Study Objective: To determine the relationship between turbidity and VAKI Riverwatcher detection efficiency.
VAKI Riverwatcher fish counters consist of two plates, 10-45cm apart, each containing opposing rows of infrared LED lights and receivers. Objects passing through the counter break the infrared light beams and generate a silhouette image. These counters are most commonly used in the UK to monitor fish passes and are usually located in the exit of Denil or Larinier passes. Where the fish pass exit is too wide to accommodate the scanner plates, an inscale is used to guide fish through the counter.
Infrared light is rapidly attenuated in water and a high sediment load in a river will serve to increase this attenuation. Rivers in lowland areas of the UK are often highly turbid during spate conditions. It is therefore important to know the turbidity limits for the effective operation of Riverwatcher counters, especially where a full count of fish is required or when the effect of river flow on fish use of the pass is being investigated.
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