Application of multibeam sonar and stationary hydroacoustic systems for 4D marine fish biomass monitoring

The vertical and horizontal diel dynamic of fish biomass assemblage inhabiting the Senigallia artificial reef (Adriatic Sea, Italy) was evaluated from July to November 1996 using an enhanced stationary hydroacoustic technique. Prior to the displacement of the acoustic equipment, an extensive seabed description of the area was conducted using a multibeam echosounder to find the more suitable places for acoustic transducers placement and to obtain a background imagery of the reef structures to be related to the fish distribution. Data analysis was conducted on three different depth layers: surface, midwater and bottom. In each layer, diel fish biomass inside the reef showed two peaks, one in early morning and the other in late evening. A slightly different behaviour was observed outside the protected area. The comparison among the layers showed that fish biomass was not homogeneously distributed along the water column throughout the sampling period, with high values in the middle and bottom layers until the end of August. On the other hand, no significant biomass variations in time were observed outside the reef showing always low values at each layer.