Comparison of in situ, ex situ, and backscatter model estimates of Pacific hake (Merluccius productus) target strength

Abstract

To convert acoustic energy into estimates of fish density, the target strength (TS) of a representative fish must be known. TS is a measure of the acoustic reflectivity of a fish, which is variable depending on the presence of a swimbladder, the size of the fish, its behavior, morphology, and physiology. The most common method used to estimate the TS of a fish is a TS-to-length empirical regression, with TS values increasing with fish length. This study uses in situ and ex situ TS measurements and a backscatter model to develop TS-to-length conversions for Pacific hake (Merluccius productus). Results from in situ and ex situ measurements had regression intercepts 4-6 dB lower than the previous Pacific hake TS-to-length regression. These differences suggest that an individual hake reflects 2.5-4 times less acoustic energy than was previously estimated.Résumé : Pour pouvoir convertir l'énergie acoustique en estimations de la densité de poissons, il est nécessaire de connaître l'intensité de la cible (TS) constituée par un poisson représentatif. TS est une mesure de la réflectivité acoustique d'un poisson et elle varie en fonction de la présence d'une vessie natatoire, de la taille du poisson, de son comportement, de sa morphologie et de sa physiologie. La méthode la plus couramment utilisée pour estimer la TS d'un poisson se base sur une relation empirique entre TS et la longueur dans laquelle les valeurs de TS augmentent en fonction de la longueur du poisson. Notre étude utilise des mesures in situ et ex situ de TS et un modèle de rétrodiffusion pour mettre au point une méthode de conversion des TS en longueurs chez le merlu du Pacifique (Merluccius productus). Les résultats de nos mesures in situ et ex situ donnent, dans les régressions, des valeurs de l'ordonnée à l'origine de 4-6 dB inférieures à celles des régressions faites antérieurement reliant TS à la longueur chez le merlu du Pacifique. Ces différences laissent croire que la rétrodiffusion d'énergie acoustique faite par un merlu individuel est de 2,5-4 fois moins importante que dans les estimations antérieures.[Traduit par la Rédaction] Henderson and Horne 1794

Accuracy of the Kirchhoff-Approximation and Kirchhoff-Ray-Mode Fish Swimbladder Acoustic Scattering Models

“…It is important that models give results that are representative of scattering from fish, therefore much work has been done to validate models by comparison with in situ and ex situ measurements. Overall, there is reasonable agreement (e.g., [14], [15], [16], [17]), but differences between modelled and measured TS of several decibel (dB) are common at broadside and off-broadside angles (e.g., [15], [18], [19], [20], [21], [22]). …”

Acoustic classification of coexisting taxa in a coastal ecosystem

“…als with similar lengths, swimbladder area and volume of herring are more than double that of hake (Gauthier and Horne, 2004;Henderson and Horne, 2007). Since acoustic backscatter intensity is a function of swimbladder cross-sectional area and shape at high frequencies (Ona, 1990), differences in morphological characteristics can result in a difference in MVBS.…”

A retrospective of the hake stocks off the Atlantic and Pacific coasts of the United States: Uncertainties and challenges facing assessment and management in a complex environment

“…Acoustic estimates of hake abundance and biomass are derived from the mean area backscatter cross section based on the target strength relationship of Traynor (1996). Recent work by Henderson and Horne (2007) suggests uncertainty in the target strength relationship for Pacific hake which could have a substantial impact on biomass estimates.…”