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Hydroacoustic fish stock assessment in the presence of dense aggregations of Chaoborus larvae

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Abstract:

A new method for eliminating reverberation due to Chaoborus larvae from hydroacoustic recordings is presented based on an assumption of a constant dependence between the area backscattering strength (sa) with a high-volume backscattering threshold (sv) and sa with a low sv threshold for fish. The idea was to analyze data with a threshold high enough to eliminate reverberation and then convert the estimate to coincide with the result that would have been achieved with a low threshold containing all backscattering from fish. The approach was validated with a secondary dataset, and the magnitude of overestimation of fish density by reverberation was evaluated using data from four surveys conducted in a clay-turbid lake, where small planktivorous fish, smelt (Osmerus eperlanus), and larvae of Chaoborus flavicans coexist in the water column. With the presented method, estimation of smelt density was possible even when Chaoborus density was >200 individuals·m–3. The analyses revealed that the overestimation of fish density could be as high as 50% if the reverberation is not taken into account. The presented method might also be applicable for eliminating reverberation due to other unwanted targets, because it is based on the acoustic properties of fish rather than those of unwanted targets.

Notre nouvelle méthode pour éliminer la réverbération causée par les larves de Chaoborus dans les enregistrements hydroacoustiques se base sur la présupposition de l'existence d'une dépendance constante entre l'intensité de la rétrodiffusion par surface (sa) à un seuil élevé de rétrodiffusion par volume (sv), d'une part, et sa à un seuil sv bas pour les poissons, d'autre part. Le principe consiste à analyser les données à un seuil assez élevé pour éliminer la réverbération et de convertir ensuite l'estimation pour la faire coïncider avec le résultat qui aurait été obtenu avec un seuil bas qui contient toute la rétrodiffusion due aux poissons. L'utilisation d'une série secondaire de données a servi à valider la méthodologie; des données de quatre inventaires réalisés dans un lac turbide à l'argile où cohabitent dans la colonne d'eau des éperlans (Osmerus eperlanus), des petits poissons planctonophages, et des larves de Chaoborus flavicans ont permis d'évaluer l'importance de la surestimation de la densité de poissons à cause de la réverbération. Avec notre méthode, il est possible d'estimer la densité des éperlans même quand la densité des Chaoborus est >200 individuels·m–3. Les analyses révèlent que la surestimation peut atteindre 50 % lorsqu'on néglige la réverbération. Notre méthode peut probablement aussi servir à éliminer la réverbération causée par d'autres cibles non désirées, parce qu'elle se base sur les propriétés acoustiques des poissons plutôt que sur celles des cibles accessoires.[Traduit par la Rédaction]

Document Type: Rapid Communication

Publication date: February 1, 2005

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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