What Do Fish Do When a River Floods?

Floods are a natural event in river ecosystems, often observed in regions with significant rainfall during the rainy season. Although one might think that such an environmental change would be disastrous for fish, many species have evolved adaption strategies to manage and even thrive during these events.

Adaptation to Flooding

When a river floods, the body of water increases in volume. The immediate reaction of many fish species is to seek calmer areas within the river or nearby tributaries where they can find refuge. This behavior helps them avoid the turbulent and fast-moving waters that could otherwise pose a risk to their survival.

Seeking Refuge

Fish often move towards the banks to find seam pockets and eddies that provide calmer water and lower velocities. Friction caused by faster-moving surface water slows down near the stream bottom, allowing fish to rest and conserve energy. These areas offer temporary safety and are crucial during high water events.

Migration and Adaptation

Some fish species are particularly adept at navigating flood scenarios. For example, in the Amazon, where annual flooding can reach up to 30 feet, fish move towards the banks where they can find seam pockets and eddies, providing calmer water and lower velocities. Other fish, like the African Lungfish, have the ability to survive periods of drought. During droughts, when water levels drop, these lungfish can ball up and become encased in mud, surviving without food or water for several years until the rains return. They can then re-emerge and immediately begin their feeding and life cycles again.

While most fish can survive and even thrive during floods, some may face challenges. When water recedes, fish that have been swept into smaller tributaries or minor streams that may dry up are at risk. Additionally, fish may be forced to move outside the banks of the river where they might encounter smoother waters. If a fish is unable to find its way back into the main channel during receding waters, it may be trapped and die on dry land.

Conclusion

Despite the significant changes brought about by flooding, fish are equipped with various survival mechanisms. These adaptations allow them to navigate and even benefit from the turbulent waters. Understanding fish behavior during floods is crucial for conservation efforts and the management of river ecosystems.