Effects of water physico-chemical properties, pollution and parasitism on farmed fish in Nyeri County, Kenya
The general objective of the study was to evaluate fish farmer management practices and determine effects of physico-chemical properties of fish pond water, heavy metals and parasites on farmed tilapia and catfish in Nyeri County, Kenya. 117 fish farmers were interviewed using semi-structured questionnaires in Nyeri Central and Tetu subcounties. Most farmers in Nyeri Central sub-county used earthen ponds while they practiced tilapia monoculture as the most dominant fish culture method compared to Tetu subcounty. Most farmers fertilized their ponds and were using commercial fish feeds. All the interviewed farmers grew different types of cash crops with most from Tetu applying fertilizers and pesticides on their crops compared to Nyeri Central. A high proportion of farmers in Tetu kept livestock and applied chemicals on them compared to Nyeri Central sub-county. Most farmers in both subcounties disposed of excess chemicals either in a composite pit or pit latrine and did not empty their ponds after harvesting fish. Water samples were collected from 15 fish ponds using one liter sample bottles and were tested for physical and chemical indicators (temperature, pH, dissolved oxygen, electrical conductivity, total hardness as calcium carbonate, nitrates, phosphates, iron, manganese, lead, copper, zinc, and cadmium). Water temperature, pH, electrical conductivity nitrates, manganese and lead did not differ significantly (p>0.05) between different pond types and among the sub-counties and were within the acceptable Food and Agriculture Organization (FAO) of the United Nations limits. Phosphates and iron ion concentration similarly did not differ significantly (p>0.05) among pond types and sub-counties but exceeded FAO acceptable limits. Fish tissue samples (muscles) were analyzed for the presence of heavy metal residues (lead, mercury and cadmium) using the Atomic Absorption Spectrometry technique. Tilapia accumulated higher concentrations of the three heavy metals compared to catfish. The levels of lead and mercury were not significantly different between the two fish species. Fish obtained from earthen ponds had higher mercury and cadmium concentrations that slightly exceeded permissible limits stipulated by European Union while lead was within European Union limits. A total of 366 fish had their eyes, skins, gills, muscles, stomachs and intestines examined for parasites using dissecting and compound microscopes. 115 out of 366 fish were infested with one species of ecto- or endo-parasite in different organs giving an overall prevalence of 31.4%. The parasites recovered in the study were Dactylogyrus spp., Clinostomum spp., leeches (Pisciola spp.), Diplostomum spp., Trichodina spp., Acanthocephalus spp., Contracaecum spp., Gyrodactylus spp. and Paracamallanus spp. Parasitic infestation rate was significantly higher (P<0.05) in tilapia relative to catfish and there were no significant differences (P>0.05) in fish infestation rates between earthen and liner ponds. Among fish raised in earthen ponds, there were significant differences (p<0.05) in the prevalence of parasitic infestation between catfish and tilapia. Gross lesions occurred in fish in the liver (grey/pale liver), skin (grayish/ black nodules as well as haemorrhagic spots on the lateral or ventral aspect), malformed dorsal fin, kinked tail and the eye (cloudiness) but there were none observed in the intestines, muscles, gills and stomach. Tilapia had a higher proportion of occurrence of gross lesions compared to catfish while a higher proportion of fish from earthen ponds had gross lesions compared to those from liner ponds. The occurrence of gross lesions in this study was not associated [(α =0.05) χ2 = 0.415 < 3.84] with presence of heavy metals concentrations above the European Union limits in fish. Histopathological lesions were present in fish tissue samples either in the liver, skin, eye, intestines, muscles, gills and stomach. The occurrence of histopathological lesions was higher in tilapia than catfish. There were no significant differences in occurrence of the lesions between fish cultured in earthen and lined ponds. The occurrence of histopathological lesions in this study was not associated [(α =0.05) χ2 = 0.415 < 3.84] with presence of heavy metals concentrations above the European Union limits in fish. In conclusion, fish farmers in Nyeri County should be equipped with knowledge and skills on fish farm management to maintain acceptable water quality, avoid spread of fish parasites and introduction of chemical pollutants to improve the productivity of farmed fish. The parasites reported in this study may affect the health and quality of fish leading to condemnation at inspection. Further studies are indicated to determine risk factors, economic impact and control of these parasites in farmed fish in central Kenya.