Frequency analysis as a tool for assessing adverse conditions during a massive Fish Kill in the St. Lawrence River, Canada

Abstract During the summer of 2001, the largest fish kill in the recorded history of the St. Lawrence River occurred. More than 25,000 dead carp were recovered. Preliminary analyses suggested hydroclimatic factors may have triggered the fish kill. Long time series of hydroclimatic variables were available upstream and downstream from the study area. In order to investigate if hydroclimatic conditions were extreme during the summer of 2001, frequency analyses were performed on time series of air and water temperature, water level, and solar radiation. During this period, the daily maximum water temperature was abnormally high (return period of 47 years) relative to other years. Air temperature was also high (return period of 22 years) and water level was very low (return period of 67 years). Results showed that hydroclimatic forcings were acting at two different time scales. First, short-term extremes are more likely to have direct impacts on ecosystems, such as lethal stress caused by oxygen depletion in shallow areas. Long-term extremes have indirect effects, which are more difficult to detect, such as immunosuppression. These results reiterate the importance of water temperature in aquatic habitat, particularly in the present context of global warming and climate change.</jats:p

Modélisation des habitats de frai et de croissance du grand brochet dans un large paysage fluvial avec une dynamique spatio-temporelle importante (fleuve Saint-Laurent, Canada)

International audienceSpawning and nursery habitats are often spatially disjunct as a consequence of specific life history stage habitat requirements and spatiotemporal habitat changes. Nevertheless, free-swimming larvae originating from spawning habitats must reach productive nurseries to maximize survival. We examined spawning and nursery habitats of northern pike (Esox lucius) over the past 50 yr to investigate how habitat connectivity and hydrological variability interact to alter the distribution of effective spawning habitat. Habitat models coupled to a least-cost approach were developed to quantify connectivity between habitats in two contrasting regions of the St. Lawrence River (Canada): a riverine corridor lake (~ 46 km) and a large fluvial lake (~ 48 km). Our simulations demonstrate that depending on hydrological conditions, between 3% to 51% of spawning habitat used by adults in the riverine corridor, and 22% to 90% in the lake, allowed larval survival up to the fifth week of development. Although rapid dewatering of spawning habitat is responsible for most spawning losses in the fluvial lake, increasing water currents were responsible for dispersing larvae away from suitable habitats in the riverine corridor. However, stable hydrological conditions led to spatial overlapping of spawning and nursery habitats favoring larval survival and growth. In addition, downstream larval dispersal by low water currents allowed larvae to reach spatially disjunct nursery habitat, especially in the lake. Our results indicate that despite the vast areas of potentially suitable habitats provided by large vegetated floodplains of fluvial lakes, the effective spawning habitats favoring early-life recruitment are much more heterogeneous and variable both spatially and temporally.Les habitats de frai et d'alevinage sont souvent disjoints spatialement en raison des exigences spécifiques de l'habitat au stade du cycle vital et des modifications de l'habitat spatiotemporelles. Néanmoins, les larves nageant librement provenant des zones de frai doivent atteindre les pépinières productives pour maximiser leur survie. Au cours des 50 dernières années, nous avons examiné les habitats de frai et de croissance du grand brochet (Esox lucius) afin d'étudier les interactions entre la connectivité de l'habitat et la variabilité hydrologique pour modifier la répartition de l'habitat de frai effectif. Des modèles d'habitat associés à une approche au moindre coût ont été développés pour quantifier la connectivité entre les habitats de deux régions contrastées du fleuve Saint-Laurent (Canada): un lac à corridor fluvial (~ 46 km) et un grand lac fluvial (~ 48 km). Nos simulations montrent que, selon les conditions hydrologiques, entre 3% et 51% des habitats de frai utilisés par les adultes dans le corridor fluvial et entre 22% et 90% dans le lac permettent la survie larvaire jusqu'à la cinquième semaine de développement. Bien que la déshydratation rapide de l'habitat de frai soit responsable de la plupart des pertes de frai dans le lac fluvial, l'intensification des courants d'eau a été responsable de la dispersion des larves loin des habitats adéquats dans le corridor fluvial. Cependant, des conditions hydrologiques stables ont entraîné un chevauchement spatial des habitats de frai et de croissance, favorisant ainsi la survie et la croissance des larves. De plus, la dispersion des larves en aval par les courants d'eau bas a permis aux larves d'atteindre l'habitat de pépinière spatialement disjoint, en particulier dans le lac. Nos résultats indiquent que malgré les vastes étendues d'habitats potentiellement propices fournis par les vastes plaines inondables végétalisées de lacs fluviaux, les habitats de frai propices au recrutement en début de vie sont beaucoup plus hétérogènes et variables, dans le temps et dans l'espace

Effets du réseau routier sur la connectivité des frayères du grand brochet (Esox lucius) au lac Saint-Pierre (fleuve Saint-Laurent, Canada)

National audienceTo help complete the picture of the pressures affecting the diverse aquatic habitats of the Lake St-Pierre floodplain (Québec, Canada), an advanced geomatics analysis was used to assess the effects of the adjacent road network on the connectivity of spawning and nursery grounds of the northern pike (Esox lucius). The results revealed adverse effects on high potential habitat when the water flow at Sorel exceeded 12,000 m 3 .sAfin de compléter le portrait des pressions agissant sur les habitats aquatiques de la plaine d'inondation du lac Saint-Pierre, une analyse géomatique avancée évalue les effets du réseau routier sur la connectivité des frayères et des nourriceries du grand brochet ( Esox lucius ). Les résultats révèlent des effets négatifs sur les habitat