Relative importance of direct and indirect effects of starvation for young walleyes

Starvation can directly affect energy levels and mortality rates in fish and may have indirect influences on behaviors essential to survival. We examined physiological and behavioral effects of food deprivation for larval (8–24 mm) and juvenile (small, 45–70 mm; medium, 80–120 mm; and large, 140–230 mm) walleyes Stizostedion vitreum encompassing critical periods in their early life history. Juvenile walleye size ranges encompass important ontogenetic diet shifts (small and medium) and the overwinter period (large). The direct effects of starvation on total body energy (J/g wet weight) and mortality rates of larval and juvenile fish were assessed in both laboratory tanks (420 L) and pond (0.4 ha) experiments. Temperature (15–25°C) affected mortality rates and the depletion of energy reserves. During the overwinter period (4°C), initial energy losses were observed that did not affect mortality through 150 d. A “point of no return” was observed for larval fish after 6–8 d of starvation. Larval and juvenile fish (all sizes) survived longer at low energy densities when temperatures were reduced. We also assessed the indirect effects of starvation on foraging efficiency and vulnerability to predation in laboratory experiments. Capture efficiency and handling time was affected for larval fish after 5 d and for juveniles after 21 d of starvation, but no effects were observed for overwintering fish. Starved fry were more vulnerable to predation than fed fish were after 6 d, and juveniles were more vulnerable after 21 d of starvation. Direct effects of starvation were more important to survival than indirect behavioral effects, but for both direct and indirect effects, the length of the period to critical starvation levels increased with fish size.