Fish feed today: what is special about it
Farmed fish and shellfish cannot find their own food, so the farmer gives it. There are four kinds of feed: live food (tiny living plants and animals), raw or fresh food (chopped small fish), moist pellets and dry compound pellets. Farms are moving from raw fish to dry pellets because pellets are cleaner, keep longer and have a known recipe.
Fish are cold-blooded. They do not burn energy to keep warm, so they turn feed into body weight better than land animals. The problem is that old recipes used a lot of fishmeal (dried ground fish), and wild fish for fishmeal are limited. So feed makers now replace part of it with plant protein such as soybean, and with new sources such as insect meal.
How fish eat, digest and use food
Feeding: fish find food by sight, smell and taste. Digestion: the gut breaks big food pieces into small parts with the help of enzymes. Absorption: the small parts pass through the gut wall into the blood. Then the body uses them.
Meat-eating fish (like sea bream) have a short gut. Plant-eating fish (like grass carp) have a long gut. Shrimp and shellfish also digest food in a special organ, but they eat slowly, so pellets must stay firm in water.
Nutrients fish need:
- Protein builds the body. Fish need ten essential amino acids from food.
- Fat gives energy. Sea fish also need special omega-3 fats.
- Carbohydrate gives cheap energy, but fish use it less well.
- Vitamins and minerals help bones, blood and health in tiny amounts.
Initial feeds: food for baby fish
A newly hatched larva has a tiny mouth and a small gut. At first it lives on its yolk bag. When the yolk is gone it needs initial feed that is small, soft and alive and moving, because larvae strike at movement.
- Microalgae (green water) feed the first food animals.
- Rotifers (about 0.1 to 0.3 mm) are the first live food for most sea fish.
- Artemia (brine shrimp) baby shrimp are bigger and used next.
- Copepods are very good natural food.
Rotifers and Artemia are fed with algae and oil first so they carry omega-3 fats. This is called enrichment. Later, the larva is weaned onto a very fine dry feed.
Feed ingredients and compound feed
Compound feed is a made recipe in which many ingredients are mixed and pressed together so each bite has all the needed nutrients. Main groups of ingredients:
- Protein sources: fishmeal, soybean meal, insect meal, shrimp or squid meal.
- Energy sources: fish oil, plant oils, wheat or rice flour.
- Extras: vitamins, minerals, a binder to hold the pellet together, and sometimes colour.
Pellets can be floating (the farmer can see if fish eat) or sinking (for bottom feeders like shrimp). Pellet size must match the mouth. Good feeding is measured by the feed conversion ratio: FCR = feed given ÷ weight gained. A lower FCR is better.
Try it: feed, water and money
In the 3D tank, move the feed slider from 0 to 10. Watch three things: how thin or healthy the fish looks, how much brown waste lies on the floor, and how the water colour changes. The best feeding is the middle: fish grow, water stays clean.
Key formulas and definitions
- FCR = feed given / weight gained
- Daily ration = body weight × feeding rate (%)
- Feed cost = feed used × price per kg
- Key terms: larva, live food, rotifer, Artemia, compound feed, fishmeal, FCR
Worked examples
1. A farmer gives 150 kg of feed and the fish gain 100 kg. Find the FCR.
FCR = feed / gain = 150 / 100 = 1.5. It took 1.5 kg of feed to make 1 kg of fish.
2. Fish in a pond total 200 kg. They are fed 3 percent of body weight each day. How much feed per day?
3 percent of 200 = 0.03 × 200 = 6 kg of feed per day.
3. A farm wants 400 kg of weight gain and its FCR is 1.2. How much feed is needed?
Feed = FCR × gain = 1.2 × 400 = 480 kg.
4. Farm A has FCR 1.2 and farm B has FCR 1.8. Both grow 500 kg of fish. Feed costs 60 per kg. Which farm spends less and by how much?
Farm A feed = 1.2 × 500 = 600 kg, cost 36 000. Farm B feed = 1.8 × 500 = 900 kg, cost 54 000. Farm A spends 18 000 less.
5. Why is a rotifer, not a dry pellet, the first food for sea bream larvae?
The larva mouth is only about 0.2 mm wide, and it strikes at moving prey. A rotifer is small enough, soft and moves, but a pellet is too big and does not move.
Common mistakes
- Thinking more feed always means faster growth. Extra feed is not eaten, it rots and lowers oxygen in the water.
- Giving big pellets to small fish. Pellet size must fit the mouth.
- Confusing FCR with growth rate. A lower FCR is better because less feed makes the same weight of fish.
- Feeding larvae only dry feed from day one. Most sea fish larvae first need live food such as rotifers.