Diel variability in fish assemblages in coastal wetlands and tributaries of the St. Lawrence River: a cautionary tale for fisheries monitoring

Both coastal wetlands and tributaries of waterbodies provide important and distinct habitat for freshwater fishes. While diel migration into and out of these systems is known to occur for some species, the resulting changes in fish assemblage composition and dominance are less well understood. To evaluate diel changes in the fish assemblages of a coastal wetland and tributary, fish community surveys were completed at ten locations during the day (noon) and night (midnight) in Cooper's Marsh, a coastal wetland in the St. Lawrence River, and the Raisin River, a nearby tributary. Catch per unit effort (CPUE) and species richness were highest in the coastal wetland during the night sampling period. Species-specific differences were also apparent with high CPUE of Mimic Shiner in the marsh at night. Differences within the river were less pronounced, suggesting less diel variability in fish assemblage structure, possibly driven by the more constrained nature of fluvial systems. These results contribute to our understanding of diel movement patterns of fishes and the natural diel variability in species assemblages that can occur. These findings also emphasize that when engaged in environmental monitoring and assessment, it is important to consider how diel variation in fish assemblage structure will influence conclusions regarding the biotic components of a given aquatic ecosystem.