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Greater Seattle Aquarium Society

Title as you wish: When to Euthanize a Fish

by Heather Candelaria
September 1998

There is the question of how to euthinize a fish [see last summer’s column], but there is the even more problematic issue of when to euthanize a fish. I think that the occasions when euthanasia are appropriate fall into very distinct categories.

When Stress can kill: I believe that fish are most concerned with the basics of biological survival---finding and consuming food, escaping predators, and engaging in procreative activities. When a fish is unable to do one or all three of these things, he is going to be under a tremendous amount of stress, which we all know, is the big killer in the aquarium. In some cases, if you do not put Mr. bubbles out of his misery he may very well die on his own, as a result of the unusual stress he is going through. The question then is not if he should die, but when and how quickly that death will occur.

Examples of this include mechanical damage making fish unable to eat, swim, or socially interact with other fish.

When Disease occurs: The most appropriate time to consider euthanasia, is when a disease occurs. In cases when diseases are highly contagious, or very hard to treat or even untreatable, then the old adage of one bad apple in the barrel, can be synonymous with one sick fish in the tank.

Common examples are fish which exhibit dropsy, fish that do not respond to medications, fish which repeatedly go through bouts of disease, fish weakened by old age which can become and easy target and breeding ground for infectious pathogens in the aquarium.

When Breeding fish: When breeding fish, especially for selective characteristics of fancy goldfish, discus or guppies, we sometimes have to euthanize or cull the fry which do not seem to be good candidates for reaching adulthood. For instance, a throwback causes some of the fry to exhibit natural or wild characteristics (which are usually dominant), or because of inbreeding, the fry show bad mutations.

For me culling fry is not really euthanasia, so much as it is recycling. The fry are not much more than potential fish, and if for some reason their potential is not very good then I usually feed them back to their mother. In this way the protein can be used in generating her next batch of fry. I believe that this is what happens in the wild with egg-scatterers---any fry that could be eaten be a predator is lost protein bearing the parents genetic imprint. If the situation is such that a predator can eat the fry, it is in the parent fishes best interest to save the protein and recycle it into other fry later on, fry that will be created in a period where predators (and the parent fish) may not be able to find and gobble them down. Even in cichlids that exhibit brood care, recycling of the fry will occur when the population has grown too large for the tank and the parents are ready to spawn again.

When fish are an ecological threat: The final time that a fish is a good candidate for euthanasia is when the fish is one which could possibly contaminate the environment and out-compete local or native species. A typical example of this is that a hobbyist cannot find a good home for their fish [see article, this issue, for instance], and considers releasing them into a lake or stream as a last resort. One should never release tropical fish into the wild! I know of several people in the fish industry who have accidentally received in fish shipments, what is commonly called the walking catfish (Clarias batrachus). These people euthanized the fish immediately. This fish can and will live in almost any body of freshwater including in temperatures below freezing, they will eat almost anything and will grow to over a foot and a half long. In addition to this amazing survival capability, they have been known to walk up to 3 miles in search of a new body of water in which to live in. Due to the adaptability and ability to migrate, this s particular fish is one that I do believe should be illegal, and euthanized on sight.

Luckily, indiscriminate mercy-killing of fish is not a problem. I cannot think of any case I’ve ever heard about that constitutes an inappropriate euthanization of a fish. We are more apt to unduly prolong the life of Mr. Bubbles than to prematurely end it.

This is because we are human, and being human we tend to care a great deal about our possessions, especially those which have given us a great deal of pleasure in the past, and particularly those which we are solely responsible for. When we look at the lives and antics of our fish, we see a world which we have created and populated, a world which we are responsible for. Whether this world thrives or suffers, whether these fish live or die, is completely reliant on us and how well we care for our aquaria.

We like to think that in the world that we created, death and sickness do not occur. We like to think we can always save the lives of our fish, and sometimes when we cannot, we take it as a personal failure.

A fact that I like to remember is that fish are not often involved in car accidents. Car accidents occur all the time and people are frequently injured and killed in these terrible occurrences, but not fish. Why? Well, because in the world we create for our fish, riding in cars is not a common occurrence, but in their watery world there are a number of other potential accidents which occur much more frequently, and just like a car accident can occur not matter how carefully you yourself are driving, things can happen in the most cared for aquaria, which will result in fish losses.

We do not really control our aquaria, the laws of physics, chemistry and biology do. One of the primary laws of biology is that if something lives, it can also die. The only thing I hope for out of a fishes death is that something is learned, or gained. Life has a great value but then so does knowledge, especially knowledge that can be applied to maintain life in the most optimal way.