The relative gonadal index: an alternative index for quantification of reproductive condition

Gonadal indices (i.e. GSI = gonadal wt/body wt X 100) commonly are used to quantify reproductive condition in fishes. These indices may be inappropriate with specimens of different sizes, however, for gonadal growth often is allometric. A new gonadal index (relative gonadal index, RGI) was developed to quantify the reproductive condition of animals independent of body size. The RGI is based on the underlying model W = alpha i X S beta i, where W is gonadal weight, S is body size (less gonadal weight if body weight is used), and alpha i and beta i are parameters to be estimated for gonadal developmental stage i. Assuming that a multiplicative lognormal error is appropriate, parameter estimates for alpha i and beta i were obtained by linear least squares regression for the log-transformed model ln(W) = beta i X ln(S) + ln(alpha i), where, in this form, beta i is the slope and ln(alpha i) is the intercept. Only if estimates of beta i do not differ significantly among ovarian developmental stages, as in our case, can a pooled estimate of beta be used to obtain the relative gonadal index, RGI = alpha i = W/S beta. Applicability of the RGI was tested using ovaries of three ecologically distinct fish species. The RGI was found to be more appropriate than the gonosomatic index for all three species.