Reservoir fish assemblage structure across an aquatic ecotone: Can river-reservoir interfaces provide conservation and management opportunities?

River-reservoir interfaces have been described as aquatic ecotones and contain strong environmental gradients of depth, turbidity and trophic resource abundance. These transitional habitats have traditionally been excluded by riverine and reservoir management schemes despite their prevalence in modern riverscapes. By systematically sampling shoreline habitats along a river-reservoir interface gradient from riverine to lacustrine zones with trammel nets in 2018-2019, strong patterns were identified in total species captured and individuals captured in the San Juan River-Lake Powell inflow, USA. Changes in assemblage structure were driven mainly by increases in relative abundance of benthic omnivores towards the riverine zone, including imperiled razorback sucker,Xyrauchen texanus(Abbott), but also by increases in predatory species, such as striped bass,Morone saxatilis(Walbaum). Inter-annual variation in species distributions along the river-reservoir interface gradient was likely influenced by variation in reservoir water level that differed by nearly 12 m between years. River-reservoir interfaces provide high-quality feeding and potentially spawning areas for both benthic omnivores and piscivores, and these areas should be considered in management and conservation efforts for species using these habitats.

Automated summary

River-reservoir interfaces serve as important habitats for benthic omnivores and piscivores, with changes in assemblage structure driven by fluctuations in reservoir water levels and environmental gradients along the gradient. Conservation and management efforts should take into account the significance of these transitional habitats for species utilizing these areas.