, the natural marks have been used in other species too, to assess an individual’s age. For instance, Risso’s dolphin (Grampus griseus) adults become lighter with age due to a loss of pigment. Thus, individuals with a moderate to a very high level of scarring are considered adults Marks of Inia could also be used to assess the age and sex of individuals.
The color, especially the pinkness of some adult males, could potentially be a proxy for Inia’s maturity. This could be investigated marks on individuals that are already sexed, aged, and artificially marked. However, these features would not appear to be useful for Inia dolphins in the Orinoco, where all individuals in our study were grey and without pink patches.
Read More – Greatest Amazon River is Home to Several Extremes / The Snapping Turtles (Chelydra serpentina) / Blue-ringed Octopus – World’s Most Venomous Marine Animals / The Eastern Bluebird (Sialia sialis)
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