Fish farming is becoming an increasingly popular agricultural activity, and water quality in these environments is a major concern. Fish parasites, such as monogeneans, respond to changes in abiotic conditions, either with an increase or decrease in population. This study aimed to identify gill monogeneans and analyse their relationships with abiotic factors during the ontogenetic development of Nile tilapia over the fish culture cycle in Mato Grosso do Sul, Brazil. Fish were sampled monthly for eight months, and a total of 200 fish were collected. The physical and chemical water parameters were measured and correlated with the abundance of each monogenean species. Over the fish culture cycle, the physical and chemical parameters fluctuated, and the water quality decreased. The parasites found included Cichlidogyrus tilapiae, Cichlidogyrus thurstonae, Cichlidogyrus sclerosus, Cichlidogyrus halli and Scutogyrus longicornis. The abundances of all species showed significant differences during ontogenetic development (body size) and C. tilapiae, C. sclerosus, C. thurstonae and S. longicornis were correlated with changes in abiotic conditions. However, C. halli was not significantly correlated with any of the evaluated physical or chemical parameters. Understanding how different monogenean species respond to changes in the physical and chemical parameters of water during a production cycle can prevent peaks in abundance and subsequent sanitary problems.

The fish farms state of Mato Grosso do Sul (Brazil) have been facing commercialization problems of the Patinga (Piaractus mesopotamicus (Holmberg, 1887) × Piaractus brachypomus (Cuvier, 1818)) due to hight acanthocephalans infection. The aim of this study was to identify the acanthocephalan species that infect this hybrid and assess the influence of management practices on the intensity of the parasite in fish farms. Samples were collected between October 2017 and June 2018. The transparency was the only water quality parameter that differed among the fish farms, and the values were below the limit established for fish production. Echinorhynchus gomesi was recorded in fish farms A, B and C. There was no significant correlation between the intensity of E. gomesi infections and the length of the intestine, and the standard length of the fish. The intensity of E. gomesi also did not affect the condition of the fish. The results of the study demonstrated that inappropriate management, such as cascading tanks, partial emptying of tanks and successive cultivations without time intervals, created conditions for the establishment of the population of E. gomesi in the evaluated fish farms.

Nile tilapia feed on plankton in natural environments. This food source can be increased in ponds through fertilization and can reduce feed expenses or improve fish performance when used as supplementary food. Organic fertilization is an alternative to commercial fertilization; however, its use increases concerns regarding water quality and sanitary aspects. This study aimed to evaluate the effects of the use of poultry litter as organic fertilizer on the physical and chemical parameters of water, gill ectoparasite metazoan abundance, and growth of Nile tilapia (Oreochromis niloticus) during the culture cycle lasting 240 days. Four earthen ponds, two fertilized with poultry litter and two non-fertilized ponds, as fertilized and non-fertilized treatments, respectively, were used. Stocking density was 3 fish per m2 with an average initial weight of 0.64 ± 0.15 g. The mean water quality values and the growth performance parameters of Nile tilapia did not show significant differences between the two treatments. The gill parasites found in the fish belonged to the class Monogenea, comprising two genera, Cichlidogyrus and Scutogyrus, with significant differences between treatments. The non-fertilized treatment showed a high abundance of parasites throughout the culture cycle months, with peak abundance in the months with low concentrations of dissolved oxygen in the water.

A total of 122 Patinga specimens were collected from fish farms (P1, P2 and P3), and only those from fish farm P1 were shown to be infected with Echinorhynchus gomesi. In addition, fish in this study were shown to have diets that consisted of 21 different food items, and Notodiaptomus sp. (Copepoda: Calanoida) was identified as a potential intermediate host for E. gomesi. K E Y W O R D S acanthocephalans, intermediate host, trophic interactions, zooplankton Parasites with heteroxenous life cycles infecting different trophic levels can reveal valuable insights into their hosts' ecology (Marcogliese, 2005; Marcogliese & Cone, 1997), as is the case of the acanthocephalans that are intestinal parasites (transmitted via ingestion), and use arthropods (insects and crustaceans) as intermediate hosts and birds, mammals, amphibians, reptiles and fish as definitive hosts (Crompton & Nickol, 1985). Particularly, species that infect fish use aquatic arthropods (Amphipoda, Copepoda and Ostracoda) as intermediate hosts to complete the life cycle, as these zooplankton groups commonly form part of the diet of fish (Kennedy, 2006). Most cases of acanthocephalans parasitism, recorded in fish farms from Brazil, are identified in round fish (Piaractus mesopotamicus, Piaractus brachypomus, Colossoma macropomum, Patinga, Tambacu and Tambatinga hybrids), with a main focus on the influence of abiotic factors on acanthocephalans colonization and the implication of these infections on the development of the host (De Matos et al., 2017;

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