The details of the spawning habitat of the pirarucu (Arapaima gigas) have been studied in a recent paper published in the most recent issue of the Journal of Fish Biology (there was limited information on the nesting habitat of the pirarucu prior to this) .
The study by Leandro Castello was conducted in the Mamirau Sustainable Development Reserve at the confluence of the Solimes and Japur Rivers.
The area is formed by vrzea, a type of floodplain that flanks the sediment-rich white water rivers of the Amazon and consisting of a complex mosaic of seasonally inundated rain forests, lakes and winding channels.
Surveys of pirarucu nesting sites in the vrzea were carried out over three years. The author found a total of 301 nests of pirarucu during the period surveyed.
The nests were holes in the ground with shapes like cooking pans with a mean diameter of 57 cm, with the depth of the water over the nest at the time of spawning about 1"15 m.
According to the author, he nests of giant pirarucu have at least three characteristics that may increase the survival rates of the young. First, the holes formed by the nests may provide the eggs with protection against predators and currents that may sweep the eggs away from the nests...Second, most nests (95%) were built in sand.
Digging the holes in sand probably requires less effort than digging in the compacted clay that dominates in the study area. Holes made of sand may increase the survival of the youngThird, the giant pirarucu make large clean circular areas surrounding the nest holes.
These clean circular areas probably assist keeping the eggs clean and free of exogenous material. They can improve visibility for the caring parent and, hence, reduce predation on the eggs. They can be related to the territorial behaviour of giant pirarucu
The author concludes here are at least five implications for the management and conservation of the giant pirarucu in the vrzea floodplains. First and most important, the fish is extremely susceptible to overfishing. The giant pirarucu are vulnerable to fishing not only when they breathe and reproduce, but also where they reproduce.
Their spawning grounds are used by fishers to move within and between their communities. Second, the giant pirarucu can be conserved more effectively through seasonal rather than spatial protection. Protecting only the lago habitats from fishing, as local fishers do, offers little protection for the reproduction of giant pirarucu.
The fish build most of the nests in ressaca, paran and lago habitats, which are widespread through the vrzea and are easily accessible to fishers. Also, the spawners and their offspring swim away from the spawning area once the offspring have the capacity.
Seasonal bans (established by Brazilian regulations) protect the reproduction of the giant pirarucu when they are both most vulnerable and most valuable for future stocks. Third, the riparian forests require protection. They are the spawning grounds of the giant pirarucu, are easily accessible and thus suffer anthropogenic impacts.
The riparian forests are protected by Brazilian law, but such regulations are rarely enforced by local communities. In the community in the study area, So Raimundo do Jarau, agricultural clearings are made behind the riparian forests. Fourth, the protection of the nesting habitat of giant pirarucu must be location-specific.
The density of their nests varies disproportionately between and within the habitats. Finally, it is uncertain whether counts of nests of giant pirarucu are reliable indicators of the abundance of their populations.
For more information, see the paper: Castello, L. (2008) Nesting habitat of Arapaima gigas (Schinz) in Amazonian floodplains. Journal of Fish Biology 72, pp. 1520"1528.