Several fish species that spawn in lotic habitats have a larval-drift phase which is a major determinant of their reproductive success. The main objective of this study was to compare seasonal, diel, longitudinal, transverse, and vertical variations in rates of lake sturgeon (Acipenser fulvescens), Stizostedion spp., Catostomus spp., Moxostoma spp., quillback (Carpiodes cyprinus), and mooneye (Hiodon tergisus) larval drift in Des Prairies River (DPR) near Montreal (Quebec), which is one of the major lotic spawning habitats of the St. Lawrence River system. Larval sampling was conducted in the spring of 1994 and 1995 for the six taxa, and on a more restricted basis for lake sturgeon in 19961998, using drift nets set at several transects, stations, depths, and periods of the day, along a 19 km long section of river beginning ca. 2 km downstream from the DPR power house. For all taxa except lake sturgeon, peak larval drift occurred ca. 1 week earlier in 1995 than in 1994. The sequence was very similar between years, beginning with Stizostedion spp., followed by Catostomus spp., then lake sturgeon, quillback, and mooneye drifting simultaneously, and finally Moxostoma spp. Generally, for all taxa except quillback, whose multimodal drift pattern suggests intermittent, prolonged spawning, larval-drift profiles showed one major seasonal mode, which was observed simultaneously at all transects. For all taxa except quillback, drift rates peaked between 21:00 and 03:00 and were minimal during daylight hours. Lake sturgeon and Stizostedion spp. larval drift rates decreased radically from the most upstream to the most downstream transect, suggesting that both taxa spawn mostly in the vicinity of the DPR power house. More studies are required to explain this longitudinal decline in drift rates, particularly for lake sturgeon. The other taxa showed longitudinal variation in larval drift rates, suggesting that they spawn near the DPR power house and (or) in the Île de Pierre Rapids, ca. 12 km downstream. At all transects, larval drift rates for the six taxa were generally higher in the right half (Montreal) of the river, suggesting that eggs are deposited mostly in this part of the river at the two major spawning areas and that larvae tend to remain in the same general corridors during downstream migration. For all taxa, though to a lesser extent for lake sturgeon, nocturnal drift rates tend to be higher near the surface than at mid-depth and near the bottom, the reverse situation being observed for diurnal drift rates.
Modelling dispersal of eggs and quantifying connectivity among Norwegian coastal cod subpopulations