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We conducted a 2-year field experiment examining the survival to hatching of Atlantic salmon (Salmo salar) in relation to the seasonal and spatial variations of silt and very fine sand (SVFS; <0.125 mm) content within a large set of artificial redds at two spawning sites of the Sainte-Marguerite River, Quebec, Canada. Each artificial redd consisted of an infiltration cube (30 cm × 30 cm × 20 cm) buried in a morpho-sedimentological unit resembling a salmon redd. One hundred fertilized Atlantic salmon eggs were inserted in a number of infiltration cubes during redd construction. The results indicate no significant relation between survival to hatching in spring and values of the sand index or total percent fine sediment <2 mm in redds at that time. However, the proportion of SVFS in the redds explained 83% of the variation in embryo survival, with a threshold at approximately 0.2% SVFS, above which survival dropped sharply below 50%. Infiltration of these very fine fractions mostly occurred under ice cover, during the low-flow winter period. However, during the spring flood period, infiltration-flushing patterns varied spatially and reflected spatial differences in local intensity of bed-load transport and fine sediment availability.Résumé : Une expérience de terrain de deux ans nous a permis d'examiner la survie jusqu'à l'éclosion de saumons atlantiques (Salmo salar) en fonction des variations saisonnières et spatiales des quantités de limon et de sable très fin (SVFS, <0,125 mm) dans un important réseau de nids artificiels à deux sites de reproduction de la rivière SainteMarguerite, Québec, Canada. Chaque nid artificiel consiste en un cube d'infiltration (30 cm × 30 cm × 20 cm) enfoui dans une unité morpho-sédimentologique ressemblant à un nid de saumon. Nous avons introduit cent oeufs fécondés de saumon atlantique dans plusieurs des cubes d'infiltration durant la construction des nids. Nos résultats indiquent qu'il n'y a pas de relation significative entre la survie jusqu'à l'éclosion au printemps et les valeurs de l'indice de sable, ni avec le pourcentage total de sédiments fins <2 mm dans les nids à ce moment. Cependant, la proportion de limon et de SVFS dans les nids explique 83 % de la variation de la survie des embryons, avec un seuil d'environ 0,2 % de SVFS au-dessus duquel la survie tombe rapidement sous 50 %. L'infiltration de ces fractions très fines se fait surtout sous la glace durant la période d'étiage d'hiver. Cependant, durant la période de crue printanière, les patrons d'infiltration et de curage varient d'un endroit à l'autre en fonction des différences spatiales de l'intensité de la charge de fond transportée et de la disponibilité des sédiments fins.[Traduit par la Rédaction] Levasseur et al. 1459

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The infiltration cube method presented in this paper allows concurrent field measurement of fine sediment content and embryo survival in an incubating environment reproducing, as closely as possible, the morphology and grain size composition of a natural salmonid redd. An infiltration cube is made up of a rectangular (30 cm square and 20 cm high) steel frame with no walls, which has a folded plastic bag attached to its base. Each infiltration cube is buried in the substrate using a procedure aimed at reproducing, as closely as possible, the construction and morpho-sedimentological structure of a natural salmonid redd. Using a tripod installed on the river bed or at the surface of the ice, the infiltration cube method provides a quick, easy and robust way to pull relatively large samples (approximately 65 kg) out of underwater substrate even during tough winter field conditions. The absence of walls on the cube also precludes bias in infiltration rates by both allowing lateral sediment transport within substrate and preventing loss of fine particles during retrieval from underwater.

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