Submerged macrophyte control with herbivorous fish in irrigation channels of semiarid Argentina

Two experiments were conducted in Río Negro province (Argentina) to study macrophyte response to different stocking rates of the grass carp (Ctenopharyngodon idella Val.). The first of these was performed in nine polyethylene-lined ponds, 12 m2 each, planted with Illinois pondweed (Potamogeton illinoensis Morong) at two fish stocking rates, 100 and 360 kg ha-1. The second experiment was set up under natural conditions in a channel full of Potamogeton pectinatusL., Myriophyllum aquaticum (Vell.) Verd. and Chara contrariaA. Braun ex Kurtz, at 100 and 200 kg ha-1. In both cases, the initial individual fish weight was between 22 and 40 g. A complete randomised block design was used in both experiments. Five samples of plant biomass were taken per experimental unit. Data were subjected to ANOVA and LSD was used as a comparative test. In both cases, the experimental period was 2 months because of the herbivory under the high stocking rate completely depleted both the leaves of P. illinoensis under pond conditions and the aboveground plant biomass in the channel. The results showed a high potential of biocontrol under tested conditions and suggest a stocking rate lower than 100 kg ha-1 as enough for local conditions and for a prolonged fish presence in the area.