Avril Harder , Freya E. Rowland, Aimee N. Reed

Thiamine deficiency complex (TDC) in fishes has been identified in an ever-expanding list of species and populations. In many documented occurrences of TDC in fishes, rates of juvenile mortality have reached 90% at the population level, with many females producing no surviving offspring. Such sweeping demographic losses and concomitant decreases in genetic diversity due to TDC can be prevented by treating pre-spawn females or fertilized eggs with supplemental thiamine. However, some fisheries managers are hesitant to widely apply thiamine treatments due to the potential for unforeseen evolutionary consequences. Specifically, these hesitations are due in part to apprehension that thiamine supplementation may impede genetic adaptation to low-thiamine conditions or may give hatchery fish an advantage over wild-origin fish. With these concerns in mind, we first review the existing data regarding genetic adaptation to low-thiamine conditions and provide perspectives on evolution-informed treatment strategies with specific population examples. We also provide practical treatment information, consider the potential logistical constraints of thiamine supplementation, and explore the consequences of deciding against supplementation. Until new evidence bolsters or refutes the genetic adaptation hypothesis, we suggest that TDC mitigation strategies should be designed to support maximum population genetic diversity through thiamine supplementation. Furthermore, we offer guidelines on when the adaptation strategy may be applicable to certain populations.