Contact DPI they talked to me about a poison that you could put in my dam to get rid of tilapia . the only problem is that it will kill everything in the dam basically the whole ecosystem and then start again . the other option that is apparently having some succes with the tilapia up here is stocking with barra as the barra are using the tilapia as food , they will never get all the tilapia but they will control the numbers a bit