HIGH MORTALITY OF POND-RAISED FISH
During the last several months (April-June), we have received
several phone calls from pond owners concerning high mortality
of pond-raised fish. These ponds are commonly adjacent to
cultivated farm land and the pond owners usually suspect poisoning
of the fish due to agricultural chemicals, such as herbicides
or pesticides which have drained into'the ponds.
In some of these cases we have detected non-toxic levels
of agricultural chemicals in pond water samples. Several
of these water samples contained toxic level of nitrites .
Nitrites are by-products of ammonia, a waste product which
is excreted through the gills of fish. Ammonia is normally
degraded to nitrites, by Ntro-somonas bacteria. Nitrites
are degraded to nitrates which are less toxic to fish by Nitrobacter
bacteria.
Nitrites are toxic to fish due to the oxidization
of iron in the hemoglobin resulting in methemog1obinemia.
This reduces the oxygen-carrying capacity of the blood.
Nitrite toxicosis most commonly occurs in the spring and summer
months, due to the rapid growth of fish with concurrent increases
in fish waste products which "overloads" the Nitrosomonas
and Nitrobacter bacteria. Treatment includes less intensive
feeding of pond fish, and the addition of salt (0.396 Nad)
to the pond.
In order to diagnose nitrite toxicosis, pond-water samples
should be collected at the first notice of fish dying. The
ADDL toxicology laboratory requests that a minimum of one
quart of water be submitted in a clean glass jar which is
covered with aluminum foil before placing the lid on tightly.
If herbicides or pesticides have been applied to crops adjacent
to the pond, please indicate which specific chemicals have
been used.
Randy White, DVM,PhD Veterinary Pathologist
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