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Daily American eel Anguilla rostrata catches and their dates of passage (starting, median and ending dates) were compared between pristine (1843-1872) and contemporary periods (1963-1990) to determine any changes and see whether these were related to environmental variations and water discharge regulation. Timing and duration of A. rostrata migration patterns differed significantly between the two periods. In the contemporary period, migrating A. rostrata were intercepted significantly earlier than in pristine times (18 days earlier on average), and ended at the same average period. Early A. rostrata migration was also significantly related to high spring flow and secondarily to high spring temperature, while migration ended later when high temperature or low water level occurred during the autumn period. A recent slight increase in the water temperature of the St Lawrence River could partially explain the earlier A. rostrata migration observed during the contemporary period. In return, the effect of high spring flow should have been more contrasted if the river discharge would have not been regulated. Recent A. rostrata production now being mainly restricted to the lower part of St Lawrence River mainstream, resulting shorter travelling distance to the estuary may explain why migrating progress was earlier during the contemporary period. [ABSTRACT FROM AUTHOR]

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]

Describes food particle size and selection in bivalve larvae. Composition of stomach contents; Implications for the variation of the size ranges of ingested algal cells; Active food selection among veligers; Difference in feeding patterns in different-sized veligers.

After being extirpated from the St. Lawrence River in the 1960s, striped bass (Morone saxatilis) were reintroduced to the estuary in 2002 and by 2008, they were naturally reproducing. To document the habitat use and feeding ecology of this reintroduced population, we examined the gut contents of 333 larvae and juveniles. Samples were collected in four estuarine habitats in 2014: the upstream freshwater section (UP), the oligohaline (O-ETM) and the mesohaline (M-ETM) estuarine turbidity maximum zones, and the downstream polyhaline section (DOWN). In June, pelagic larvae developed in the UP and the O-ETM, feeding mainly on copepods such as Eurytemora affinis. The O-ETM exhibited better suitable feeding conditions compared to the UP, likely due to the presence of Bosmina sp. as a primary prey. After July, striped bass shifted to larger prey items, consuming mainly dipteran pupa in upstream littoral habitats and gammarids and mysids in downstream habitats. In the early summer, the UP provided a high-quality nursery habitat and as the season progressed, the smallest juveniles dispersed downstream and improved their feeding success by exploiting a new feeding niche. This observation suggests that being distributed throughout the estuary may increase the potential survival of striped bass early life stages. [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]

The St. Lawrence estuary (SLE) supports important nursery habitats for many fish species, including the newly re-established population of striped bass (Morone saxatilis). The present study aims to describe pelagic (5 m isobaths) and littoral (<1.5 m-deep) nearshore habitats of the SLE and to document their use by fish larvae and age-0 juveniles, especially for striped bass. From June to September 2014, 162 pelagic and 188 littoral stations were characterized in terms of biophysical conditions (turbidity, salinity, temperature, dissolved oxygen and chlorophyll- a concentration), abundance of main preys and fish assemblages. Striped bass larvae identification was confirmed with genetic analyses. In the pelagic and littoral zones, four estuarine habitats were defined based on turbidity and salinity: an upstream freshwater section (UP), an oligohaline (O-ETM) and a mesohaline (M-ETM) estuarine turbidity maximum zone, and a downstream polyhaline section (DOWN). Our results emphasized the spatial partitioning of fish early life stages among contrasting habitats, potentially to minimize interspecific competition. In the SLE, striped bass is using the wide spectrum of habitats available. At the onset of striped bass development, the UP and the O-ETM were the main nursery habitats, which is consistent with the known location of spawning grounds. Results suggest that the O-ETM provided the most suitable conditions for the species with optimal physical properties and abundant food resources. Later in the season, the M-ETM provided a favourable trade-off habitat offering a less optimal physical environment although potential for new prey. • Along de salinity gradient, four distinct estuarine habitats were identified in the St. Lawrence Estuary. • Contrasting habitats support distinct assemblage of fish early life stages along the salinity gradient. • Along the growing season, striped bass (Morone saxatilis) is using the wide spectrum of habitats available. • The O-ETM provided the most suitable conditions for striped bass with optimal physical properties and abundant food resources. • The dispersion of striped bass further downstream may be a mechanism to partition the estuary, and to minimize competition. [ABSTRACT FROM AUTHOR]