Physiological and Behavioral Stress Responses of Walleyes Transported in Salt and Buffered-Salt Solutions

Abstract

To evaluate the value of transporting fingerling walleyes Stizostedion vitreum in buffered sodium chloride, we monitored blood chemistry, behavior, and water quality in conjunction with two 5-h transport trips. The control salt mixture was 0.50% sodium chloride (NaCl), and the buffered treatment was 0.45% NaCl and 0.05% sodium bicarbonate (NaHCO₃). We also collected posthaul blood samples from walleyes 2 h after they were placed into cages in East Okoboji Lake, as well as daily during a 72-h recovery period in hatchery raceways. Behavior was tested before and after hauls to quantify any disorientation that might occur as a result of hauling procedures. The addition of NaHCO₃ to haulingtank water significantly increased the pH and alkalinity of the water compared with the unbuffered control. Overall, buffering produced significant effects on water chemistry but only minor effects on physiological measures of stress and the acid–base balance. In part, the effects of ram-air ventilation in removing excess CO₂ and the fact that the water had moderate alkalinity levels to begin with negated the beneficial effects of buffering. Buffering is likely to confer benefits in situations where is alkalinity is low and there is poor ventilation of the hauling tank. Before and after hauls, we tested the preference of walleyes for a black or white tank background with a simulated predator (black brush). In both hauls, before the haul fish in all groups chose a black background more often than the white background. After hauling, fish in the control group sometimes showed preference for a white background, but fish in the buffered water never showed a preference for a white background. Many fish stocked directly into the lake remained near the lake bottom close to the boat ramp for at least 5 h after stocking. The most conspicuous changes in blood chemistry occurred in fish 2 h after they were stocked into floating cages. Our results suggest that fingerling walleyes are disoriented following a 5-h haul and would benefit from a recovery period prior to stocking.