A histological description of gonadal development and sex differentiation in the coho salmon (Oncorhynchus kisutch) for both untreated and oestradiol immersed fry

Gonadal development is described in detail for coho salmon ( Oncorhynchus kisutch ) between hatching and 1000 degree-days, post-hatch, to aid sex reversal research. Germ cell morphology and sequence, vascular and reproductive duct development, and gross characteristics of the gonads are presented. Sex differentiation occurs by 380 degree-days, post-hatch (800 degree-days, post-fertilization) and is direct to male and female. Oocytes proliferate rapidly after differentiation while the testes enter a period of quiescence. Fry immersed for short durations in oestradiol (200 μg ⁻¹) are also examined. Hormone immersion advanced sex differentiation by 70 degree-days. The immersions were applied early, at 20 and 90 degree-days, post-hatch, yet still altered the sex ratio and timing of differentiation. Definitive germ cells, which are abundant during this period, may be the type most receptive to steroid treatment.