Performing an autopsy (necropsy) on dead fish might sound grouse, but it is actually a very good idea. By examining your fish after its demise, you may be able to figure out what killed it, and if other fish develop the same symptoms in you aquarium, it will be easier for you to diagnose the problem and carry out the right treatment. If there is no treatment and the chances of spontaneous recovery are slim, you may wish to euthanize you fish to save if from further suffering and prevent the disease from spreading.
Even when you know what killed your fish it can be a good idea to examine it closely once it has died, since this will provide you with a deeper understanding on which conditions that are normally occurring in your fish and wish conditions that are not.
This is only a very brief article about fish autopsy. If you want to start examining your fishes, there are several detailed books and articles available that will provide you with more comprehensive guidelines. It is also a good idea to consult a comprehensive fish disease book since a myriad of different organisms can infest a fish and they can be tricky to tell apart for the untrained eye.
If a threatened or endangered fish dies in your aquarium, you should ideally contact a qualified veterinarian or fish export to perform the autopsy for you. If there is a breeder organization or similar, you can contact them for further advise.
A fish autopsy will normally start externally. Use a binocular to carefully dissect eyes, skin and fins. Keep an eye out for external lesions and bleeding. If you have access to a microscope, you can use it to check external scrapings. A rich profusion of different organisms can cause external damage to fish, including parasites such as Argulus, isopods, copepods, monogenetic trematodes, parasitic dinoflagellates, and various protozoa.
When you have checked eyes, skin and fins, it is time to cut off the tail and drain the blood from the gills. The goal is to remove the gill arches without leaking any blood that might obscure the gills during the inspection. The autopsy should ideally be carried out as soon as possible when you notice a dead fish in the aquarium, because if you do it early, most gill parasites will still be alive and easy to detect since they will continue to move around.
Before you start cutting the fish up for the internal part of the autopsy, carefully check the mouth and throat of the fish and look for abnormalities.
When you proceed to examine the inside of your fish you will often stumble over organisms that may seem dangerous, but that had little to do with the death of your fish. It is true that internal disease can cause fish death, but it is also true with that a lot of organisms that live inside fish, such as internal worms and protozoans, the host and the parasite are to well-adapted to each other that they can co-exits until the fish dies of other reasons.
The first step of the internal examination is normally to carefully cut the fish from the vent to the isthmus to get to the internal organs. Be very gentle when you cut and make sure that you do not damage any internal organs or major vessels. Inspect the fish for internal bleedings, such as free blood in the coelom, damaged vessels, or bleeding organs. Also keep an eye out for tumors, blood clots, blocked ducts and vessels, a ruptured or punctured digestive tract, and lesions. Any organ that looks strange, e.g. that is enlarged or discolored, should be inspected in detail. As you can see, prior knowledge from “healthy” fish is important; otherwise it will be difficult to notice abnormalities. Internal abnormalities in aquarium fish are often the result of bacteria, tumors or a blocked duct. Internal worms that normally cause no problem can prove fatal if they migrate through a vital organ or get stuck in a duct or vessel.
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