Relação parasito-hospedeiro-ambiente durante o ciclo de cultivo de tilápia do Nilo (Oreochromis niloticus) em sistema semi-intensivo

Abstract

The phytoplanktonivorous feeding habit of tilapia is one of the reasons for the advancement of the cultivation of this species in Brazil and worldwide, as it minimizes feed costs, giving space to alternative foods, such as natural plankton. For plankton growth it is commonly used fertilizer which, in the case of organic, is a low cost alternative to the producer, using waste from other farms. Concerns about water quality and pathogens regarding organic fertilization are constant and studies on the parasites in these rearing systems are scarce. Thus, the objective of this work was to evaluate the growth of Nile tilapia and gill metazoan in nurseries containing organic manure (poultry litter) and the use of monogenean as bioindicators of changes in water physical and chemical variables in fish farming. Four ponds were used in fish farm in Laguna Carapã, state of Mato Grosso do Sul, Brazil, two containing fertilizer organic, poultry litter (fertilized treatment) and two without fertilizer (unfertilized treatment). The management and the density were the same for both treatments and the fish collections were done monthly, after the biometrics, and the condition factor was calculated. The zootechnical performance was also evaluated through the weight gain (GP), survival, feed conversion and feed offered, then the gills were removed and the parasites quantified and identified. To evaluate the use of monogeneans as bioindicators, the collections were carried out monthly, using 200 Nile tilapia specimens in fish farm. The physical and chemical variables of the water were also measured before the fish collection, thus correlating the abundance of the species with the physical and chemical variables of water. All parasites found belonged to the Monogenean class of species: Cichlidogyrus halli, C. tilapiae, C. thustonae, C. sclerosus and Scutogyrus longicornis, for both treatments. The average abundance of Cichlidogyrus and Scutogyrus was higher in nurseries without fertilizer, with significant differences between treatments (p = 0.14), being 2.945 specimens in the fertilized treatment and 5.792 in the fertilized, with abundance peaks in February/2018 and March/2018 for unfertilized treatment, in conjunction with decreased dissolved oxygen. There were no significant differences in fish weight gain, but in fertilized ponds there was a saving of 90.48 kg of feed, and survival was higher, thus evidencing the positive effects of using organic fertilizer in adequate doses in system semi-intensive. Regarding the bioindication, the C. tilapiae, C. thustonae, C. sclerosus and Scutogyrus longicornis were sensitive to environmental changes during the cultivation cycle, being indicators of variations in physical and chemical parameters. Thus, they have proven to be a low cost and relevant alternative for monitoring the quality of fish water.