The gonadosomatic index, abbreviated as GSI, is the calculation of the gonad

Gonad

The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

 mass as a proportion of the total body mass. It is represented by the formula: GSI = [Gonad Weight / Total Tissue Weight] x 100 (Barber & Blake 2006). It is a tool for measuring the sexual maturity

Sexual maturity

Sexual maturity is the age or stage when an organism can reproduce. It is sometimes considered synonymous with adulthood, though the two are distinct...

 of animals in correlation to ovary

Ovary

The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...

 development and testes development.