“We know from a decade of doing survival studies that migrating juvenile salmon are dropping out of the system pretty much everywhere in the Delta,” says UC Santa Cruz fisheries biologist Brendan Lehman. “Physical habitat features are potentially aggregating predators and prey in ways detrimental to salmon smolts and steelhead.” The scientists report in the December 2019 San Francisco Estuary and Watershed Science that artificial light and submerged aquatic vegetation pose the most severe and widespread risks to native fishes. Plants like Brazilian waterweed form dense underwater mats across vast reaches of the Delta. These mats have nurtured a rise in predatory bass populations and force prey fish into exposed channel centers. The artificial lighting pervasive along docks and buildings attracts prey fish while improving the success of sight-hunting predators. The scientists will spend upcoming field seasons examining how lighting and aquatic vegetation affect encounters between juvenile salmon and their predators.