Fish Diet – What Do Fish Eat?

What Do Fish Eat? From Piscivores To Plankton & Everything Else

So what do fish actually eat?

Well, let’s see. Fish need to eat in order to grow…

And somewhere, at sometime, everything that lives in the waters of the world – that is larger than a single bacterium – is eaten by fish.

But they also eat things that don’t live in the water – such as plant material, insects and other animals that fall into the water for one reason or another.

There are a number of words that scientists use to define an animal’s diet. The first four words in the list define the the diet in a general way and the last three define the degree of variety in an animal’s diet.

Note that these words apply to all animals, not just to fish. However, terrestrial detritivores feed off the floor of the forest or grassland and not off the sea floor.

Types Of Diet In Fish

Scientific Term Definition of Term Example Species
Detritivore Feeding on dead material off the sea floor. Yellowfin – Xenocypris argentea
Herbivore Feeding on living plant material. Milk Fish – Chanos chanos
Carnivore Feeding on living animals. Pike – Esox lucius
Omnivore Feeding on a mixture of plant and animal material. Common Carp – Cyprinus carpio
Monophagus Feeding on only one species or type of food.  
Stenophagus Feeding on a small variety of organisms. Carolines parrotfish – Calotomus carolinus
Euryphagus Feeding on a wide variety of organisms. Sea Raven – Hemitripterus americanus

Of Piscivores And Plankton Eaters

Some more important words, when considering what fish eat, are:

  • Piscivore: someone or something that eats fish.
  • Obligate: without choice. Able to do only the specified thing. Thus an obligate piscivore eats fish and only fish. If there are no fish it starves.
  • Facultative: with a choice, the opposite of Obligate. Thus a facultative piscivore will happily eat fish, but if their are no fish (or if fish are scarce) it will eat something else like prawns or crabs.
  • Nekton: Larger aquatic organisms that swim actively rather than just drifting, such as fish and squid.
  • Plankton: Smaller to very small aquatic organisms, which while they might swim a little, are still completely at the mercy of the current – and go wherever it takes them. There are two sorts Plankton. Zooplankton = animals and Phytoplankton = plants.

Phytoplankton comprises most of the plant material in the oceans and occurs in staggering quantities. It is the basis of the oceanic food web.

Phytoplankton includes Green Algae, Blue-green Algae and Diatoms.

Scientists have shown that there is a fundamental relationship between the presence of phytoplankton and catches of commercially viable fish. Basically, more phytoplankton means more zooplankton – which after one or two more steps equates with more fish.

marine fish zooplankton diet
Marine zooplankton floating in the ocean

Scientists have also shown that damming a river can drastically reduce the phytoplankton populations of the sea, or part of the ocean it flows into. This is because damming the river allows a lot of the dissolved organic salts to accumulate in the sediment of the dam, instead of flowing out into the sea (Karpevich, 1958 – In “The Ecology of Fishes” by G. V. Nikolsky).

As you can see by the date of the authority, this knowledge has been around a long time. Yet despite this, and a mass of other negative evidence, mankind is still busy damming the world’s rivers.

The vast majority of fish are euryphagus carnivores feeding on insects (fresh water species), crustaceans (particularly copepods and shrimp species but also crabs) and molluscs (squid, octopus etc.) as well as on other fish.

A few species of course also feed on mammals, in particular the Great White Shark, Carcharodon carcharias, regularly eats seals.

A specimen caught off south Australia was reported to have the skeleton of a horse in its stomach. Equally unusual is the report of a Greenland Shark, Somniosus microcephalus, with a hornless reindeer in its stomach (sharks often swallow their prey whole).

What Do Detritivore Fish Really Eat?

When considering the diet of fishes, we we need to think a little. The term detritivore for instance is not nearly as simple as it seems. In reality it is a rather a feeding method than a feeding type.

Such fish suck in the accumulated detritus of the sea floor – and with it some of the sea floor as well. When digesting the food, they do not select out the animal or plant aspects separately but consume any organic matter they find.

In this way they are usually feeding on not only a mixture of dead plant and animal matter, but also consume any of the small organisms that live on and in the sea floor and a variety of micro-organisms. Thus a detritivore is also likely to be a euryphagus omnivore.

Detrital feeding is much more common in fresh waters than it is in marine environments. In the Grand Lac of Cambodia an analysis of the commercially caught species showed that of the 56 species involved, 25% were detritivores.

In marine environments Grey Mullet is a good example of a detritivore.

Gut Size Relationship To Diet

The length of the gut (see fish digestive system) of a species of fish, or any other animal, reflects its diet.

Plant material is often very abundant – and easy to acquire – but it is harder to digest, because of the cellulose content. It also supplies less energy per gram.

Meat is harder to acquire and often requires chasing or subduing even after it has been found. But it is more easily digestible than plant material and gives a better energy pay out per gram.

So carnivores have shorter guts than herbivores. This is so that they will have a greater surface area over which nutrients can be absorbed. It allows them to keep the food in the gut for a longer period of time allowing micro-organisms more time to work on it.

This relationship between diet and gut length is eloquently shown by a study published in 1995 by Kramer and Bryant on the relationship of a fish’s body length to the length of its gut, using 21 species of fish from Panama.

Dietary Type Ratio of body length to gut length
Carnivore 0.7 to 0.9
Omnivore 1.1 to 2.2
Herbivore 5.4 to 28.7

In the Chondrichthyes and some ancient fish, such as the Sturgeon and Lungfish, the internal surface area of the gut is increased by the possession of a spiral valve within the intestines. Imagine a corkscrew running along the centre of the intestines.

Fish change their diets as they grow. Obviously something that is a suitable food item for a 1cm long carnivore will not be suitable for a 20cm long carnivore.

Young Pike, Esox lucius, feed primarily on planktonic crustaceans, but as they grow larger they change over to fish. If they are not offered any fish, they stop growing and die.

monkey goby fish diet change
Monkey goby fish (Neogobius fluviatilis)

The diet of the Monkey Goby, Neogobius fluviatilis pallasi, an important commercial species of the Caspian Sea, changes over time as well. When young, they feed on both Mysids and Amphipods (these are both groups of Crustaceans) however as they grow older they eat less and less Mysids, until as full grown adults they feed entirely on Amphipods.

This reflects not only the fact that the Amphipods will grow heavier than the Mysids, but also the fact that the Mysids are pelagic and fast swimmers – making them more difficult for larger predators to catch than the slower moving benthic Amphipods.

Size of Fish 5 – 7 cm 7 – 9 cm 9 – 11 cm 11 – 13 cm 13 -15 cm
Percent Amphipods in Diet 0.168 0.32 0.351 0.671 0.744
Percent Mysids in Diet 0.243 0.118 0.03 0.04 0

This is just an example from a study done in the 1930s. While it illustrates a general principle quite well, we must remember that many other factors such as level of competition and relative abundance (which will change with the seasons and with time of year) also effect the predators choice of prey.

Furthermore, not only do fish feed on different items of food as they grow, but some species even change their feeding category. Thus Yellowfin Xenocypris macrolepis, which is mentioned above as a detritivore is an active carnivore in its juvenile stages.

Ten amino acids are essential to fish, meaning there are ten amino acids they must take in as a part of their diet because they cannot synthesize them themselves.

The amino-acids which are essential for fish are (the last 8 of these are also essential amino acids for human beings):

  • Arginine,
  • Histidine,
  • Isoleucine,
  • Leucine,
  • Lysine,
  • Methionine,
  • Phenylalanine,
  • Thronine,
  • Tryptophan,
  • Valine.

What Next?

Well, I hope this has helped to answer the question “What do fish eat?”.

Perhaps now you’d like to learn more about electric fish.

Gordon Ramel

Gordon is an ecologist with two degrees from Exeter University. He's also a teacher, a poet and the owner of 1,152 books. Oh - and he wrote this website.

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