Tilapia were introduced into Queensland in the late 1970s, and over the last 5 years have spread significantly throughout that state and are now seriously threatening invasion of the Murray-Darling Basin. This project was set up to collect critically-needed ecological and life-history data to inform a coordinated management program.

Progress:

Monthly sampling was conducted at two impoundments (Tinaroo and Herbeton), in an unregulated river (Mulgrave), an urban canal (Kewarra) and a set of artificial lagoons (Paradise Palms). Sampling techniques used included netting, electrofishing and trapping. Over a 15 month period 5,500 tilapia were caught. Some 500 gonads were sectioned and stained for histological examination - confirming the extended reproductive seasonality of both species of invasive Tilapia (Oreochromis mossambicus and Tilapia mariae).

Preliminary data analyses have shown some big differences in life history and population parameters to what has been previously documented in the literature. There were also some variations between local populations. For example, the average batch fecundity of a 'stunted' population of O. mossambicus was found to be 300 eggs per fish compared to a normal population that had over 2,000 eggs per fish. Similarly, for T. mariae, eggs counts of more than double that documented in the literature have been recorded. Three thousand otoliths were also examined for data on growth and population age structure. The results of this experiment indicate that the opaque banding on the otoliths of both species is laid down annually, ie. is a relatively reliable indicator of age.

This project is using computer modelling to evaluate a range of management scenarios, and will make recommendations on appropriate strategies for use in Queensland. Work is underway to adapt the CarpSim software for use with tilapia. This model has been successfully populated using parameters from a north Queensland O. mossambicus stock. A number of management scenarios for this population have already been tested.

Efforts are now being concentrated on modifying the CarpSim model for use on stocks of T. mariae. A number of areas where the CarpSim software could be modified to make it more useful as a tilapia management tool have been identified. These include simulating the impacts of predator introductions and the ability to model the outcomes of multiple, simultaneously applied control measures. These enhancements are currently being considered by the software developers.