# Comparing Macroscopic and Quantitative Histological Methods to Determine Sexual Maturity in the Female European Plaice, Pleuronectes platessa Linnaeus, 1758

## TL;DR

This paper compares two methods for determining sexual maturity in female European plaice, finding that the stereological method is more accurate than the macroscopic method.

## Contribution

The paper introduces a novel approach to reduce subjectivity in stereology by checking cellular homogeneity across ovarian sections and implementing a reading protocol.

## Key findings

- The macroscopic method correctly identifies sexual maturity in only 40.4% of cases.
- The stereological method provides a lower size estimate for sexual maturity (20.6 cm) compared to the macroscopic method (28.6 cm).
- Developing individuals are often misclassified as immature using the macroscopic method (47.9%).

## Abstract

To implement regulations in fisheries, like the minimum catch size for a species, data on the species’ biological parameters must be collected and analyzed. Among these parameters, sexual maturity of the fish is evaluated. Multiple methods exist, and in this paper we used the macroscopic method (visual appreciation of the sexual organ to classify the fish into a maturity phase) and the stereological method (identifying and counting cells inside the fish’s sexual organs to determine the maturity phase). The methods put forward try to minimize the subjectivity brought on by the human agent that will determine the maturity phase, through the implementation of guidelines and calibration exercises. Beyond lowering the human assessment bias, this article also shows that the macroscopic method, used to collect data that will later be exploited to set up fishery regulations, accurately determines the sexual maturity of female plaice only 40.4% of the time. This leads to estimations of size at first maturity of 28.6 cm for the macroscopic method compared to 20.6 cm when using stereology, differences in sizes that could have a major impact when setting up fishery regulations.

Two methods (macroscopic and quantitative histology) to determine sexual maturity are described in this paper, with an emphasis on the methods used in stereology. Based on a point counting grid (Glagolev’s method), ovarian cellular structures were quantified throughout the whole cross section of 151 Pleuronectes platessa individuals. The novelty of this paper is checking the cellular homogeneity in the anterior, median and posterior sections of both ovaries from 15 fish, accompanied by a reading protocol to limit subjectivity during stereology readings. Once the ovarian structures were quantified for 151 individuals, a model was set up following the standardized staging grid of the International Council for the Exploration of the Sea (ICES), classifying each sampled individual into a maturity phase. These results showed that 23.4% of the time immature (A) individuals were correctly identified through the macroscopic method, but 47.9% of developing (B) individuals were classified under A, while individuals that had finished spawning were correctly identified as such 80.4% of the time. Finally, the maturity ogive and the length at which 50% of the population is sexually mature (L50) were calculated, with an L50 of 28.6 cm when using the macroscopic method and an L50 of 20.6 cm with the stereological method.