One of the first accounts of piranhas in the wild was provided in 1800 by the German explorer, Alexander von Humbolt, who observed that people were sometimes attacked when they bathed
in rivers. Piranhas have sharp teeth and powerful jaws and can inflict a nasty bite if threatened. But are they really as vicious as they are portrayed, or could it be that piranhas form schools for the same reason as other fish that is to gain protection from predators? Our research project, based in the flooded forest of Amazonia in Brazil, set out to discover the truth about one of nature’s most charismatic animals.
The Mamirauá Sustainable Development Reserve, situated on the Solimões (or Amazon) River approximately 500km west of Manaus, and just south of the equator, was established to conserve the
flooded forest (várzea) ecosystem. It covers 1,124,000 ha, an area more than half the size of Wales, and is home to countless animal and plant species. There are two species of river dolphin and
around 500 species of fish including the piracucu, Arapaima gigas the world’s largest scaled freshwater fish in the reserve. To set this in context Britain has only about 40 native freshwater fish species. The red-bellied piranha, Pygocentrus nattereri, is one of 13 species of piranha found at Mamirauá.
As the name implies, these forests are inundated for part of the year the result of meltwater from the Andes and seasonal rainfall and water levels rise by around 12m. In April and May the waters invade the forests covering all but the highest ground, and fish swim amongst the branches of the trees. After this time water levels fall so that by October and November the main channels are well-defined and the forest becomes a patchwork of isolated lakes.
Like the other inhabitants of the flooded forest, the red-bellied piranha’s ecology and behaviour is tuned to the seasonal rise and fall of the water. Males and females pair up around the time the lakes and rivers reach their lowest level (typically October) and also again when there is a brief pause (the ‘repiquete’) during the rising of the waters in February and March. Unexpectedly for a fish with
such a fierce image, piranhas build simple nests and even show some parental care. A female can lay as many as 10,000 eggs though only a fraction of these will survive to adulthood. The newly-hatched fish soon take refuge in the tangle of vegetation formed by floating meadows and remain there until they reach about 5cm in length.
At this point at which they will be around 5-6 months of age – they form shoals with other piranhas and swim in the open water. Although fish and invertebrates (including insects) are important food items, nearly half of their diet can be plant material. Piranhas become sexually mature at about 16cm, which in this habitat occurs when they are around 24 months old. Mature individuals tend to be found in the vegetated areas at the water margins.
Piranha behaviour changes as they grow. Young fish prefer to hide in cover rather than associate with other piranhas. Once they begin to shoal the smaller individuals try to swim next to larger fish. Larger mature individuals, by contrast, place themselves in the centre of the shoal, where they will be safer if predators attack. Small fish are the first to forage; larger fish hold back. Our explanation for these size-related responses is that small fish choose the habitat and behaviour that allows them to grow most rapidly. However, once they reach maturity, survival and the production of offspring takes precedence over growth.
We tested our hypothesis that piranhas shoal to avoid predators by exposing groups of different sizes to a simulated attack by a model cormorant. As expected fish in larger shoals regained their normal behaviour more rapidly than those in small shoals. Moreover, piranhas are generally less stressed in large shoals than in small ones. Shoaling behaviour as a means of predator defence is an adaptive choice for piranhas in Mamirauá as they are constantly at risk of attack from a wider range of predators including large fish, such as the pirarucu, dolphins, caiman and aquatic birds. Shoals are largest during low water the time when predation risk is most intense.
The Mamirauá Institute for Sustainable Development (Instituto de Desenvolvimento Sustentável Mamirauá – IDSM) has been working for the conservation of the várzea for the last 16 years.
During this time the IDSM has promoted conservation in cooperation with local people, and is committed to improving the quality of life of the human population in the reserve as well as protecting its biodiversity. Around 25,000 people live in Mamirauá Reserve and depend on its natural resources for their livelihood. Although traditional fishing and harvesting methods do not threaten biodiversity, technological advances during the last 150 years resulted in a steep increase in the rate at which the reserve’s resources were being exploited.
This trend has been reversed following the introduction of sustainable methods by Mamirauá Institute. Fish populations have increased, markets are supplied on a regular basis and the people in the local villages are now better off. Projects supported by Mamirauá Institute include the sustainable harvesting of ornamental fish, and ecotourism.