Tilapia for biological control of giant salvinia

In August 1999, giant salvinia (Salvinia molesta Mitchell) was found along the lower Colorado River in irrigation drainages. To investigate the slow spread and apparent control of giant salvinia in this region, the herbivorous fish, tilapia (Oreochromis niloticus Trewavas), was examined as a biological control agent. The study was conducted in a 5,000-L recirculating system. One of four treatments was assigned to each of twenty 200-L tanks after they were stocked with tilapia at a density of five fish per tank. The first treatment group, the giant salvinia control, contained giant salvinia only; group two, the fish control, contained fish only and was fed a commercial diet; group three, the giant salvinia and fish minus feed treatment, contained fish and giant salvinia; and group four, the giant salvinia and fish plus feed treatment, contained fish and giant salvinia, as well as being fed a commercial diet. Changes in giant salvinia biomass were statistically different among all treatments (p < 0.0001, F_{2, 12} = 49.4370), with greatest change occurring in the giant salvinia and fish minus feed treatment (-491 g). Average fish growth was 13.7 g greater in the giant salvinia and fish plus feed treatment than either the fish control treatment or the salvinia and fish minus feed treatments. These findings could explain why giant salvinia has had limited dispersal on the Lower Colorado River, as tilapia are ubiquitous.