An Original Theoretical Investigation of the Evolutionary Ecology of Host-Parasite Interactions in Farmed Tilapia Populations in South West Nigeria

The intensification of tilapia aquaculture in South West Nigeria has created conditions favouring the rapid evolution of ectoparasites, particularly Gyrodactylus species, yet the evolutionary ecology of these host-parasite interactions in farmed fish populations has not been theoretically investigated or empirically documented in Nigerian aquaculture. This study makes original theoretical and empirical contributions to understanding the evolutionary dynamics of Gyrodactylus-tilapia host-parasite interactions in commercial fish farms in Ogun and Oyo States. The study integrates population genetic analysis of Gyrodactylus populations sampled from 18 farms with experimental infection trials designed to measure virulence evolution under different host density and host immune background scenarios. Gyrodactylus isolates from high-density intensive farms and low-density extensive systems were compared for virulence traits including reproductive rate, transmission rate, and induced host mortality. Population genetic analysis using microsatellite markers revealed significant genetic differentiation between Gyrodactylus populations from intensive versus extensive farms (FST = 0.16), consistent with divergent selection pressures. Experimental infection trials demonstrated that intensive-farm Gyrodactylus isolates produced significantly higher host mortality at equivalent infection intensities compared to extensive-farm isolates, providing direct evidence of virulence evolution under intensive aquaculture conditions. A theoretical model is proposed predicting virulence evolution trajectories as a function of stocking density and parasite transmission rates. The study provides the first empirical evidence of virulence evolution in fish ectoparasites in Nigerian aquaculture and introduces original theoretical predictions for managing virulence evolution through farm management interventions.

Keywords: host-parasite evolution, Gyrodactylus, virulence, tilapia aquaculture, South West Nigeria