General ecological and geographic patterns

Parasite abundance was calculated by dividing the total number of Gyrodactylus sp. worms found on farmed “tilapia” by the total number of fish examined in each farm and state surveyed (Table ​(Table1);1); i.e., no precise ecological data are available for individual gyrodactylid species, as not all parasite specimens were identified taxonomically and no ecological data are presented for Gyrodactylus sp. infections of native fishes. Significant numbers of parasites were dislodged from fish during transport, so we decided against presenting data on prevalence of infection (% of the sample infected). The proportion of parasite species reported for each state was calculated from the subsample of worms characterized morphologically and molecularly (Table ​(Table1).1). From this partial data (subsample), a summary map was constructed for visualization of this information using QGIS 3.6.2 software⁵⁶.

To compare mean abundances of Gyrodactylus sp. between regions, data from farms in these regions was pooled (East = 14 farms, North-West = 14, Centre-South = 12), and the Kruskall-Wallis non-parametric test was performed to detect differences between these groups. Paired Mann–Whitney U tests were used to identify significantly different groups. Statistical tests were performed in R 3.6.0⁵⁷.