Fish Necropsy
by Dr. Adrian Lawler
Extracted from the staff Operational Handbook which Dr. Lawler wrote as Aquarium
Supervisor for the J.L. Scott Marine Education Center and Aquarium, of Biloxi, MS
Aquarticles
An examination, or necropsy, of dead or dying organisms should be done unless it is
apparent the animal was killed by another animal in the tank. But even killed fish should
be examined periodically for skin and gill parasites so we know more about what is going
on in a tank. If a threatened or endangered species dies from an unknown cause, a
qualified veterinarian should be responsible for the main necropsy.
There are numerous parasitology books which detail methods of necropsy. For aquarium
purposes a thorough necropsy of fish is not necessary as most internal worms and
protozoans do not usually cause acute problems or death (the host and parasite being
well-adapted to each other). It is the external parasites (monogenetic trematodes,
parasitic dinoflagellates, Argulus, copepods, isopods, various protozoa) which can
reproduce rapidly in closed systems and the bacteria which cause both internal and
external infections that cause the greatest problems in an aquarium. Additionally,
normally free-living algae, flatworms, and protozoans can "bloom" in a tank
environment so that their released toxins (algae) or attacks on skin and fins and gills
(flatworms, protozoans) can cause stress or death of affected fish.
In examining fish, whether dead or dying, an external examination with a binocular
dissecting scope of eyes, skin, and fins should be done first (before external parasites
fall off or are otherwise dislodged); a skin scraping placed on a microscope slide and
examined may also be necessary. One should also look for external lesions or bleeding.
Unless trained in fish parasitology, one will probably not be able to recognize and
identify what he is seeking (anything attached to or coming off skin that is not part of
the fish), so appropriate texts on fish diseases and parasites should be available for
assistance.
After examination of eyes, skin, and fins, the tail should be cut off to drain the
blood away from the gills so that the gill arches can be removed without leaking blood and
obscuring the gills under examination. If the fish is fresh dead, most of the gill
parasites (monogenetic trematodes, copepods, isopods, and protozoa) will still be alive
and moving - so they can be detected. From experience (or visible damage to gills, or
researching appropriate literature) one can determine if a fish has a lethal load of
parasites. The mouth and throat of the fish should also be examined for anything abnormal.
After the external examination is done cut the fish carefully (to avoid cutting organs or
major vessels) from vent to isthmus to expose the internal organs. Look for internal
bleeding (blood free in coelom, ruptured mesentary vessels, bleeding organs), lesions,
tumors, discolored organs, enlarged organs, blocked ducts, blood clots, punctured
digestive tract, and anything abnormal. As internal worms usually cause few problems
(unless, e.g., nematode migrates through a vital organ or worms block a duct or vessel)
what one will usually see if there is an internal problem is the result of an internal
bacterial infection, blocked duct, or tumors. Look at the liver, spleen, and kidneys for
evidence of bacterial infection, fish TB, discoloration, bleeding, or tumors.
Various taxonomic or fish disease books should be consulted in order to identify
organisms found. If a problem of significance arises and assistance is needed consult with
microbiology, microscopy, or parasitology groups of professional laboratories for
bacterial, viral, or internal worm help, respectively.
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