The fish in our aquariums rely solely on us to meet their dietary needs, and their long-term health and well-being are sure to suffer if we aren’t careful to provide foods in the proper amounts and of the appropriate nutritional balance. In the following excerpt from The Salt Smart Guide to Preventing, Diagnosing, and Treating Diseases of Marine Fishes, author Jay Hemdal explains how improper feeding can lead to the all-too-common problem of fatty liver disease in captive marine fishes—and what you can do to prevent it.
From Chapter 3: Husbandry, Environment, and Your Fishes’ Health
“A major chronic health problem facing most long-term captive marine fishes is “fatty liver disease,” or liver degeneration. Fish do not assimilate fats well, so oftentimes, if a fish’s diet is too high in fat, it will then be deposited in various tissues, especially the liver. Unlike with mammals, this fat is not readily usable during times of starvation. Even if food is withheld from a fat-laden fish, very little of the fat is ever reconverted into usable energy.
Some fish do not show outward signs of obesity, especially sedentary species such as grouper. In these fish, fatty liver disease may only be diagnosed after death, when sections of the liver will show oil droplets and pieces of the liver itself will float in sea water.
Fatty liver disease is most common in adult fish whose growth rate has slowed considerably. Younger fish tend to grow fast enough to minimize fat deposition—excess calories are turned into muscle tissue instead. With long-term captive marine fish, fatty liver disease is the leading cause of (or contributor to) death. Virtually every lionfish held in captivity longer than three years will show evidence of fatty liver disease upon necropsy.
What techniques are available to solve this problem? Nothing can be done for a fish once fatty liver disease has become evident. Prevention is the only course of action an aquarist can take. Simply putting the fish on a “diet” may not work, in that this may result in increased aggression towards other fish in the aquarium. The best solution is:
- Avoid feeding herbivores diets high in meat proteins.
- Train carnivores to feed on low-protein, low-fat diets, such as squid or gelatin diet.
- Avoid feeding any saturated fats to fish. Overall dietary fat levels should be less than 20% in actively growing fish and less than 12% in adult fish.
There has also been a theory that feeding freshwater fish to predatory marine fish will cause malnutrition and fatty liver disease. Most of the rationale behind this has to do with slightly different fat profiles between marine and freshwater prey fish species. Credence is given to the theory when so many marine aquarists report that they lost their lionfish to liver degeneration after feeding them live freshwater goldfish. In reality, they would have run into the same trouble had they fed any single fish species as a diet for their lionfish. Feeding carnivores too frequently can also cause fatty liver disease. The general recommendation is to feed larger carnivores only twice a week.”