Fish take food from the aquatic environment for life sustaining, body building and growth. Different species of fish feel comfortable in different aquatic environments. The foods that fish consume from different aquatic environments are varied. Fish consume a large number of living ingredients and inert substances as food. Fish eating techniques are more diverse and complex in nature than other animals. The physical and chemical properties of water and various physiological factors affect the food intake and feeding of culture fish.
Food, eating habits and aquatic environment control the growth and production of fish. For this reason, it is necessary to have an idea about fish food and aquatic environment for modern fish farming and improved management. Nutrition and food intake play an important role in the growth and survival of fish in farming methods. The method of feeding fish is quite complicated. Food intake and swallowing occur through the interaction of fish and its environment.
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The Role of Different Types of Factors in Food Intake
Various factors of the aquatic environment affect the food intake of fish. These factors can be mainly divided into the following two categories:
1. A factor that creates internal stimuli
2. Factors that stimulate food intake
Internal Stimulus Factors
Factors that stimulate the physiological process and affect the food intake of fish are called internal stimulus factors. The factors that create internal stimuli are:
- Illuminated period of the day;
- Feeding schedule;
- The interval between meals;
- Fixed time of day;
- Time and nature of previous food application;
- Water acidity and alkalinity;
- Dissolved oxygen;
- Rhythmic activities inside the body.
Stimulants in Food Intake
Factors that stimulate the fish’s senses and affect food intake are called stimulants in food intake. Fish can sense such stimuli through the senses. e.g.
- Feed quality;
- Meal size;
- The duration of floating of food in water;
- Taste of feed;
- Feed odor;
- Feed coloring;
In addition to the above factors, fish feed intake is regulated by other factors, viz
- Disease and germs;
- Parasitic attack;
- Deformed body;
- Age and growth rate;
- Reproductive stage;
- Environmental pollution;
- Stress caused by other fish;
- Human intervention;
- Stress caused by other predators (kite, snake, Lizards);
Different species of farmed fish usually feed on daylight. Fish that are attracted to taste and smell and they take food usually eat at night. Light is needed for food hunting. For this reason, predator fish usually eat in an illuminated environment, i.e. in daylight. The longer the illuminated period of the day, the higher the rate of food intake of fish.
As the temperature of the atmosphere changes, so does the temperature of the aquatic environment. Therefore, the water temperature varies in different seasons of the year. When the water temperature rises within the optimum range, the rate of food intake of fish increases. Similarly, as the temperature decreases, the rate of food intake of fish decreases. Therefore, the tendency of fish to eat in winter decreases and the tendency of fish to eat in summer increases and the amount of food consumed also increases.
Food demand fluctuates at different stages of the fish’s life cycle. Different physiological activities and different stages of the life cycle affect the food intake of fish. During the breeding season, fish usually consume less food than usual. Many fish almost stop taking food during breeding. Fish food demand is highest in the fry and juvenile stage.
Bangladesh is a country of six seasons. Different seasons cause different changes in our natural environment. Therefore, seasonal variation also affects the rate of food intake of fish. For example, during the monsoon season, the quality of natural water improves. This brings a special kind of stimulus to the life of the fish. The fish became excited. As a result, the metabolic activity of fish increases and the tendency and demand for food increases. In winter the illuminated period of the day decreases. It also reduces the production of natural fish food. Fish food demand decreases in winter due to environmental reasons.
Visible features such as color, size and movement of food also affect the food intake of fish. Sudden changes in light intensity play an important role in the process of feeding fish. The perch exhibits a rhythmic process in food intake according to the intensity of light. Such fish show maximum activity in taking food during sunrise and sunset.
Effects of Water Quality on Food Intake
Different types of biochemical changes take place in the body of fish to manage their life. Different physico-chemical properties of water affect their change. Water has an optimal level of these physical chemical properties for fish food intake, survival, physical growth, reproduction and proper management of vital metabolic functions. The optimal levels of these qualities in the aquatic environment are as follows:
Table: Optimal levels of different qualities of water
The name of the element
Dissolved oxygen (DO)
Free carbon dioxide(CO2)
Different qualities of water affect the tendency and demand of fish for food. Some of the major regulators of water have a brief effect on the diet of fish.
As the temperature rises within the optimum range, the rate of metabolic activity increases. As a result, the demand for fish food also increases. Similarly, as the water temperature decreases, the food demand of fish decreases comparatively. Various experiments have shown that when the water temperature drops below 10 0 C, the carp stops eating and at 30-35 0 C, the carp’s diet is elevated. Tilapia fish consume food at a rate of 10% of body weight at a temperature of 24-25 0C but stop eating when the water temperature drops to 15-160 C .
Dissolved Oxygen (DO)
Oxygen affects the food needs of fish at a proportional rate. As the amount of dissolved oxygen in the water increases, the food demand of the fish increases and when the amount of oxygen decreases, the food demand decreases. When the dissolved oxygen level in water is 10-15 mg / L, the food demand of common carp reaches the highest level. The optimum level of dissolved oxygen in water is 6-7 mg / liter for cultivable fish.
Water pH has a significant effect on the food needs of fish. When the pH value of water decreases, the food demand of fish decreases. If the pH of a reservoir is less than 6.0 or higher than 9-9.5 and it lasts for a long time, the demand for fish decreases. In such a situation the growth of fish comes to zero. Fish food demand is highest between pH level 6-7.5.
Free Carbon dioxide (CO2)
In the process of photosynthesis, carbon dioxide plays an important role in the natural food production of fish. As the amount of carbon dioxide in the water decreases, the production of natural food decreases. This does not meet the food needs of fish. Again, too much carbon dioxide increases the acidity of the water increases. As a result, the tendency of fish to eat and the demand for food decreases.
Light and Turbidity
Sunlight and water turbidity affect the fish’s food needs. In turbid water, effective light cannot penetrate to a certain depth. As a result photosynthesis and production of natural food is affected. The colloid of muddy water stops the gills from getting stuck in the fish gills. This makes it difficult for the fish to breathe and disrupts normal life. As a result, the tendency of fish to eat and the demand for food decreases.
Appetite and Satiation
Appetite is the desire for food and satiation is the fulfillment of that desire. Appetite and gratification are interrelated. The following is a diagram of the relationship between appetite and satiation.
Appetite and satiation are important for fish farmers and they play an important role in setting the right food list and schedule. Metabolic, physiological and hormonal influences are responsible for fish food behavior and appetite control.
Detection of Fish Appetite
Most of the fish farmers create the demand for fish food by determining the appetite of the fish. It is very easy to observe the feeding process in the case of fish that come to the surface on the water and feed. Hungry fish swim fast to find food and eat fast until they are satisfied. After taking food, the fish go to the bottom of the water in groups.
Physiology of Appetite
The physiology of fish appetite is similar to that of other higher vertebrates. In this case two types of strategy can be seen:
(1) When the stomach is empty or full, the brain receives stimulation through the nerves located in the stomach or intestinal wall.
(2) Different types of nutritional changes in the blood (glucose, lipids, amino acids) also stimulate appetite.
When the stomach is full or partially full, the amniotic region of the brain’s hypothalamus region receives stimuli from the stomach and reduces food intake and stops at one stage.
Factors for Appetite
Different types of regulators or factors affect the appetite of fish, e.g.
- Temperature variations;
- Variation of dissolved oxygen;
- Food size;
- Predatory fish;
- Interference of humans or other organisms;
The appetite of the fish mainly depends on the zero time of the stomach. Carnivorous fish consume high energy foods and their intestines are small whereas herbivorous fish consume low energy foods so they need more food as they have large intestines.