###### Abstract  
Monitoring fish species populations in very turbid environments is challenging. Acoustic cameras allow work in very poor visibility but are often deployed as a fixed observation point, limiting the scope of the survey. A BlueView P900-130 acoustic camera was deployed in rocky marine habitats off the coast of French Guiana in order to assess the total abundance, size structure and spatial distribution of a demersal fish population. The relevancy of using an acoustic camera to achieve these three objectives was evaluated by comparing acoustic data to those obtained from fishing surveys. The detection and identification of large demersal fish species were possible with the shape and size of the acoustic signal and acoustic shadow silhouette as well as swimming behavior. Mobile surveys combined with stationary surveys increased the probability of distinguishing individuals from inanimate objects. Estimated total length based on the acoustic signal underestimated the actual length of fish measured on deck, but the data showed the same trends in spatial and temporal variation. Acoustic cameras overcame the extreme lack of visibility by increasing knowledge of fish use of habitat, therefore providing much more efficiency in the effort, more accurate data on the abundance, size structure and spatial distribution than the fishing method. Thus, despite few limitations, acoustic camera surveys are far superior to fishing surveys in evaluating large demersal fish stock status. f +
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which was formerly used for surveillance and inspection of structures in the marine environment [8]. They are being used increasingly to study marine fish populations and were adapted for fish enumeration or identification under conditions of low visibility, such as poor lighting or high turbidity [6; 7; 9; 10]. These acoustic systems provide high-quality images with similar resolution as standard videos [3]. Among existing materials, fishery scientists have principally used the Sound Metrics Corp. DIDSON dual-frequency identification sonar [1; 8; 11; 12; 13; 14; 15; 16; 17; 18; 19]. DIDSON allows researchers to operate at several frequencies, the highest of which is 1.8 MHz, which produces high image quality with a 29\({}^{\circ}\) field of view. The maximum distance for this frequency is about 12 m, but this distance may be increased to 30 m with a lower frequency of 1.1 MHz [9]. Fish species can also be distinguished with a Teledyne \"BlueView P900-2250\" high-frequency multibeam sonar and at a frequency of 1 MHz, even pectoral and caudal fins could be discerned to identify individuals and fish species [20]. BlueView sonar equals DIDSON in the quality of images [21]. Both types of acoustic cameras are deployed to count pelagic fish in horizontal and stationary surveys (in one site) in shallow waters. As example, it is possible to count moving salmonid fishes [1; 18; 22], and to measure reef fish abundance [23]. With appropriate software (Sound Metrics software or ProViewer software), it is possible to measure the length of individuals [8; 15; 19] and to study their behavior [24; 25; 26; 27]. A double protocol, using both static and mobile acoustic cameras was developed to study the abundance of the Kamchatka steelhead (_Parasalno mykiss_) in a river that allow describing the size structure of the species [17]. Recently, the spatial distribution of fish populations has been described using acoustic cameras. Indeed, the density and spatial distribution of pelagic fish was assessed by deploying an acoustic camera in a lake [28]. Few studies focused on demersal fish even if identification through acoustic shadows is possible [3]. However, immobile individuals on the bed cannot be detected and may be problematic for abundance estimates [29].