The effect of thermal stress on walleye fry and fingerling mortality

We performed laboratory and field tests on the relationship between temperature stress, fish size, and mortality in walleye Stizostedion vitreum. Laboratory treatments included fish size (fry and two sizes of fingerlings), temperature increase, acclimation temperature, and tempering rate. All variables significantly influenced walleye mortality. Smaller sizes of walleye were more vulnerable to thermal stress than large walleyes. Fry (mean total length, TL = 9.3 mm) acclimated to 14°C experienced increased mortality when transferred to final temperatures of 16–26°C, corresponding to temperature increases of 2–12°C. Small fingerlings (44.0 mm) experienced little mortality at final temperatures less than 31°C. Large fingerlings (95.9 mm) had markedly lower mortality rates, with high mortality only when temperatures were increased by greater than 11°C (final temperatures 32°C and greater). Both increased acclimation temperature (24°C versus 20°C) and slower tempering rates (0.02°C/min versus 1.50°C/min) significantly reduced mortality for small and large walleye fingerlings at temperatures (31–32°C) just below upper lethal values. Results of laboratory experiments were confirmed in field enclosures. Mortality at 24 h after stocking ranged from 0–100% for fry and fingerlings, and positive significant relationships were observed between mortality and lake temperature at stocking for fry and small fingerlings. Survival of stocked walleye can be improved by matching hatchery temperatures to ambient lake temperatures, and by avoiding stocking or by using long tempering periods when ambient temperatures are greater than 22°C (for fry stocking) or 30°C (for fingerling stocking).