This study examined factors which can affect the quality of fluorescent oxytetracycline (OTC) marks on walleye (Stizostedion vitreum) otoliths. A 1 -time exposure to a strong ultraviolet (UV) light source, such as is done when viewing under a UV microscope, significantly decreased mean OTC mark qualities from an initial value of 2.8 (3.0 is maximum and 0.0 is no mark) to <1.0 3 months following the initial examination. Mark intensity continued to decrease over time (<0.5 after 6 months, 0.3 after 12 months). Otoliths that were stored in an unlit environment consistently had higher intensity marks over time than those stored in a lighted environment. Dimethyl sulfoxide (DMSO) was examined as a possible potentiator to improve OTC mark intensity. Otoliths that were marked by immersion in 200 ppm OTC and 0.81 % DMSO did not exhibit higher quality marks than those which were immersed only in 200 ppm OTC. Fish growth rate immediately surrounding the time of marking appeared to affect the quality of OTC marks. In addition, Walleyes that were fed to satiation throughout the study possessed a mean mark quality of 2.6, while those that were starved or fed 1% of their body weight/day had mark qualities of 2.0 or 1.9.
