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Commercial catches of two ecologically distinct sympatric smelt (Osmerus mordax) populations segregated along the two shores of the St. Lawrence middle estuary exhibited inverse patterns with periodicities on the order of 30 years. The influence of water level in the St. Lawrence River and air temperature, chosen to reflect variations in hydrology and climate, differed markedly between the two populations. Analyses revealed that both water level and temperature were generally positively related with north-shore smelt landings and negatively related with south-shore smelt landings. For both populations, a number of significant climatic factors contributing to variance in smelt landings were lagged by one to three years relative to the year of landings, indicating that climatic variables influenced smelt recruitment. The contrasting role of hydroclimatic variables in driving these abundance cycles is likely related to differential exploitation of estuarine habitats; the south-shore population is associated with shallow shoal habitat, whereas the north-shore population is associated with deep channel habitat. The responses of the two smelt populations also reflect the fundamental ecological differences existing between shoal and channel habitats, indicating that future climate change may differentially affect other populations or species that are segregated between these two habitats.Les débarquements commerciaux de deux populations d'éperlans (Osmerus mordax) arc-en-ciel sympatriques, distinctes du point de vue écologique et ségréguées le long des deux rives de l'estuaire moyen du Saint-Laurent, montrent des variations cycliques inverses et une périodicité de l'ordre de 30 ans. L'influence du niveau d'eau du fleuve et de la température de l'air, choisis dans l'étude pour refléter les variations du régime hydrologique et du climat, diffèrent de façon marquée pour les deux populations. Les analyses révèlent que le niveau de l'eau et la température sont généralement corrélés positivement aux débarquements de la rive nord et négativement à ceux de la rive sud. Quelques un des principaux facteurs climatiques contribuant pour une grande part aux variations des débarquements des deux rives étaient décalés dans le temps, indiquant que les facteurs climatiques influencent le recrutement des éperlans. L'effet contrasté des variables hydroclimatiques sur les cycles d'abondance est vraisemblablement liée à l'utilisation différentielle des habitats de l'estuaire; la population de la rive sud est associée à un habitat peu profond et celle de la rive nord est à un chenal profond. Les réponses de ces deux populations d'éperlans reflètent aussi des différences écologiques existant entre des habitats peu profonds et un chenal profond, indiquant que les changements climatiques futurs pourraient avoir des effets différents sur d'autres populations ou d'autres espèces, qui sont ségréguées entre ces deux types d'habitats. [ABSTRACT FROM AUTHOR]

RNA/DNA ratios and total lengths were compared to determine growth patterns of age-0 yellow perch (Perca flavescens) in managed and natural habitats of a large fluvial lake (Lake Saint-Pierre, St. Lawrence River, Quebec, Canada) over seasonal and yearly temporal scales. In 2002, the RNA/DNA ratio responded to degree-days accumulated over periods of 7–8 days before sampling, while in 2003, no relationship with temperature was established. The growth patterns obtained each year probably reflect indices responding to different limiting variables. In 2002, temperature would have been limiting, whereas in 2003, other factors such as prey availability, food quality, and competition may have influenced growth. In addition, the discrepancy between total length and RNA/DNA ratio observed in 2003 may reflect a differential time of response to limiting variables. These results together show that the two indices reflect growth at different time scales and suggest that their combination can help identify shifts between limiting environmental variables. Also, growth in managed wetlands during springtime was systematically superior to that in the natural environment, supporting the contention that managed wetlands are highly productive habitats. In natural habitats, growth rates were higher on the south shore by summer, which is consistent with the established north-south productivity gradient in Lake Saint-Pierre. [ABSTRACT FROM AUTHOR]