Visual Acuity of Two Tunas, Katsuwonus pelamis and Euthynnus affinis

Two tunas, Katsuwonus pelamis and Euthynnus affinis, were trained to discriminate between vertically and horizontally striped images. The striped images were projected on a plate of opal glass which rested against a window of the experimental tank. Visual acuity depended upon the width of the stripes and the distance between the fish and the window at the instant the projector was turned off. The fishes were adapted to an illumination of 170 luxes. At lower luminances of the white stripes, the visual acuities of the two species were similar. At higher luminances of the white stripes, K. pelamis had a greater visual acuity. Maximum visual acuity for K. pelamis was about 0.180 and for E. affinis about 0.135.