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Channel head dynamics: capelin (Mallotus villosus) aggregation in the tidally driven upwelling system of the Saguenay - St. Lawrence Marine Park's whale feeding ground

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Abstract:

Capelin (Mallotus villosus) tridimensional distribution at the head of the Laurentian Channel in the St. Lawrence estuary was investigated using 38- and 120-kHz acoustic surveys in the summers of 1994, 1995, 1997, and 1998. The results are interpreted with the help of a high-resolution tridimensional tidal circulation model. Total biomasses were small (93–4583 t) and showed rapid fluctuations, whereas mesoscale distribution was more constant. Capelin tended to occupy the very end of the channel head, especially the slopes and shallows surrounding the basins. This pattern did not coincide with the krill distribution, but the two total biomass series were significantly correlated. Capelin tidal dynamics is characterized by herding of capelin against the channel head slopes by the starting flooding currents, followed by an upwelling over the sills and shallows during maximum flood currents, and a return to the channel by the surface outflow during ebb. Each side of the channel head has a distinct capelin retention tidal cycle involving passive advection, swimming, and the two-layer estuarine circulation. This capelin distribution and tidal dynamics closely match the local fin whale (Balaenoptera physalus) and minke whale (Balaenoptera acutorostrata) distributions observed from the whale-watching fleet and typical tidal feeding strategies at the channel head.

La répartition tridimensionnelle du capelan (Mallotus villosus) à la tête du Chenal Laurentien dans l'estuaire du Saint-Laurent a été examinée à l'aide de missions acoustiques (38 et 120 kHz) aux étés 1994, 1995, 1997 et 1998. Les résultats sont interprétés à l'aide d'un modèle tridimensionnel à haute résolution des courants de marée. La biomasse totale était faible (93–4583 t) et fluctuait rapidement, alors que la répartition à méso-échelle était plus constante. Le capelan tendait à occuper la toute fin de la tête du chenal, spécialement les talus et les hauts-fonds entourant les bassins. Cette répartition ne coïncidait pas avec celle du krill, mais les deux séries de biomasse étaien t significativement corrélées. La dynamique du capelan pendant un cycle de marée est caractérisée par un rassemblement contre les talus à la tête du chenal par les courants au début du flot, suivie par un saut au-dessus des hauts-fonds pendant le maximum du flot et d'un retour au chenal par les courants de surface sortant pendant le jusant. Chaque côté de la tête du chenal est caractérisé par une boucle distincte de rétention du capelan sur un cycle de marée, chacune faisant intervenir l'advection passive, la nage et la circulation estuarienne en deux couches. Cette répartition du capelan et sa dynamique en fonction du cycle de marée sont étroitement appariées avec la répartition locale des rorquals communs (Balaenoptera physalus) et des rorquals nains (Balaenoptera acutorostrata) observée par la flottille des bateaux d'observation ainsi qu'avec leurs stratégies typiques d'alimentation en relation avec le cycle de marée.

Document Type: Research Article

Publication date: February 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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