Evaluation of blue tilapia (Oreochromis aureus) for duckweed (Lemna minor) control in South Carolina's private waters

We investigated the potential of Blue Tilapia ( Oreochromis aureus) as a means for biological control of duckweed in agricultural impoundments, and assessed the potential for naturalization of this non-native fish in coastal regions of South Carolina. Additionally, we studied growth characteristics and nutrient accumulation abilities of duckweed (Lemna minor) growing in agricultural impoundments in the piedmont and coastal plain. Observations of tank feeding trials indicated juvenile Blue Tilapia consumed 36 -- 86% of their body weight in fresh duckweed every 24 hours. Tilapia were stocked in agricultural impoundments in the piedmont and coastal plain during the 2012 and 2013 growing seasons. Duckweed population densities (19, 310 kg/ha) at time of stocking prevented survival of tilapia due to anoxic conditions. As a result, we were unable to measure duckweed control by tilapia in impoundments. Cold tolerance trials were conducted using juvenile Blue Tilapia (38 -- 54 mm TL) to determine LT50 and LT100. Observed lethal temperatures were LT 50 = 8.8° C and LT100 = 6° C. Investigations of duckweed growth characteristics in agriculture impoundments from April to October revealed monthly changes in duckweed water surface coverage and biomass. Duckweed coverage of ponds was highest in August (98.75%) and lowest in October (77.8%). Duckweed biomass was highest in July (2.93 kg/m2) and lowest in October (1.76 kg/m2). Attempts to predict duckweed biomass in terms of percent surface coverage were unsuccessful due to variability of biomass as surface coverage neared 100%. Duckweed tissue was analyzed to determine nutritional quality, nutrient concentrations, and bioconcentration ability. Observed crude protein (20.38%) and fat (2.38%) contents indicate duckweed was comparable to other plant sources of protein in livestock feeds. Nutrient concentrations observed in duckweed tissue in descending order were: K, Ca, Fe, Mn, P, Mg, Zn, and Cu. Analysis of nutrient accumulation indicated duckweed bioconcentration was significant for all measured nutrients. We ranked nutrient accumulation for macro and micro nutrients. Rankings were as follows: primary macro (P > K), secondary macro (S > Ca > Mg), and micro (Mn > Fe > Zn > Cu). Based on results of this study, Blue tilapia have potential as a biological control option for duckweed when used responsibly. Additionally, duckweed has potential as a feed source for livestock, biomass production, and bioremediation of waters.