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Background. The purpose of this study was to determine the timing of gonadal differentiation in two gadoids: Atlantic cod, Gadus morhua L., and haddock, Melanogrammus aeglefinus (L.). This information is required to develop a practical protocol for the production of monosex populations of these species for aquaculture. Materials and methods. Cultured larvae and juveniles were collected weekly, measured (total length; TL), prepared histologically and then examined microscopically for the presence of characteristic stages of gonadal differentiation. Results. In Atlantic cod, undifferentiated gonads were present by 18 mm TL, at 84 days post hatch (dph), and definitive germ cells by 19 mm TL (90 dph). Ovarian cavities were first observed at 27 mm TL (102 dph), and by 35 mm TL (112 dph) anatomical divergence into two types of gonads was clear. In haddock, undifferentiated gonads were observed at 21 mm TL (64 dph) and an ovarian cavity was evident at 29 mm TL (71 dph). Conclusion. Gonadal differentiation in Atlantic cod and haddock occurs at roughly the same size in both species (27 and 29 mm TL, respectively). For successful sex reversal, the administration of steroids should therefore begin at approximately 25 mm TL, shortly after weaning onto dry feed. |
