Although the individual variability in the number of A. ocellatum per fish was
considerable, the reduction in the number of parasites after the formalin treatments
was evident. After 1 h of exposure, the 51 mg l 1 formalin treatment (Fig. 3) reduced
significantly the number of parasites on the skin (97%) and gills (68%) of the fish in
comparison the controls ( P= 0.001 and P= 0.009, respectively). The comparison
between controls before and after the treatments did not show significant differences
for skin and gills (P= 1.000 and P= 1.000, respectively). Hundreds of trophonts were
collected from the bottom of the container after the formalin treatment. Only three fish
died after 20 min of exposure, representing less than 2% of all exposed animals. All
the remaining fish were swimming normally and began to feed the day after the