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Ontogenetic changes in the longitudinal distribution of two species of larval fish in a turbid well-mixed estuary
Authors:R. Laprise   J. J. Dodson
Affiliation:Département de Biologic, UniversitéLaval, Sainte-Foy, Québec, Canada G1K 7P4
Abstract:The relationship between vertical migration, estuarine retention and species-specific patterns of longitudinal distribution of the pelagic larvae of rainbow smelt, Osmerus mordax , and tomcod, Microgadus tomcod , was investigated in the upper section of the St Lawrence Middle Estuary. We hypothesized that the species-specific use of the vertical pattern of current exhibited by the two species results in the partitioning of the estuarine habitat in the longitudinal plane while assuring retention. Important differences in the longitudinal distribution of tomcod and smelt larvae were related to the ontogeny of their vertical distribution in the water column. In June, small tomcod and smelt larvae are generally associated with waters of salinities less than 5%o. As the larvae grow, their vertical distribution patterns change, leading to a horizontal separation of the two species. Tomcod juveniles migrate downstream into colder, more saline waters, whereas larger smelt larvae migrate upstream into warmer, tidal fresh waters. Ontogenetic changes in vertical distribution serve to concentrate larvae in specific conditions that may optimize physiological conditions and also permit population persistence.
Keywords:larval fish    retention    estuary    behaviour    vertical and longitudinal distribution
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