About Species Location Results Taking Part Publications



Interactions with Man

Man’s relationship with dolphins living in fresh water is much more intimate than that with their marine cousins. The reason for this is obvious – the rivers and lakes in which dolphins live are surrounded by land, and these waterways are often heavily used by expanding human populations as a means of transportation and source of food. Conflicts inevitably arise between the two ‘user groups’ and, equally inevitably, it is the dolphins which usually lose. Fortunately for all, waterways in the Tefé region have not been heavily degraded by riverine industry or damming, but boat traffic is locally heavy and fishing is carried out at both subsistence and commercial levels, leading to accidental entrapment and death of dolphins.


One of the objectives of our study is to assess the level and type of Man-induced impact on dolphins in the area of Mamirauá, and in particular to discover if such impact is sustainable. We might expect two different types of impact to be occurring – direct and indirect. Direct impacts are obvious and would include drowning in fishing nets or dolphins being struck by boats. Indirect effects are less apparent but could potentially be as serious at the population level. They include dolphins being displaced by human activity such as engine noise, the negative effects of chemical pollution on health, and the reduction in food supply brought about by fisheries.

It is too early for us to be able to give definitive answers to all of these questions, but progress has been made in some areas:

Direct effects: Two of our marked botos are known to have died in fishing nets, and many other dolphins have been found dead near Tefé with evidence of net entanglement. One young dolphin was found to have a large fishing hook impaled in its head. Fishing appears to be responsible for the vast majority of Man-induced dolphin mortality; we have found no evidence of any other direct impacts. Work continues to quantify fishery-related mortality and to relate it to dolphin population size. Some extra mortality can probably be sustained by a healthy dolphin population, but even a one or two percent rise in the adult mortality rate could induce population decline in a long-lived, slow-reproducing species like the boto.

Indirect effects: Botos seem to be quite resilient to most human activity on rivers and lakes in this region; some of the highest densities of botos are to be found in Tefé harbour itself, where noise and pollution levels are at their highest. Within Mamirauá, there is evidence that botos avoid floating houses in narrow waterways, so care should be given in the choice of where to site new human habitation. Mamirauá is extremely fortunate in that it does not have dams nearby, because the catastrophic impact of dams on the riverine environment is probably the main cause of the loss of river dolphins in much of Asia.

With the above picture emerging, our research in this field is now focussing on fishery-related dolphin deaths and in particular the damage caused by large-mesh monofilament nets. In the Mamirauá area, as in the World at large, more cetacean (whale and dolphin) deaths are caused by accidental drowning in these nets than by all other factors put together, including deliberate hunting.

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Dolphin hunting

As long as anyone can remember, botos had always been protected from hunting in most of the Amazon basin by myths about them being reincarnated humans, or arising from the water at night and impregnating forest-dwelling women in their sleep. Shortly after the dawn of the new Millennium, we became aware that this protection was being eroded, and by 2004 it was apparent that large numbers of dolphins were being harpooned near the Mamirauá Reserve and further afield. In 2006 the hunting is widespread, common and unsustainable. The number of botos in and around the Reserve is declining rapidly, and hunting is likely to blame.

The evidence for the killing is now everywhere. Most distressingly, we have found harpooned, mutilated and dying calves, some of which have rope tied around their tail stocks. We have photographs of harpooned dolphins in wooden canoes, and we have heard many first-hand accounts of local villagers seeing dolphins being hunted and cut up for bait. Also, at the same time as the piracatinga fishery started locally, we began to lose marked botos with increasing regularity. Dolphins that we had seen month after month for many years suddenly disappeared, and we know from witnesses that marked botos have been killed by a fisherman at Nova Macedonia – just outside the Mamirauá Reserve boundary.



This is a young male boto known to us as 'Vani'. He was first seen swimming alone on the river, and noticed because of his strange dorsal fin and body scars. A week later we captured him, and were shocked to discover that his appearance was due to being mutilated with a knife. Furthermore, Vani had a rope tied tightly around his tail stock that was already cutting in to the skin, and would have resulted in his tail falling off due to lack of a blood supply. The rope must have been placed there to hold the dolphin until he was needed for bait - a practice that is apparently quite common now. Fortunately we were able to cut the rope, and Vani was seen a year later on the river again, growing well and fully recovered from his wounds.

The dolphins are being killed for one reason – to be used as bait for a particular species of catfish that feeds on dead animals. This fish, the piracatinga (Cetopsis coecutiens) is known locally as the ‘vulture of the water’ and is considered inedible in Brazil due to its ‘dirty’ habits. But in neighbouring Colombia it is much sought-after, to the extent that it has been almost wiped out in some areas. In consequence, traders regularly cross the border into Brazil, bringing with them the culture of killing dolphins (and caimans) as bait, and buy up thousands of tonnes of this abundant fish.

Projeto Boto is heavily involved in helping to stop this illegal and inhumane hunting of dolphins. We are working with the Mamirauá Institute and the Brazilian Environmental Protection Agency to identify the hunters, and have them caught red-handed. One difficulty is that the killing occurs at night in the flooded forest, so is difficult to detect. Another is that the men involved often threaten the lives of anyone who tries to stop them, and of course we cannot have our interns placed in danger. One thing in our favour is that the harpooning is carried out by a small minority of the local people, and most are vehemently opposed to it. We are optimistic that the hunt will eventually stop, but the question is how many dolphins will suffer a brutal death before this happens.

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Getting to know individual dolphins

The study is based on a suite of research techniques which have been developed and adapted specially for use in tropical flooded forest habitat (see da Silva and Martin, 2000). Central to this work is the ability to identify more than 200 botos individually. Daily observations from small boats are far more valuable if they relate to dolphins of known age, sex and reproductive status. Individual identifications permit us to follow the lives of dolphins over periods of years, recording important events such as the birth and weaning of a calf and finding out where each animal spends its time. It allows us to discover which other dolphins any particular individual likes to associate with, and whether it has seasonal behavioural characteristics such as movements or migration.

Identification is based on marks placed on the upper body or dorsal fin using a technique called freeze-branding which is commonly used for horses, cattle and even small mammals. This requires a cooled metal block to be placed on the skin for a short time, and in dolphins it leaves a light patch which may remain for the lifetime of the animal. While the marking takes place, we measure, weigh, photograph and sample the animal to provide information on many other topics. The dolphin is then released and quickly resumes its normal behaviour. One result of this marking work is the realisation that far more individuals use the Reserve than would be apparent if we could not tell the animals apart. The average number of botos within the study area is about 50, yet we have already marked more than 200 different animals and many are still unmarked. The implication is that many hundreds of botos use Mamirauá during the year, and a large proportion of these do so for relatively short periods of perhaps only a few days or weeks.

One well-known adult male (boto number 22 ‘HH’) is a good example of this. He was first marked in November 1994 and remained in the Reserve for 8 months before disappearing for almost 3 years. In June 1998 we encountered him again, playing with a rock in the heart of the Reserve. Then he moved away for another year, was seen again in July 2000, re-captured in October 2000 and has been seen in early 2001. This old male is one of many with an interesting history, but we remember him well because sometime before we met him for the first time he must have become entangled in fishing net and was subsequently badly injured. Now he has only one flipper, and is easily recognisable by the deep healed wound across his back.

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How many dolphins, and where do they go?

Another important element of Projeto Boto is to find out how many dolphins of both species occur within the Reserve and in the rivers and lakes nearby.


For this, we use two different types of boat - a small aluminium skiff for work in the narrow channels within the forest and a 20m river boat for the larger rivers and lakes. Each type of survey requires its own precise methodology for the collection of sightings data, and our results have been fascinating. We have found that at certain times of year the density of botos within rainforest channels is greater than has been recorded for any other type of dolphin in the World, and also that this species much prefers the edges of the main rivers (Solimões and Japurá) to the centre. That botos prefer lake systems like Mamirauá to the large rivers is demonstrated by the fact that there are fewer dolphins per kilometre of large river than per kilometre of forest channel, despite the much greater width of the river. Tucuxis also prefer the edge, especially where streams meet the river, but they are sometimes found in deeper water too.

The final technique employed in this study is that of radio telemetry, or radio-tracking as it is also known. We have fitted 56 botos with small radio transmitters (Fig. 8 - photo of a boto w/ VHF radio), of which 3 communicated with satellites to show the animal’s position and 53 were tracked locally using receivers mounted above the forest canopy on towers or trees (Fig. 9 - photo of tower or platform). Results showed that most botos in the central Brazilian Amazon make local movements of 10s of kilometres in response to cyclical water level changes, but do not make large-scale migrations. Daily movements vary from almost nothing (dolphins remaining in one small area) to more than 30 km. That said, some of our marked animals have been seen up to 100 km away from the site of capture, and one radio-tagged boto was seen by a team of German researchers in Peru, more than 1000 km from Mamirauá. We do not know the identity of this dolphin, but we do know that it subsequently returned to the Reserve because all the tagged animals were recorded by our research team after the observation in Peru.


This is one of the VHF receiving towers we erected above the canopy to pick up radio signals from tagged dolphins. The signals allowed us to follow animals night and day for many months at a time
without disturbing them, and revealed insights that could not have been gained with eyes alone.

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Births and deaths

An understanding of any animal population requires knowledge of the balance between births and deaths, i.e. the rates of reproduction and mortality. We have used the ability to identify individual dolphins to provide this information, and keep a detailed record of the observations of every animal so we can see, for example, when a female appears with a calf for the first time, or when an animal suddenly disappears and may have died. Although we are far from having all the answers we need, some things are already clear. Firstly, botos are long-lived creatures, and an old dolphin could be as much as 30 or even 40 years of age. Consistent with this, botos grow slowly, reach sexual maturity late, and produce calves at intervals of several years. In 2000 we had the excitement of seeing a female we had known as a calf produce her own first offspring, and it seems that the earliest botos can ‘start a family’ is at 6 or 7 years of age. Males probably have to wait a bit longer. The interval between calves is often 4-5 years, but older mothers may take much longer than this, and several of the females we know have not produced a single calf in at least 8 years. A typical mother is “JA”, who had a male calf “P Horizontal P” in 1993 and produced another male calf “+ Horizontal P” in 1999, some 6 years later. Youngsters begin to take solid food within their first year, but continue to take milk for at least 2 years and often much longer. We are finding that some immature botos still associate with their mothers after 7 or more years, but it is unlikely that they are still suckling at this advanced age.

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Social behaviour

Uniquely among aquatic mammals, and in common with Man, botos use objects in socio-sexual display. Here, an adult male dramatically smashes weed against the water surface; roses are in short supply in the rainforest!

One particularly interesting part of our work concerns the social behaviour of Mamirauá dolphins. Little is known of cetacean social structure generally, and knowledge of freshwater dolphins is particularly poor. Our observations have shown that, in contrast to most marine dolphins, botos do not form family groups and that the only close, stable association is between a mother and her calf. Evidence for this was found by looking at the membership of large aggregations of dolphins which often occur at favoured localities. The ability to identify individuals allowed us to see that botos frequently joined and left such groups, so group membership varied day-by-day and hour-by-hour. Such groups can be considered analogous to human shoppers at a supermarket; they each come looking for food, mothers bringing their youngsters. The dolphins may meet relatives and acquaintances while there, but leave independently or with one or two temporary friends, often to move on to the next supermarket up the road (or in this case the river!).

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© Projeto Boto 2006