Although all the fish ponds studied were relatively shallow (0.4–
1.3 m), there may be direct effects of water depth on snail populations,
e.g. temperature fluctuations may be less in deeper ponds because of
the larger volume ofwater.Water temperature also influence transmission
of FZT by affecting parasite intra-snail development rate and
survival of emerged cercariae, although it is uncertain whether temperature
conditions vary sufficiently between ponds within these depth
ranges to affect cercariae survival in ponds. Finally, another factor that
may play a role in regulating snail populations is the association between
water quality and pond volume. Nhan et al. [26] showed that a
substantial amount of the organic material added as fish food in growout
ponds, accumulates in the sediment, and depending on water exchange,
various physico-chemical water quality parameters are affected,
e.g. oxygen content and subsequent algal growth. This may be
slightly different for nursery ponds as the standing crop of fish is relatively
low at least initially in the nursing cycle. Studies on feed utilization
and accumulation in sediments and how this affects water quality
and subsequent snail populations in nursery ponds are thus required