The Tahuayo River is a "black" water river. The water is a dark tea color due to the presence of acids in plant material which have leached into the river. However, the chemistry of the Tahuayo often resembles that of the region's sediment-rich, white water rivers. This is partly because the Tahuayo is located within the huge floodplain of the Amazon River, allowing water from the Amazon into the Tahuayo. This combination of both black and white water characteristics makes the Tahuayo a diverse and productive aquatic ecosystem.

The upper Tahuayo River system includes many lakes and swamps, along with seasonally and permanently flooded forests.
These flooded forests help sustain populations of many of the most important fishes eaten by the people living in the rain forest and in the city of Iquitos, an urban center of a half-million people. Studies of ribereño diets in northeastern Peru show that fish constitute 60% to 70% of the protein they consume. Some of the most important fish species found in the upper Tahuayo include paiche(Arapaima gigas), boquichico (Prochilodus spp.), sabalo, arohuana (Osteoglossum bicirrhosum), carachama (Hypoptomus spp.), tucunaré (Cichla ocellaris), gamitana (Colossoma macropomum), paco (Piaractus brachypomum), acarahuazu (Astronotus ocellatus)and burjúrqui (Cichlasoma bimaculatum). This area is also home to the pink river dolphin (Inia geoffrensis), gray dolphin (Sotalia fluviatilis), Amazonian manatee (Trichechus inungui), caymans, turtles, stingrays and electric eels.

The paiche , reaching two meters in length and 125 kilos, is one of the most intensively hunted Amazonian fish. Large specimens are increasingly uncommon-both researchers and fishermen have noticed a decline in the average size. To encourage conservation, the Peruvian government made it law that no females with young could be taken, and in the Pacaya-Samiria National Reserve, no paiche under 1.6 meters in length. Paiche are air-breathers, and therefore surface periodically. This makes them easy to find, catch in nets, or harpoon. It is difficult to know the sex or size of the paiche until it is killed. While the intent of the law is good, it is impractical. Seasonal fishing bans on paiche also exist. But in Peru, it is nearly impossible to enforce many conservation laws. There are few effective forest police, or economic alternatives for people involved in the fishing industry.

Jim Penn and Greg Neise