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Diurnal variation in bottom trawl survey catches: does it pay to adjust?

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The diurnal bias of bottom trawl survey catches is studied with the purpose of adjusting for it and thereby improving the accuracy of abundance estimates. The correction term is estimated with uncertainty and thus increases the variance of the resulting abundance estimate. To investigate this adequately, we use a stochastic model describing diurnal fluctuations and examine the annual variation of the diurnal amplitude as a function of species and length. The diurnal amplitude is fairly stable for large fish, and for these, the bias-corrected estimate leads to a moderate increase in variance. For small fish, the diurnal amplitude is unstable, however, and the correction of diurnal bias occurs at the expense of a large increase in variance. This unstable amplitude also leads to a large year-to-year variation in catchability for small fish. For haddock (Melanogrammus aeglefinus), the diurnal amplitude depends heavily on fish length, indicating a strong decrease in catchability with decreasing fish length.

Nous avons étudié l'erreur systématique reliée à la période de la journée dans les inventaires faits à partir de récoltes au chalut de fond dans le but de trouver un ajustement et ainsi améliorer les estimations d'abondance. L'estimation du terme de correction comporte de l'incertitude, ce qui augmente la variance des estimations d'abondance. Un modèle stochastique qui décrit les fluctuations diurnes et l'examen de la variation annuelle de l'amplitude diurne en fonction des espèces et des longueurs nous ont permis d'étudier ce problème adéquatement. L'amplitude diurne est assez stable chez les gros poissons et, pour eux, une estimation corrigée entraîne une augmentation modeste de la variance. Pour les petits poissons, cependant, l'amplitude diurne est instable et toute correction de l'erreur amène une importante augmentation de la variance. Cette amplitude instable cause aussi une grande variation inter-annuelle de la capturabilité des petits poissons. Chez l'Aiglefin (Melanogrammus aeglefinus), l'amplitude diurne est fortement reliée à la longueur des poissons, avec une forte décroissance de la capturabilité en fonction de la diminution de la longueur du poisson.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: January 1, 2002

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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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