Spatial risk estimation of parasitic infestations of pond and cage cultured Nile tilapia (Oreochromis niloticus) in the Lake Victoria Crescent, Uganda.

Abstract

In pond and cage culture systems, parasites live concurrently with fish. However, parasitic infestations arise when this relationship is interfered with. This comes about when there is poor water quality, poor nutrition, inappropriate fish handling, low water exchange etc., which exacerbate the parasite proliferation. Parasites affect fish by disrupting fish nutrition, metabolism, secretory functions of alimentary canal, osmo-regulatory system, damaging the nervous system whereas reducing fecundity, resistance to stressors and increasing fish's susceptibility to other infections. As a result, misfortunes such as mortality, increased costs associated with mortality, morbidity and treatment, reduced growth, low feed conversion ratios and rejection of products happen. The overall aim of this study was to evaluate risk factors and their contribution to parasite infestation in the Lake Victoria Crescent and then create quantitative risk maps for estimating the risk of parasite infestation at fish farms. The study included active Nile Tilapia pond and cage farming sites located in the 10 districts in the Lake Victoria Crescent .

Using Spatial Areal Unit Modelling with Conditional Autoregressive Priors, without spatial component, out of 15 risk factors (water quality and farm management practices), only intermediate hosts had a significant effect on parasite infestation and only intermediate hosts was included in the final model. There was no spatial clustering in parasite infestation among fish farms (only localized infestation present) since, there was no spatial autocorrelation in the residuals from the final model. Based on the fitted values from the final model, a map presenting the estimated parasite infestation in fish farms in the Lake Victoria Crescent was produced and 71% (23 of 32) of fish farms had a high estimated parasite infestation.

The parasite infestation risk map produced provides a very informative visual summary of the extent of parasite infestation in the Lake Victoria Crescent. The best farm management practices and good water quality identified may not only be utilized by fish farmers as a parasitic infestations preventive measure but also used by policy makers in coming up with contingent plans to prevent the emergency of parasitic infestations.