4.2. Sampling Design and Sample Collection

Fish were sampled from two cohorts at three time points (transfer, pre-treatment with praziquantel and harvest). Transfer sampling occurred when the fish were transferred from the towing pontoon to a ranching pontoon, pre-treatment sampling was done a couple of days before praziquantel treatment and harvest sampling was at commercial harvest. The exact dates were dictated by commercial objectives, as a result, the cohort which was transferred earlier was harvested later (Table 4).

Fifteen fish were sampled from each cohort at each sampling point. At transfer and pre-treatment, the fish were caught on line and at harvest standard commercial harvest procedures were used. At transfer, fish weight and length were measured and the presence of ectoparasites (monogenean Hexostoma thynni and two species of copepods Pseudocycnus appendiculatus and Euryphorus brachypterus) determined as an additional measure of the cohort health. The characteristics of the two cohorts are presented in Table 5. Samples of gills and internal organs were fixed in 10% phosphate buffered formalin and processed for histology using routine procedures. Heart and gill samples were collected to determine the severity of infection with blood flukes using adult counts in heart flushes [16] and real-time qPCR [13]. Unless processed fresh (heart flushes), the samples were stored frozen after being fixed in RNAlater. The samples were obtained from commercially harvested fish, therefore, according to local regulations and no Ethics approval was required.