In cultured juvenile Pacific bluefin tuna (Thunnus orientalis), reducing the mass deaths caused by collision or contact with tank or net walls at night is a priority for seedling production. Pacific bluefin tuna is a visually dependant species, although its scotopic vision is poor. We recorded electroretinograms to investigate the visual function with growth in the dark-adapted eyes of juvenile Pacific bluefin tuna. Peak wavelengths of spectral sensitivity [38-62 days posthatch (dph), 77-167 mm standard length (SL)] were observed between 474 and 494 nm. Visual light sensitivity has a tendency to increase slightly with growth at 28-64 dph in individuals that measured 29-175 mm SL. However, visual temporal resolution did not significantly increase with growth at 38-62 days dph in individuals that measured 77-167 mm SL. These results suggest that the mass death continues between 28 and 64 dph because of low visual function and increasing swimming speed with growth.