The correct answer is option 1: Zone of degradation. When the absence of fish is observed along a river, it indicates that the river is facing degradation. This means that the quality of the river water has declined to a level where it is no longer suitable for fish to survive. Degradation can occur due to various reasons such as pollution from industrial or agricultural activities, excessive sedimentation, or changes in water flow. These factors can impact the oxygen levels and water quality in the river, making it inhospitable for fish and other aquatic organisms. So, the absence of fish serves as an important indicator of environmental degradation in a river ecosystem.