The Rainy Season Transforms the Freshwater Fish Habitat

The tropics, home to so many fish species, have distinct and predictable seasons. During the rainy season, the aquatic habitat is transformed by huge volumes of water. For around four months, the Amazon region experiences torrential rain.This, combined with the accompanying meltwater from the Andes, causes the river to rise by anything up to 40 feet (12m) and to flood beyond its banks as much as 10 miles (16 km) from the main channel. It covers thousands of square miles of floodplains, which may remain submerged for several months. For fish, the annual flood brings rich pickings in terms of food and they rapidly build up fat reserves and grow. For this reason, the species that live in rivers in the tropics very often time their breeding to coincide with the beginning of the rainy season, thus ensuring that their offspring hatch and prosper among the flooded forests.

How do fish respond to the annual rainy season?

The end of the dry season is marked in many regions of South America, Asia, and Africa by an upstream migration of adult fish. For example, large adult clown loaches (Chromobotia macracanthus) move up the channels of the rivers where they live in Sumatra to spawn. The young hatch at the beginning of the rains and grow rapidly on the flood plains. In the early weeks of their life, they lack the power to swim against the current and drift gradually downstream, eventually reaching the areas of the river from where their parents set out a few months before. The initial adult migration therefore counteracts the potential problem of larvae drifting out of the river entirely, and it is also suggested that moving upstream to spawn reduces the risk of predation to the eggs and young. Some migrations are thought to be quite substantial, with larger species moving miles upstream. Even the smaller fish travel considerable distances.

How do fish match their life cycles to the changing seasons?

The floodplains can remain underwater for months, but as the wet season gives way to the dry, water levels start to drop. Gradually the river returns to its original course, leaving thousands of isolated sink holes and ponds in its wake. These can be extremely tough environments, low in oxygen and fraught with danger as the river's predators become constrained to small pools. In the ditches of Southeast Asia, the male croaking gourami builds a bubblenest when the water levels are at their lowest and temperatures well above 79°F (26°C). If he has timed it right, the rains will arrive with their bounty by the time the young have developed sufficiently to feed on larger food items. Even back in the main river channels, conditions can be tough and food scarce during the dry season. However, most of the returning fish have built up considerable reserves during the floods to see them through this period. Those surviving fry that hatched at the beginning of the wet season are now adult, awaiting their turn to spawn with the coming of the next rains.

How can I persuade fish from these habitats to come into breeding condition?

As so many of the fish kept in aquariums, including several of the scatter-spawning tetras and barbs, spawn during the rainy season, it is often well worthwhile attempting to simulate their natural conditions, especially when trying to persuade difficult species to breed. By the end of the dry season, water levels are low and the amount of dissolved organic matter in the water means that oxygen levels are also low. The advent of the rains brings a change in water chemistry as the soft rainwater dilutes the stale river water. Water levels and oxygen levels rise and the colder rainwater causes the temperature to drop. Finally, food ranging from infusorians to the fry of other species is suddenly in abundance.