Project Amazonia: Characterization - Biotic - Fish

Introduction

The Amazon River basin has a large number of fish and other aquatic life forms. The immense diversity of species in the Amazon River basin can be demonstrated by observing the number of frogs in the basin. For example, at a single site in Amazon rainforest in Santa Cecilia, 81 species of frogs have been recorded¹. For comparison, there is approximately the same number of frog species in the entire United States¹. Furthermore, every year, about 35 species of fish are discovered and named in the Amazon basin¹. Many new species have even been discovered unintentionally as a side effect of studies on known species. This diversity of the fish population of the Amazon River basin is due to three factors:

1. The size of the Amazon River basin, which enables many species of fish to flourish.
2. The location of the Amazon River basin near the equator, which is conducive to fish growth. Because the basin absorbs so much energy from the sun, a large aquatic flora population exists that is able to maintain a large fish population. In addition, the position of the rainforest near the equator results in a fairly constant amount of energy absorption from the sun. Thus, there is little seasonal variation in the length of a day and daily temperature.
3. The low extinction rates of the Amazon Basin rainforest. Additionally, since the extinction rate is lower than the rate at which new species are introduced to the basin, the net number of species is increasing¹.

Miscellaneous

Fish from the Amazon are a popular export to Asian countries, especially Japan. They are also a key element in the diet of people living along the Amazon River. Because of the high protein content of their diet, inhabitants along the river are much less likely to be malnourished than rural people in regions without fisheries.

As the Amazon River rises, fish move through river channels into the floodplains. Some fish, such as the tambaqui, are specially adapted to the flooded forest environment. A keen sense of smell leads the tambaqui to fruit which has fallen from the tree tops to the water. The tambaqui are genetically adapted, with powerful jaws and teeth that enable them to consume fruit. Not only do they gain and store fat to last them through the dry season, but in the process they propagate the tree species by providing a dispersing mechanism for the seeds.

Over the past 15 years, there has been a steady decline in the size of many fish. This, together with increasing agriculture, raises concern about over-fishing and habitat depletion, especially in the lower Amazon where extensive agricultural production already exists and continues to expand².

Affects on fish populations by water management

The reproductive success of both native and non-native river fish populations is related to the water flow of the previous year. Therefore, water flow manipulations can be a powerful tool for managing fish populations. Such manipulation can be accomplished using dams, flow diversions, and river channelization. Conversely, the implementation of artificial flow control means can have an adverse effect on fish populations³.

Damned rivers can be divided into four main segments:

1. The upstream segment, which is largely unaffected by the dam.
2. The segment immediately behind the dam.
3. The segment immediately downstream of the dam; this segment of the river is most affected by the dam. In this section, native fish populations are the most severely affected, to the point that they may be dominated by non-native species.
4. The segment downstream of the dam. With increasing distance from the dam, and with the influx of other rivers and streams, the effect of the dam in this segment is decreasingly severe. Correspondingly, native fish populations are more successful with increasing distance from the dam³.

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