Importance of Sample Selection
Good sample selection is critical. Moribund
fish showing typical disease signs are preferable.
If several fish can be killed when evaluating a
population, fish should be chosen at different
stages, including at least one with early stage
disease. Fish with late stage disease often have
secondary complications that mask initiating
causes.
For large populations of low disease prevalence
with mortality as the only sign, random
sampling may result in the selection of healthy
fish for evaluation. If necropsy samples reveal no
significant findings (and environmental and
management factors have been carefully evaluated
and ruled out), evaluation of fresh dead
fish can be helpful. In general, fresh dead fish
should have relatively clear eyes, good coloration,
red to pink gills, and should not have a bad
odor. Because of the rapidity with which dead
fish are cannibalized, autolyze, and/or take on
secondary organisms (such as fungi, bacteria,
and parasites including the ciliates Tetrahymena
or Uronema), tile significance of lesions on dead
fish should be regarded cautiously.
Although moribund fish are ideal specimens
for post mortem work ups, fresh dead, frozen,
and formalin-fixed specimens can provide useful
information. 1 Fresh dead fish examined immediately
or kept in a plastic bag in the refrigerator
or on ice for 6 to 12 hours can provide almost as
much information as moribund fish) Frozen
and formalin-fixed fish are much less informative)
Common protozoal parasites and monogeneans
often die or leave a dead host. Cultures
collected from fish that have been dead in water
for as little as thirty minutes to sixty minutes may
be contaminated due to decomposition and autolysis.
Basic Fish Anatomy and Physiology
Fish species vary widely in their anatomy and
physiology. Several popular fish species from two
important families, the Cyprinidae (koi-Cyprinus
carpio and goldfish-Carassius auratus) and the
Cichlidae (discus-Symphysodo,~ sp. and OscarAstronotus
ocellatus) highlight some of the similarities
and differences in gross anatomy. Several
good references provide additional species informationY,~-9
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