Age, but not salinity, affects the upper lethal temperature limits for juvenile walleye (Sander vitreus)

The walleye (Sander vitreus) is a North American freshwater fish that has been introduced into waters outside its native range for sport fishing purposes. Heat has been identified as a potential method to produce sterile walleye as a means of controlling introduced populations. However, in order to design experiments to study the effects of elevated temperature on developing walleye germ cells, it is necessary to establish the ultimate upper incipient lethal temperature (UUILT) for different juvenile age classes. An additional variable that was studied was the influence of adding modest amounts of salt (i.e., NaCl) to the water to increase the UUILT. To address these questions, the UUILT was estimated for sub-yearling (57-95 mm total length) and yearling (235 mm total length) walleye at salinities of 0.2 (control), 2.5, 5.0, and 10.0 parts per thousand (ppt) by increasing the temperature by 1 °C per day in each of these treatments until death. The UUILT for sub-yearling fish (34.1 °C) was significantly (p < 0.05) higher than that for yearling fish (31.6 °C) in this study. The UUILT for yearling walleye at 2.5 ppt (31.0 °C) was significantly (p < 0.05) lower than those for yearling walleye at the other salinities tested (31.6 °C), which were not different from one another. Increased levels of salinity were not found to have a significant effect on the UUILT of sub-yearling fish. These results demonstrate that salinity has a negligible effect on the upper thermal tolerance of juvenile walleye, but show that the upper thermal tolerance changes with age during the juvenile life history period.