Biology of Finfish and Shellfish

4.3.3 Maturity stages

The term ‘maturity stages’ is unique but has accepted meaning in Fisheries Biology. It is taken as a measure to observe the degree of ripeness of the ovaries and testes of a fish and not whether the fish has sexually mature or not. The term first maturity describes a fish which is spawning for the first time. In other animals, the term maturity is used when an animal reaches maturity (the ability to reproduce) once again.

To determine the cycle of maturity of gonads, the most common method is to define the stages of sexual maturity and observe them with large number of samples at weekly intervals. The species to be studied should be brought to the laboratory. The fish has to be weighed accurately and the total length of each specimen should be recorded. The criteria for assessment of maturity are the colour of gonads, the size of gonads in relation to body cavity, presence or absence of oil globule in the ovum, nature of perivitelline space, nature of ova and diameter of the unspawned eggs. The ovaries undergo maturation process quicker than testes; but the stages can be ascertained more easily in ovary. The ovaries should be preserved either in formalin or in modified Gilson’s fluid for further studies on ova diameter and fecundity. To know the distribution pattern of ova in an ovary, ova should be taken from the anterior, middle and posterior regions of few ovaries during different stages maturity. If the distribution pattern of ova is uniform in all the three regions, then the ova can be taken in any place irrespective of the regions. If the distribution is not uniform, the ova should be taken from all the three regions and then it should be pooled.