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Bimodal population size distributions and biased gillnet sampling

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Abstract:

Bimodal size distributions have been commonly observed in Arctic char (Salvelinus alpinus). We document an example of such bimodality caused solely by biased gillnet sampling. The observed bimodality was a direct artefact of the sampling method resulting from an abrupt increase in gillnet catchability of fish larger in total length than between 25 and 30 cm. Mean gillnet selectivity (catchability) of char in the upper mode of the observed bimodal size distribution was about 40 times higher than the corresponding value for char in the observed lower mode. Fish of intermediate size, lacking in the gillnet samples, were present in the population and readily obtainable by electrofishing. The observed size difference in gillnet vulnerability is likely to result from behavioural changes following ontogenetic niche shifts.

On a souvent observé des distributions de taille bimodales chez l'omble chevalier (Salvelinus alpinus). Nous présentons un exemple d'une telle bimodalité qui est causée uniquement par un échantillonnage faussé au filet maillant. La bimodalité observée est un artefact direct de la méthode d'échantillonnage qui résulte d'une augmentation abrupte de la probabilité de capture dans le filet des poissons plus grands que 25 à 30 cm de longueur totale. La sélectivité moyenne (probabilité de capture) du filet maillant pour la capture des ombles du mode supérieur de la distribution bimodale des tailles est environ 40 fois plus élevée que pour celle des ombles du mode inférieur de la distribution bimodale observée. Les poissons intermédiaires, absents des échantillons récoltés au filet maillant, sont présents dans la population et facilement capturés par pêche électrique. La différence de probabilité de capture au filet maillant observée en fonction de taille est vraisemblablement due à des modifications de comportement consécutives à des changements ontogéniques de niche.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: November 1, 2004

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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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