Fish Diets in the Wild: What's There To Eat In Freshwater Habitats?

In their natural habitats, the majority of tropical aquarium fishes are opportunists, or generalist feeders. If they come across a tempting morsel, they investigate it, often by mouthing it; if it is edible they will eat it, if not they will eject it, spitting it back out of their mouths. In this way, they sample a huge variety of potential foods, learning what is and what is not food. As a result, wild fish often have an incredibly diverse diet.

What's a typical wild diet for a fish such as a tetra?

With its iridescent blue and vivid red colors, the cardinal tetra is one of the most striking fishes in the world. Huge numbers are collected and exported each year from their Amazonian home, but although they are a familiar sight in aquarium shops, little is known of their life in the wild. One of the most important steps in finding out about the biology of any species is to find out what it eats.

Researchers examining the diet of cardinal tetras from flooded forests and stream tributaries of the Rio Negro found that the fish were naturally eating a very broad range of foods. The stomach contents of the fish included crustaceans, such as Daphnia species and copepods, the larvae and adults of flies, mosquitoes, beetles, and bugs, as well as ants, shrimps, fish fry, eggs, algae, and fibrous plant matter. The smaller fish — those measuring 1 inch (2.5 cm) — also preyed extensively on tiny rotifers. Perhaps most surprisingly, the results also showed up scales and parts of other; larger fish, which suggests that the cardinal tetras are not averse to scavenging on dead fish.

Do fish have favorite foods?

Just like us, individual fish often have their own preferences. For instance, they may become used to a select number of prey species. They learn where these are most likely to be found and through practice become efficient at hunting and eating them. Quite often a change in diet can occur as a release from competition. If a large number of fish all compete for the same food resource, it can pay to switch to a new food type. This happens over and over again in nature, meaning that fish from the same species may specialize in different food.

If evolution gets to work on this, it can produce two morphotypes, each of which is specialized in its own diet. (Morphotypes are members of the same species with different appearances.) The cichlid Herichthys minckleyi is an example of this. Two of these morphotypes coexist in their Central American home, one specializes in eating molluscs, especially snails, the other typically snacks on larvae. Each eat their own favored prey far more efficiently than the other. Most interestingly of all, the two morphotypes look quite different from one another, as each has evolved characteristics that aid their diet specialization, including different teeth and mouths,

Do fish change behavior at feeding time?

Aquarists often notice how, when they add food, their fish go into a feeding frenzy. Their behavior switches from slow and placid to fast and frantic as they try to cram in as much food as possible before it disappears inside their tankmates. This feeding behavior also occurs in the wild when fish find a rich patch of food, where they take their cue from the feeding of others — “if they're eating it, I can too.” Such observation helps fish to find their own next meal. However, the feeding frenzy is a dangerous time for the fish. They may be injured as they thrash around or very often they are themselves eaten by a predator taking advantage of their distraction as they feed.