Review of the Gonad Index (GI) and an introduction to the concept of its «critical value»: application to the skipjack tuna Katsuwonus pelamis in the Atlantic Ocean

With special reference to tunas and using recent results obtained on the skipjack tuna Katsuwonus pelamis, the authors point out that Gonad Index (GI, defined as the ratio between the weight of the gonads and the size of the fish to the cube) is an inaccurate indicator of each of the different sexual maturity stages commonly used to describe the sexual maturation process of marine fishes. They also show that it would be of interest to determine a GI critical value in order to distinguish females in anadvanced stage of maturation from the others. Using diameter measurements of skipjack tuna oocytes they adjust a non linear model between GI and the diameter of the most advanced group of oocytes. This permits the estimation of a GI critical value. This critical value is discussed.