The growth of a fish (fish growth) results from the consumption of food, its assimilation and the construction from it of the organism’s body. The growth process is specific for each species of fish, as for any other organisms.

The knowledge of ageing fish has made it possible to estimate the rate of growth. A study of growth in fish offers system ecologists many opportunities to enquire into the dynamic balance and states of change in aquatic ecosystem.

However, growth is influenced by food, space, temperature and other factors. Furthermore, since fish are poikilothermic and live permanently immersed in water, they are very affected by changes in their ambient medium.

Forms of Fish Growth

There are some important forms in which the growth of fish is estimated:

Growth in length – this estimate growth rate of individual fish called the True Growth Rates. Growth in length quit never cease.

Growth in weight – fishermen are usually interested in the weight of fish catch not the numbers. And indeed the value of a catch of a given weight often increases when the average size of the fish in the catch is greater i.e. when there are fewer individuals. The growth in weight is an important aspect of growth of fish.

Population growth– Population growth refers to change in the number of fish observed at different times of a particular fishery.

Method of Studying Fish Growth

Though, once the age of a fish has been determined, the study of growth will appear superficially simple. In practice, there are numerous difficulties encountered. Fish growth may, therefore, be keenly studied by the following methods:

1. Experiments in Rearing and Holding Device

Fish of known age or size, which are placed in ponds or tanks, can easily be recaptured at a later date. Otoliths, scales and other structures can be examined for the number of marks to be expected.

2. Tagging and Marking Method

When fish are tagged or marked with a numbered tag of metal or plastic or injected with tetracycline which produces a mark in the structure of the otoliths.

At recapture, the number of supposed age marks on the otoliths should coincide with either the known age or with the number of years the fish has been at liberty.

3. Age-Length Data

The age of each fish in a random sample, caught with a gear with the least bias, and used to sample the habitat thoroughly, is determined. Then the mean length, with confidence limits, is calculated for each age group.

4. From Petersen’s Method of Age Determination

The modal length of each age group obtained from the size frequency method, as apply to age determination.

5. By Cohort Analysis

Modal length of a particular age group ( usually of a specially strong year class) are estimated at yearly intervals with this method, and size frequency analysis, the growth rate can be indicated without actually knowing the age of the fish.

In summary, most fish continue to grow throughout their lives. Consequently, age and growth have been one of the most intensively studied aspects of fish biology because they are good indicator of the health of individuals and populations.

Rapid growth indicates abundant food and other favourable conditions, whereas slow growth is likely to indicate just the opposite. Important influences of diet and environmental and social factors work via hormone-mediated mechanisms to regulate fish growth.

Age and growth studies in fishes have become a well –studied integrator of fish well-being in aquatic habitats and an important measure of production success to the fish culturists.

Fish age and growth are critical correlates with which to evaluate many other biological; their studies provide information on stock composition, age at maturity of a fish, life span, mortality and production rate etc.

Direct estimate, anatomical method and length-frequency distribution methods are the basic methods of age determination; these are validated to know whether the growth increments seen are in fact annual in nature or follow some other cycle.

Cohort analysis, capture & recapture method, age-length data and Petersen method are the principal of measuring growth. There are different forms of growth: growth in- length, growth in-weight and growth in- population.