Can Fish Sense Hunger, Low Oxygen, and How Do They Swim Straight?

Metabolic Rate and Oxygen Need

The rate at which an animal uses up energy, produces heat and waste products, and consumes oxygen is called the metabolic rate. An understanding of the factors that modify the metabolic rate is of primary importance to the aquarist.

Since fish are cold-blooded, they differ fundamentally from mammals in that their metabolic rate increases as the temperature rises and are hungriest when warm. Humans consume a great deal of energy, which is supplied by foods and drinks, in order to maintain a constant body temperature that is often well above the temperature of the body’s surroundings.

A fish, on the other hand, doesn’t have a warming mechanism to do this but merely obeys a fundamental chemical law which causes the body processes to go faster the higher the body temperature becomes due to the temperature of the water that surrounds the body itself. Thus, a fish turns food into energy at a much higher rate in warm water than in cold water.

Another factor influencing the metabolic rate is activity. A resting fish needs less energy (food) than an active fish. The higher the temperature, the more energetic a fish tends to be, so that an elevated temperature acts doubly in causing higher energy consumption in most species – the fish is using more energy not only because it is warmer but also because it has to swim more to catch and to consume and digest more food. This action has an upper limit, however, and is probably determined by the lowered solubility of oxygen in warmer waters.

Thus, at about 80 degrees F, the average fish reaches its maximum oxygen consumption and maximum appetite. This is also the prime temperature to induce breeding activity in most species and to induce the quickest birth cycle in livebearer species.

A further factor influencing metabolism is age. Young fish are growing relatively faster than older fish, and also they use up oxygen and foodstuffs faster per unit of body weight.

Remember that female livebearers will need more oxygen than younger fish or males. Keep this in mind as you manage your aquarium.