EFFECTS OF STOCKING DENSITY AND SEASONALITY ON DIGENEAN TREMATODE AND MONOGENEAN INFECTIONS IN NILE TILAPIA (Oreochromis niloticus, Linnaeus 1758) REARED IN CAGES IN UHANYA BEACH IN LAKE VICTORIA, KENYA

Abstract

There exists a balance between, parasites, their fish hosts and the environment. However, when this balance is broken parasites spread rapidly in culture facilities inducing severe epizootics. This study was conducted from December 2017 to May 2018 with an objective of highlighting stocking density and seasonality as drivers of monogenean and digenean trematode parasitism in Oreochromis niloticus in cages in Uhanya Beach, Lake Victoria, Kenya. The study adopted a systematic random sampling technique to sample an aggregate 600 fish during the rainy and dry seasons. Parasites isolated from the sampled fish were the monogenean Dactylogyrus and the digeneans; Tylodelphys, Clinostomum and Neascus. Dactylogyrus sp. was predominantly abundant in the fish from all the 10 cages. Mann-Whitney U-test revealed that mean intensity of Dactylogyrus sp. infestation considerably augmented with higher stocking density of fish (p < 0.05; df =298). On the contrary, Clinostomum sp., Tylodelphys sp. and Neascus sp. exhibited significantly reduced infestation with increased stocking densities (p < 0.0001). Temperature significantly differed between the seasons (df = 28; p < 0.0001). All the individual fish showed allometric growth and the general fish condition as depicted by condition factor (Kn) was significantly better in cages with stocking densities of 2500 individuals/ Cage (df = 298; p < 0.05). Kruskall-Wallis test indicated significant variation in prevalence of attack between the parasite species both in wet season (H = 17.793; df = 3; p < 0.0001) and dry season (H = 30.226; df 3; p < 0.0001). There was a general positive correlation between water temperatures and mean parasite intensity, however, this relationship was weak in all the parasite species (r2 < 0.5). The relationship between intensity of parasite infestation and fish condition factor was not also weak and significant in all the parasite species (Dactylogyrus sp.: r2 = 0.017; Clinostomum sp.: r2 = 0.001; Tylodelphys sp.: r2 = 0.008; & Neascus sp.: r2 = 0.026). The study recommends proper site selection for cages, water quality management and sourcing of fish seed from certified hatcheries to prevent the spread of parasitic diseases.