Use and limits of three assessment methods of fish size spectra and abundance in two tropical man-made lakes

INRA, Station Hydrobiologie Lacustre, UMR CARRTEL, BP 511, 74203 Thonon les Bains, France
Fisheries Research (Impact Factor: 1.9). 02/2007; 83(2-3):306-318. DOI: 10.1016/j.fishres.2006.10.005


A comparative study, combining three different assessment methods (fish gillnet sampling, artisanal fisheries surveys and hydroacoustics) was conducted in Mali where two man-made reservoirs (Sélingué and Manantali) are particularly suited for investigating the impact of fishing effort on the fish assemblage. These two ecosystems have relatively similar areas, edaphic and environmental properties but are subjected to different levels of fishing exploitation (low at Manantali, high at Sélingué). The comparison is based on two indicator parameters: the abundance indices and the size spectra distributions, obtained by the three methods at two contrasting hydrological seasons (April and October). The results were compared first between the two seasons, and then between the two lakes. The present work is based on two main hypotheses: (1) that there is a higher fish abundance in October associated with smaller overall sizes, after spawning; (2) a lower abundance and smaller sizes in the Sélingué reservoir than in Manantali, because of the much higher fishing pressure in Sélingué. The relevance of each method to the selected indicators is discussed. On the one hand, the three methodologies on the whole gave similar conclusions and they also complement each other. On the other hand, some results do not match the hypotheses because of biases due to difficulties and technical limitations of each method in such ecosystems (shallow water with vegetation and stumps of former forests).

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    • "Fish sampling techniques have biases, and their results are therefore difficult to interpret (Robinson et al., 1996; Kube cka et al., 2009). Thus, it is necessary to compare different approaches (McClatchie et al., 2000; Coll et al., 2007), using standardised protocols, to measure variation in the amount of biomass and changes in the spatial distributions of fish populations. Comparisons in space and time can be based on this information (Yule, 2000). "
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    ABSTRACT: Two techniques, the conventional purse seine method and a novel method using vertical hydroacoustic surveys at fixed positions, were applied simultaneously in the Gambia River estuary to sample a fish population. Environmental parameters were measured simultaneously. Four surveys were performed during different hydrological seasons to reflect the main sources of variation in the environmental conditions of the estuary. A total of 153 samples resulted from these surveys. To identify the environmental conditions in which the two methods yield similar pictures of the fish population, a regression tree was constructed in which the dependent variable was the difference between the standardised values of the catches from the purse seine and the estimated biomass from the acoustic samples. A highly significant correlation was found between the two approaches using a set of data (128 samples) that excluded the samples with specific environmental conditions (downstream samples with high transparency, and upstream ones with high transparency, high depth and high temperature). We conclude that, except under these specific environmental conditions, vertical hydroacoustic samples recorded at a fixed station can serve as a proxy of the catches from a purse seine and can allow the monitoring of fish biomass in estuaries without damaging the environment.
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    • "Hydroacoustic offers an opportunity to adequately describe spatial and temporal patterns of fish distributions in freshwater habitats (Rose, 1992; Chen et al., 2009). Hydroacoustic techniques have the advantage of a rapid, economic and extensive coverage of the water cross section (Call et al., 2007; Guillard and Verges, 2007). The bucket-like morphology of Lake Alchichica precludes the application of standard methodologies (i.e., nets) to study the abundance and distribution of the fish; hence "
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    • "Hydroacoustics are known to provide a reliable method for estimating fish populations in the marine environment , and also in lakes (e.g., Brandt 1996; Wanzenböck et al. 2003; Coll et al. 2007; Guillard et al. 2006b; Mehner et al. 2007), dams (Djemali et al. 2009; Tuser et al. 2009), rivers (Hughes 1998; Kubecka & Duncan 1998; Matveev 2007), estuaries (Guillard et al. 2004) and lagoon channels (Brehmer et al. 2006b). Hydroacoustic assessment using echosounding with the acoustic beam pointing straight down is the most commonly method (Simmonds & MacLennan 2005). "
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    ABSTRACT: Guillard J, Balay P, Colon M, Brehmer P. Survey boat effect on YOY fish schools in a pre-alpine lake: evidence from multibeam sonar and split-beam echosounder data. Ecology of Freshwater Fish 2010: 19: 373–380. © 2010 John Wiley & Sons A/S Abstract – Hydroacoustic methods are widely employed by fish scientists for assessing fish stocks. The method most often used is echosounding, beaming vertically. Nowadays the multibeam sonar, and therefore the 3-D presentation of fish schools, has yielded better knowledge of school morphology. Using the data collected simultaneously by both sonar and echosounding in a lake, we have identified boat-induced behavioural changes in small pelagic fish schools. Using high resolution sonar data, we showed that the fish schools detected under the boat have a significantly larger volume than those alongside the boat. This finding is explained according to behavioural response due to the theoretical characteristics of the boat diagram sound pressure, and the existence of a strong thermocline. Then we compared two descriptors, the height of the fish school and the backscatter energy. We found significant differences, which reveal vertical fish school compression occurring simultaneously with the horizontal and sidelong escape behaviour.
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