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

Abstract

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.

The last snapshot of natural pelagic fish assemblage in Lake Turkana, Kenya: A hydroacoustic study

“…The significant diel change in TS distributions, with a higher number of small fish during the day and the increased number of larger fish at night, was apparently the direct consequence of this phenomenon. Similar diel changes of acoustic fish parameters were also observed by Szczucka (1998) in Lake Tanganyika and Djemali et al (2009) in Tunisian reservoirs. The overall decrease of acoustic fish density in OW during night observed in this study seems to be also affected by the previously mentioned vertical shift to the uppermost water layer.…”

“…Therefore, the day survey is usually more representative than that from night. A simultaneous survey with a horizontally oriented echosounder can achieve more accurate estimates of fish abundance, especially in lakes where the majority of fish occupy the upper few meters of the water column (Djemali et al, 2009;Knudsen and Saegrov, 2002). Unfortunately, this method is difficult to use under the windy conditions (Mous and Kemper, 1996) that prevail in African Great Lakes; furthermore, it was also not feasible for our survey due to technical and logistical limitations.…”

“…Unfortunately, this method is difficult to use under the windy conditions (Mous and Kemper, 1996) that prevail in African Great Lakes; furthermore, it was also not feasible for our survey due to technical and logistical limitations. For these reasons, horizontal hydroacoustic surveying remains a rarely employed method in African waters (Djemali et al, 2009) as compared to the routine vertically-oriented hydroacoustic technique (Getabu et al, 2003;Everson, 2006;Rufli and Vitullo, 1982;Szczucka, 1998).…”

Hydroacoustic survey of fish density, spatial distribution, and behavior upstream and downstream of the Changzhou Dam on the Pearl River, China

“…In the distant future it would be ideal to take into account complicated factors such as biological (food availability, predation risk and competition), environmental (oxygen concentration and water temperature) and Fish Sci (2011) 77:891-901 899 physical (water turbulence, visual and hydrodynamic habitat heterogeneity) information when the methodology of fish resource estimation and dam obstruction assessment has been developed and enhanced [15-18, 21, 22]. In comparison, horizontal beaming is considered to be a crucial component of acoustic fish stock assessment in freshwater lakes and reservoirs [23,24]. A conventional hydroacoustic method is limited by the low volume of water sampled near the vertically aimed transducer, which can be problematic when the aim is to detect fish located near a surface or at a relatively shallow water depth.…”

How pulse lengths impact fish stock estimations during hydroacoustic measurements at 70 kHz

“…Over the past few decades hydroacoustical methods have been increasingly used both at sea and in fresh water in order to acquire the detailed information about these aquatic ecosystems, and particularly about their living resources (Godlewska et al 2004;Simmonds et al 2009;Trenkel et al 2009;Warner et al 2009;Guillard et al 2010). Hydroacoustic instrumentation has now matured to be used routinely in a number of applications, including fisheries and ecological studies of ecosystem quality (Knudsen et al 2006;Mehner et al 2007;Djemali et al 2009;Kaartvedt et al 2009). However, the internationally accepted standards need to be created and outlined to a Corresponding author: guillard@thonon.inra.fr ensure comparability of results.…”