Sensory capacities and perceptual challenges faced by gillnet bycatch taxa result from fundamental physiological limits on vision and constraints arising within underwater environments. To reduce bycatch in birds, sea turtles, pinnipeds and blue-water fishes, individuals must be alerted to the presence of nets using visual cues. Cetaceans will benefit but they also require warning with cues detected through echolocation. Characteristics of a visual warning stimulus must accommodate the restricted visual capacities of bycatch species and the need to maintain vision in a dark adapted state when foraging. These requirements can be provided by a single type of visual warning stimulus: panels containing a pattern of low spatial frequency and high internal contrast. These are likely to be detectable across a range of underwater light environments by all bycatch prone taxa, but are unlikely to reduce the catch of target fish species. Such panels should also be readily detectable by cetaceans using echolocation. Use of sound signals to warn about the presence of gillnets is not recommended because of the poor sound localisation abilities of bycatch taxa, cetaceans excepted. These warning panels should be effective as a mitigation measure for all bycatch species, relatively easy to deploy and of low cost.