Reproductive phase determination in male meagre (Argyrosomus regius, Sciaenidae): testis development and histologic corroboration of a gross anatomical scale

Reproductive stage determination of male gonads has received sparse attention in fish biology literature with few studies detailing the building of gross anatomical-and histologic scales. The meagre (Argyrosomus regius) is one of the world's largest sciaenids and supports a significant regional fishery in European and North African waters whose reproductive patterns are yet to be fully investigated. In the present study, we derive a macroscopic grading system for meagre testis using semi-quantitative graphs that feature the testis variability along the species size range and time of the year. We then describe the histological stages and reproductive phases of male testes and determine the extent to which they corroborate the anatomical scale. Our results indicate that gross anatomical analyses are accurate in assessments of the meagre spawning season but may not precisely distinguish the testes of well-mature fish and first spawning virgins. Furthermore, we show that milt expression varies widely with size and misclassifies as immature many smaller fish in spawning-capable condition. We discuss these findings in terms of their contribution to the understanding of testes development and the uncertainties involved in determining the size-at-maturity of male fish using gross anatomical scales.