Fish biomass estimates in non-stratified tunisian reservoirs: acoustic and gillnet sampling.

Hydroacoustic monitoring (SIMRAD EK 60 split beam echosounder, 120 kHz) with gillnet sampling were performed in three shallow water Reservoirs in spring. Gillnets catches showed high presence of mullet in Sidi Saad Reservoir and roach in Mellegue and Bouhertma Reservoirs. NPUE varied between 45 and 431 fish/1000 m² of gillnet panel and BPUE from 3.1 to 29.9 Kg/1000 m². The results obtained in this study indicate that total fish biomass in Sidi Saad (458 kg ha-1) was approximately three times higher than the biomass in Bouhertma and Mellegue Reservoirs. Big fish was detected in the upper layer of all Reservoirs and particularly near the dam for a water depth less than 3 m in Bouhertma Reservoir. Fish biomass and density were related to depth in Sidi Saad Reservoir and decrease from dam to the tributary. A significant linear correlation was showed between acoustic fish biomass/BPUE and acoustic density/NPUE.