AbstractThe assumption for hermaphroditic fish species that mature individuals of the terminal sex arise directly from mature individuals of the primary sex has led to the use of sex ratios as a proxy for age at maturity (A50). The timing of transition and deficient energy reserves, however, can result in a delay between transition and spawning. To test the assumption of female maturity and investigate the relationship between maturation and energy reserves, common snook, Centropomus undecimalis, a protandrous hermaphrodite, were collected from rivers, estuaries, inlets and offshore habitats on the east coast of Florida during 2010–2015. Immature females were observed every month, with lowest proportions during the peak spawning months of July and August. When calculated based on sex ratio, A50 (8.1 years) overestimated the age at which 50% of the female population was, in fact, mature (4.1–4.9 years). Best‐fit models indicate that mesenteric fat index (IF) and hepato‐somatic index (IH) were significantly affected by gonad phase, month and size and weakly by habitat. In post hoc analysis, immature female IF did not differ significantly from developing and regenerating females, but immature female IH was significantly lower than that for all mature phases except animals in the regressing phase. Although immature females may have sufficient energy in terms of fat, it appears that energy is not allocated to reproductive processes, as evidenced by lower IH. Nonetheless, approximately 95% of females were spawning‐capable during peak spawning months, suggesting that the energy threshold at which immature females reach maturity is met by most females each spawning cycle.