Suitability of the gonadosomatic index in Peruvian Anchoveta (Engraulis ringens): Sexual maturity, a 5% critical threshold, and seasonal variation

Determining gonadal maturity phase is crucial for estimating reproductive indicators of fishery resources. In Peruvian anchoveta, the Einarsson and Flores (1961) scale and the updated scale of Buitrón et al. (2015) have been used over time; however, changes in scale usage, together with the subjectivity of morphometric methods and the time required for histological analyses, limit the precision of maturity-phase assessment. This study proposes the Gonadosomatic Index (GSI) as an alternative approach to evaluate gonadal maturity. Information from 192,069 individuals collected across 116 surveys (1986–2022) was compiled. We assessed the performance of GSI using the 5% critical threshold (GSIc) to describe the reproductive process. Sigmoid models (logistic, Gompertz, Hill, and Weibull) were fitted to estimate GSI at 50% and 100% sexual maturity (GSI50, GSI100), and seasonal variation was examined relative to GSIc. In addition, the ability of GSI to classify gonadal maturity phases was evaluated using seven classification models. Random forest models achieved up to 82% accuracy (α = 0.01), identifying GSI as the most important predictor. Sigmoid models estimated GSI50 between 2.5–2.9% and GSI100 between 4.1–5.3%, with no significant differences between spawning seasons according to Student’s t-test (α = 0.05). A one-sample Z test (α = 0.05) confirmed that GSIc is a valid reference point for assessing gonadal maturity in anchoveta.