Accuracy of gonadosomatic index in maturity classification and estimation of maturity ogive

Determination of the stage of sexual maturation of an individual fish is commonly assessed using either microscopic or macroscopic staging. Microscopic staging examines either the internal cellular structure (histology) or the external appearance of the whole-oocyte (whole mount). Macroscopic staging is the visual evaluation of the external gonad development according to a standardised reference scale. Histology is the most widely used microscopic method and highly accurate but expensive and time consuming because it requires specialised equipment. Macroscopic staging is cheaper and faster but is associated with higher degree of error and bias. This study describe a methodological approach to assessing maturity stages of individual fish that is both inexpensive, rapid and more accurate than macroscopic staging. We have named it the gonometric method because it is based on direct measurements of the gonodasomatic index (GSI), which expresses gonad weight as a proportion of total or somatic weight. Immature, mature-active, and mature-inactive fish were classified using a logistic multinomial model giving a threshold value of GSI named here as GSI(cut-off). The gonometric method was applied to four fish species that had different reproductive strategies, including demersal and pelagic species. For all four species the gonometric method estimated the maturity ogives with comparable precision to microscopic methods (p > 0.05). Across species, GSI(cut-off) values correlate with growth parameter of the von Bertalanffy growth function and length at 50% maturity, suggesting that the scaling of the GSI(cut-off) value is underpinned by fundamental allometric principles. Further testing of the gonometric method is required, but our results suggest that it has considerable potential for providing a faster and more accurate alternative to maturity staging without the need for extensive training requirements to ensure consistency across individual observers.