Shades of Green: Earth's Forests -- Fish

Fish in a forest? Well, maybe not right in the forest -- not usually anyway -- but in the rivers and streams that run through forests. Rivers, ponds, and bogs are all integral parts of natural forests, and there are often complex relationships between the animals in the water and the animals in the trees.

Bears try to catch spawning salmon in temperate forest rivers. Photograph courtesy Philip Greenspun.

The areas around forest streams and ponds are called "riparian zones", and include the water source plus the wetlands or moist forest in the surrounding area. These places are extremely diverse habitats and home to many animals and plants. Fish live in the more open water, amphibians lay their eggs in shallow ponds or marshes, and the variety of insects that live near the water attract foraging reptiles and birds.

Salmon are some of the more common fish inhabiting forest streams in certain areas. Every year, the population of mature salmon migrates from the ocean into rivers in northern temperate zones. Here they lay their eggs when they find a suitable location with clear, silt-free water and the right sized stones on the riverbed. The water quality of streams, and salmon habitat, is linked to the health of the forest. Trees' roots keep river banks from eroding and prevent the water from being clogged with silt. Fallen trees which occasionally become lodged in streams make the habitat more diverse by creating pools which are ideal for insects and water plants: fish food.

Right: Salmon are capable of swimming up streams with very rough water. Below: Ideal spawning streams have very clear water and pebbly bottoms. Photos courtesy Al Walters.

Fish themselves become food for all sorts of animals in the forest. Bears are especially well known for attempting to catch the salmon which are trying to swim upstream to lay their eggs. Spawning is the last part of the salmon's life cycle, and soon afterwards they die naturally. As their bodies are gradually washed downstream, they become food for many other species, including birds such as ospreys, eagles, and ravens.

In the Amazon rainforest, the fish don't stay in the rivers. Or rather, the rivers don't stay in the rivers. During huge seasonal floods, rivers spill over their banks and leave some areas of forest under 10 meters of water. Over 100,000 square kilometers of the Amazon basin can be flooded at once. Nutrients in the river sediments are carried out into the forest. And so are the fish, which swim through the trees, feeding on the fruits, seeds, leaves, flowers and insects that are suddenly available to them during the flood.
About 200 species of fish in the Amazon are fruit- and seed-eaters, far more than in any other forest. Even piranhas have been known to feed on seeds! The flooded forest is such a rich source of food that many fish species feed heavily during the floods and build up reserves of fat to last them between the seasonal flood cycles. Sometimes they don't have to wait long--in some areas of the Amazon, the forest stays flooded for as long as ten months every year.
Many Amazonian fruits and nuts float on the surface of the water, which allows fish to easily locate them and gobble them up as they swim through the trees. Fish are actually an important seed disperser in the flooded forest. Some seeds from palm trees are encased in thick shells that most animals can't open. Some fish, such as the tambaqui (Colossoma macropomum), have evolved large, molar-like teeth that are specialized for grinding palm nuts and other hard seeds.

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In the Amazon rainforest, the fish don't stay in the rivers. Or rather, the rivers don't stay in the rivers. During huge seasonal floods, rivers spill over their banks and leave some areas of forest under 10 meters of water.	Over 100,000 square kilometers of the Amazon basin can be flooded at once. Nutrients in the river sediments are carried out into the forest. And so are the fish, which swim through the trees, feeding on the fruits, seeds, leaves, flowers and insects that are suddenly available to them during the flood.
About 200 species of fish in the Amazon are fruit- and seed-eaters, far more than in any other forest. Even piranhas have been known to feed on seeds!	The flooded forest is such a rich source of food that many fish species feed heavily during the floods and build up reserves of fat to last them between the seasonal flood cycles. Sometimes they don't have to wait long--in some areas of the Amazon, the forest stays flooded for as long as ten months every year.
Many Amazonian fruits and nuts float on the surface of the water, which allows fish to easily locate them and gobble them up as they swim through the trees. Fish are actually an important seed disperser in the flooded forest.	Some seeds from palm trees are encased in thick shells that most animals can't open. Some fish, such as the tambaqui (Colossoma macropomum), have evolved large, molar-like teeth that are specialized for grinding palm nuts and other hard seeds.