Our results show that pond depth is a significant predictor of FZT infection
in juvenile fish and that this association is likely due to the effect
of water depth on snail population density. The associations although
significant are, however, weak and it is obvious that many other factors
could play a role in determining level of infections in fish and density of
host snails. Only few studies have looked atmicro-distribution of snails
within ponds. In the Red River delta, Boerlage et al. [21] were able to
find snails in all sections of ponds, but they did not estimate densities.
Hung et al. [22] observed that density of thiarid and viviparid snails decreased
with distance (and depth) from the pond edge, i.e. density of
thiarid snails at 50–100 cm, 100–150 cm and 150–200 cm from the
shore was 0.81, 0.63 and 0.51, respectively, of that in the section
0–50 cm from the edge. Our sampling focused on the sloping sides of
the ponds and this may have biased snail density. We cannot rule out
the possibility that snail distribution pattern differs between shallow
and deep ponds. This requires further investigation. Variation in snail
density among ponds may be related to the area of location and various
on-farm practices related to pond management