Aquatic Ecology

, Volume 43, Issue 4, pp 1121–1131 | Cite as

Hydroacoustic fish biomass assessment in man-made lakes in Tunisia: horizontal beaming importance and diel effect

  • Imed DjemaliEmail author
  • Rachid Toujani
  • Jean Guillard


We used a Simrad EK60 echosounder equipped with two split-beam transducers to develop a sampling strategy for assessing fish resources in Tunisian man-made lakes. Day and night surveys, using vertical and horizontal beaming, were carried out between December 2006 and February 2007, a period when fish catchability is high. Four reservoirs with differing surface areas and bathymetries were selected. Echogram analysis revealed that fish communities were mainly composed of individual targets. A few schools were detected near the surface during daylight, but these schools dispersed slightly at night. In these multispecies reservoirs, considerable day and night differences in density existed, but with no clear trend. Target strength (TS) distribution mode values detected at night were always lower or equal to daytime values. Biomass estimates were significantly higher during daytime in three reservoirs, corresponding with higher TS modal values. In the other reservoir, the biomass estimate was significantly higher during nighttime corresponding with higher mean density during this period. Using only a vertically aimed transducer in our study reservoirs would have led to an underestimate of density and biomass by 5–100% and 20–100%, respectively, depending on the man-made lake. We conclude that acoustic sampling in our reservoirs must be done during day and night and that both vertical and horizontal beaming must be used to obtain the best possible picture of the fish stocks.


Echosounder Day and night Reservoir Water column Horizontal beaming 



The study was financed by the Tunisian Ministry for Higher Education and Scientific Research, and was carried out as part of the research project “GRAVID”. We would like to thank Dr. F.R. Knudsen from Simrad Horten in Norway for his kindness and advice, and also Dr. H. Balk from the University of Oslo for his kind assistance. We would like also to thank Dr. L. Cardona from the University of Barcelona for providing information about the behaviour of mullet. We express our gratitude to the people in charge of regional fishing in the districts of Béja, Nabeul, Siliana and Bizerte for their assistance. Special thanks to the two anonymous referees for improving the manuscript and to Daniel Yule for his great help and advice.


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Copyright information

© Springer Science+Business Media B.V. 2008

Authors and Affiliations

  1. 1.Institut National des Sciences et Technologies de la MerTunisTunisia
  2. 2.Institut National de la Recherche Agronomique, Station d’Hydrobiologie Lacustre, UMR CARRTELThonon les BainsFrance

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