BAFF systems are primarily used to guide downward migrating fish (such as salmonid smolt) away from the entrance to a water channel, so they remain in the main river, or guiding smolt from a major flow into a fish pass channel, typically at the entrance to a hydro-electric facility. The systems can also be used for other applications, including accurately blocking a river to upward migrating fish (such as for Asian Carp) or for guiding fish towards fish collection or census stations.
A BAFF system can be considered as being analogous to a conventional angled fish screen, but uses an air bubble curtain to contain a sound signal, effectively creating a “wall of sound” (an evanescent sound field) that can be used to guide fish in a river, and if required into fish passes so that the fish can pass around structures such as dams.
The sound is generated either pneumatically or electronically and is contained within the bubble curtain as a result of refraction, since the velocity of sound in a bubble-water mixture differs from that in either water or air alone. The sound level inside the bubble curtain may be as high as 170 dB re 1mPa, typically decaying to 5% of this value within 0.5-1m from the bubble sheet. It can be deployed in much the same way as a standard bubble curtain, but its effectiveness as a fish barrier is greatly enhanced by the addition of a repellent sound signal.
The characteristics of the sound signal can be completely controlled if a Sound Projector Array (SPA) system is used to provide the sound signal and further stimulus can be provided by the addition of FGS’s High Intensity Lights. This creates a BAFF system using SILAS™ technology, as successfully trialled at Head of Old River and Georgiana Slough in California, USA.