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Effects of clay turbidity and light on the predator–prey interaction between smelts and chaoborids

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Abstract:

The effects of clay turbidity and light on the predator–prey interaction between planktivorous smelts (Osmerus eperlanus) and phantom midge (Chaoborus flavicans) larvae were studied by means of laboratory experiments. Irrespective of light intensity, fish-mediated mortality of chaoborid larvae was highest at intermediate turbidity (20 nephelometric turbidity units (NTU)). Increases in light intensity enhanced the feeding rate of smelts at very low light intensities. A regression model describing the dependence of smelt-mediated mortality of chaoborids on light intensity and turbidity was fitted to the data. The model suggested that turbidity exceeding 30 NTU combined with light intensity below 0.1 µE·m–2·s–1 provides an efficient daytime refuge for chaoborids even in the presence of planktivorous smelts. In the field studies, the depth distribution of chaoborids followed the predictions of the model. The depth at which chaoborid density was highest depended on the existence and location of the thermocline, the densest swarms occurring beneath the turbidity maximum in the metalimnion. The smelts occupied water layers above the chaoborids, suggesting that the chaoborids used the steep turbidity gradient in the thermocline as a shelter against predation.

Des expériences de laboratoire nous ont permis d'étudier les effets de la turbidité due à l'argile ainsi que de la lumière sur les interactions prédateurs-proies entre les éperlans (Osmerus eperlanus) planctonophages et les larves de chaoboridés (Chaoborus flavicans). Dans les expériences, quelle que soit l'intensité lumineuse, la mortalité des larves de chaoboridés due aux poissons est maximale au niveau intermédiaire de turbidité (20 unités néphélométriques de turbidité (NTU)). Aux intensités lumineuses très faibles, un accroissement de l'intensité lumineuse accroît le taux d'alimentation des éperlans. Nous avons ajusté aux données un modèle de régression qui décrit la dépendance entre la mortalité des chaoboridés due aux éperlans, d'une part, et la lumière et la turbidité, d'autre part. Le modèle indique qu'une turbidité supérieure à 30 NTU combinée à une intensité lumineuse inférieure à 0,1 µE·m–2·s–1 procure aux chaoboridés un refuge de jour efficace, même en présence d'éperlans consommateurs de plancton. Dans les études sur le terrain, la répartition des chaoboridés en fonction de la profondeur suit les prédictions du modèle. La profondeur présentant la plus forte densité de chaoboridés dépend de l'existence et de la position de la thermocline; les regroupements les plus denses se retrouvent sous la profondeur de turbidité maximale dans le métalimnion. Les éperlans vivent dans les couches d'eau supérieures à celles où sont les chaoboridés, ce qui laisse croire que les chaoboridés utilisent le fort gradient de turbidité dans la thermocline comme un refuge contre la prédation.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: October 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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