Longevity of Fishes in Captivity 215 



Conditions in aquaria at best are still artificial since move- 

 ments of fishes are restricted, and for this reason there cannot 

 be any escape from environmental stress. It is apparent, then, 

 that fishes that survive aquarium conditions are those that 

 can withstand shock stresses and can acclimatize to a variety 

 of exaggerated environmental factors. But even such fish as 

 these are often at the limits of their tolerance, and any sudden 

 change in one or more of the physical, chemical and bio- 

 logical factors often results in death or increased suscepti- 

 bility to infections. Invariably these infective agents are ex- 

 ternal (gill and skin) protozoan and helminthic parasites; only 

 rarely are they bacterial or mycotic organisms. This would 

 indicate that once the fish is acclimatized to its new environ- 

 ment (captivity), its resistance is increased and the parasite load 

 diminished to a point where immunity is maintained by pre- 

 munition. Diseases caused by internal parasites are often self- 

 limiting and in some instances may spontaneously disappear. 



Once a balance has been established between fish, parasites 

 and environment, other diseases of a non-infectious nature 

 may develop. It may not be surprising that the greatest 

 single cause of mortality is associated with nutrition. Fishes, 

 like other animals, are herbivorous, carnivorous and omnivor- 

 ous and all need an exogenous source of vitamins and other 

 nutriments. The main food source in the New York Aquarium 

 consists chiefly of commercial-grade fresh and frozen fish, 

 clams and Crustacea. The kind of fish used for feeding 

 depends entirely on their availability on the market and in 

 collecting areas. Feeding oily fish (mackerel, herring, etc.) over 

 relatively long periods to species that normally eat invert- 

 ebrates and non-oily fish frequently results in liver damage, 

 commonly referred to as fatty degeneration but properly 

 called fatty "metamorphosis" or "fatty change". The 

 pancreas, kidneys and other organs may also be involved in 

 this type of damage. Fatty changes may be due to a relative 

 anoxia, the result of prolonged passive congestion. Other 

 diseases of fishes indicative of disturbances in carbohydrate 



