Landscape genetics is being increasingly applied to elucidate the role of environmental features on the population structure of terrestrial organisms. However, the potential of this framework has been little explored in aquatic ecosystems such as large rivers. Here, we used a landscape genetics approach in order to (i) document the population structure of the yellow perch (Perca flavescens) by means of genetic variation at microsatellite markers, (ii) assess to what extent the structure was explained by landscape heterogeneity, and (iii) interpret the relevance of interactions between genetics and landscape for management and conservation. Analysis of the genetic variation among 1715 individuals from 16 localities and distributed over 310 km in the freshwater section of the Saint Lawrence River (Québec, Canada) revealed a relatively modest level of genetic structuring (F(ST) = 0.039). Application of the Monmonier's algorithm combining geographical and genetic information identified three zones of restricted gene flow defining four distinct populations. Physical barriers played a more important role on gene flow and genetic structure than waterway geographical distance. We found correlations between genetic differentiation and presence of distinct water masses in the sector of Lake Saint-Louis (r = 0.7177, P = 0.0340) and with fragmentation of spawning habitats in the sector of Lake Saint-Pierre (r = 0.8578, P = 0.0095). Our results support the treatment of four distinct biological units, which is in contrast with the current basis for yellow perch management. Finally, this study showed that landscape genetics is a powerful means to identify environmental barriers to gene flow causing genetic discontinuities in apparently highly connected aquatic landscapes.

Discharge fluctuations in the St. Lawrence River (Canada) affect reproduction habitat for Northern pike (Esox lucius Linnaeus). We developed a spatially explicit model for that large-scale river system to predict spawning habitat surfaces available for egg deposition and the potential mortality by dewatering occurring during the embryonic-larval stages. The spatial model used simulated current velocity, wetland type and water temperature at a high spatial resolution over the entire fluvial St. Lawrence River, Canada. Those three variables were integrated into a habitat suitability index (HSI) and weighted according to the literature. A new thermal preferendum curve, based on a field experiment, was included in the HSI, reflecting that the probability of observing pike spawners in a given area increased with temperature. The reproduction chronology was predicted every year with an original model based on air temperature in order to reconstitute the historic habitat surfaces for the period 1960-2000. The results revealed that discharge had a substantial effect on both suitable habitat for egg deposition and potential mortality following dewatering. The best and the largest spawning habitats were identified, as well as the most limiting regions in the river. The present findings have already been used to prepare a new discharge regulation plan for the Lake Ontario-St. Lawrence River system.

ABSTRACT. We documented the vertical distribution, species composition, size and feeding of bivalve veligers in vertically stratified and well-mixed waters of the Baie des Chaleurs, Quebec, Canada. Larvae aggregated at certain depths in both types of water column. When a pycnocline was present, the mean depth of veligers was generally below it. Veligers' vertical distribution was not related to that of water chlorophyll a except at night in stratified conditions. In stratified waters, gut chlorophyll a content of larvae was associated with water chlorophyll a concentration. In mixed waters, larval gut

Due to the increasing interest in biochemical indices such as the RNA-DNA ratio used to measure fish growth, fish often need to be stored frozen with dry ice (i.e., À808C). The objectives of this study were to (1) quantify the effects of dry ice on both the length and weight of larval and juvenile yellow perch Perca flavescens preserved for storage periods of 15 d and 7-8 months, (2) compare these effects with those of two commonly used preservatives (a 10% solution of formalin and a 75% solution of ethanol), and (3) provide equations to convert the lengths and weights of larval and juvenile yellow perch preserved with dry ice, formalin, and ethanol back to their initial unpreserved values. For all preservation methods, fish weight was more affected than length. The smallest length reduction was observed with formalin (short term: 2.1% and 0.1% for larvae and juveniles, respectively; long term: 10.1% and 1.2%), followed by dry ice (short term: 4.0% and 1.4%; long term: 7.2% and 3.9%) and ethanol (short term: 9.6% and 1.2%; long term: 11.7% and 1.2%). The smallest weight reduction was also observed with formalin (short term: 21.9% and 2.2%; long term: 23.2% and 3.9%), followed by dry ice (short term: 54.0% and 11.1%; long term: 52.8% and 8.4%) and ethanol (short term: 61.1% and 22.0%; long term: 66.0% and 26.0%). Except for one case, all of the regression equations that were built to convert the lengths and weights of larval and juvenile yellow perch preserved with dry ice, formalin, and ethanol back to initial measurements were highly significant.

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Larval dispersal is a crucial factor for fish recruitment. For fishes with relatively small-bodied larvae, drift has the potential to play a more important role than active habitat selection in determining larval dispersal; therefore, we expect small-bodied fish larvae to be poorly associated with habitat characteristics. To test this hypothesis, we used as model yellow perch (Perca flavescens), whose larvae are among the smallest among freshwater temperate fishes. Thus, we analysed the habitat association of yellow perch larvae at multiple spatial scales in a large shallow fluvial lake by explicitly modelling directional (e.g. due to water currents) and non-directional (e.g. due to aggregation) spatial patterns. This allowed us to indirectly assess the relative roles of drift (directional process) and potential habitat choice on larval dispersal. Our results give weak support to the drift hypothesis, whereas yellow perch show a strong habitat association at unexpectedly small sizes, when compared to other systems. We found consistent non-directional patterns in larvae distributions at both broad and medium spatial scales but only few significant directional components. The environmental variables alone (e.g. vegetation) generally explained a significant and biologically relevant fraction of the variation in fish larvae distribution data. These results suggest that (i) drift plays a minor role in this shallow system, (ii) larvae display spatial patterns that only partially covary with environmental variables, and (iii) larvae are associated to specific habitats. By suggesting that habitat association potentially includes an active choice component for yellow perch larvae, our results shed new light on the ecology of freshwater fish larvae and should help in building more realistic recruitment models.