Quantitative histology for maturity staging : Timing is everything

Accurate modeling of exploited fishery stocks relies first and foremost on data quality. Among the biological parameters collected yearly for stock assessment purposes, sexual maturity is key when estimating variables such as the Spawning Stock Biomass or factors like maturity ogive or the size/age at first maturity of a fish stock. In the following paper, a method based on quantitative histology is used to determine the sexual maturity phase of 317 female Striped red mullets Mullus surmuletus from either the Bay of Biscay (n=103) or English Channel (n=214) stocks. Going beyond identification of the most advanced germline cell to classify an individual into a maturity phase, cellular structures were quantified on ovarian cross sections. This allowed for verifying cellular homogeneity within the gonad, implementing a reproducible reading method for reader calibration with a difference of less than 3% in cellular features identification, and finally classifying individuals into a maturity phase. When using histology, the cellular structures used to identify individuals that have spawned disappear just after the spawning period, leading to an over-estimation of immature (A) individuals outside of the spawning period. This underlines the importance of timing the sampling during the spawning period to avoid over-estimating immature individuals.