Factors Associated with Increased Walleye Production in Lined Compared to Earthen-Substrate Ponds

Walleye (Sander vitreus) fingerling production can be enhanced when the pond substrate is altered from earthen soils to an exposed liner;however, few differences in water chemistry, prey densities, and food habits have been identified that could potentially explain this production difference. Lack of an explanation led to further comparison of walleye fingerling culture between substrate types during 2016. Two ponds of each type were stocked with 1- to 3-day-old walleye fry, but lined ponds received an additional 104,166 fry/ha and were stocked 1 or 2 days later than earthen ponds (May 10). Walleyes in lined ponds achieved a growth advantage on May 31 (+2.5 mm) and on June 6 (+3.3 mm), but lengths were not significantly different at harvest (June 15). Lined ponds increased walleye yield and number 1.8-fold while not reducing individual fish size at harvest compared to earthen ponds. Walleye food habits were generally similar between pond types with a greater number of zooplankton being consumed through June 6 followed by an increase in larval chironomid consumption at harvest (June 15). After May 30, lined ponds exhibited lower dissolved oxygen and higher ammonia-nitrogen concentrations even though earthen ponds received additional (157 kg/ha) alfalfa (Medicago sativa) meal fertilizer. Also, at this time pH declined abruptly and remained below 8.2 in lined ponds. Moreover, the enhanced production in lined ponds coincided with walleyes that exhibited an increased pelagic (δ13C) signature during the culture period (2‰ to 3.3‰ more negative). Efforts to improve production in earthen-substrate ponds should focus on increasing the pelagic signature of the food web through fertilization strategies which increase ammonia-nitrogen (≈0.3 mg/L) and decrease dissolved oxygen (≈6.5 mg/L) while maintaining a pH between 8.0 and 8.2 during the second half of the rearing interval.