Recommendations for construction in fishways

The effectiveness of the systems (optical and acoustic) used to monitor fish passes depends mainly on the hydrological conditions prevailing in the water. The work of optical systems is strongly dependent on the degree of water transparency. Transparency of water in practice does not have a major impact on the correct operation of acoustic systems.
Often, however, the design of monitoring systems is based on the use of both optics and acoustics. In these cases, the degree of aeration and water transparency is important.
The basic requirement for both types of monitoring systems is to keep as little water as possible where the monitoring equipment is installed. Air bubbles in the water cause significant disturbances in the operation of the systems. Especially in the case of fast currents, larger air bubbles take the form of a spindle-like shape, from which the echo can be incorrectly recognised as an echo from a fish.

Such a shape of air bubbles in conditions of low water transparency may cause misidentification of the image from the camera and the qualification of larger bubbles as quickly moving fish.

Causes of reflections in water
The basis of all acoustic systems is the directional transmission of sound at different frequencies through the water and the recording of its reflections by appropriate receivers. The reflection (echo) occurs on the boundary layers between the water and the water bed or fish. Similarly, any air bubbles in the water cause very pronounced boundary layers. In conditions where the water is not aerated, the sound wave penetrates through it without hindrance, enabling correct transmission and correct readings of data about objects in the water. But if air appears in the way of a sound wave in the form of small or large bubbles (with sizes from less than 1 mm to more than a few cm), the sound wave is suppressed and reflected. As a result, either it is impossible to correct the echo recording or the fish are difficult to distinguish against the background noise.

Causes of aeration in water The first source is surface water aeration caused by the wind, which causes a swell, as well as rainfall. As the wind speed increases, the waves that are created mix the water and air. It is possible to see a white, foamy surface on the water containing air bubbles. The second source of aeration is the rapid movement of water in limited spaces.



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