The somatic growth, sexual maturation and fecundity of individually marked first‐time spawning female Atlantic cod Gadus morhua were examined under different varying temperature and feeding regimes over the months preceding spawning. A negative correlation between somatic and oocyte growth was found, reflecting the changing energy allocation pattern. Nevertheless, the somatic growth of mature individuals was at least as high as those of immature fish over the period of vitellogenesis. Potential fecundity was positively correlated with body size, but neither temperature or feeding regime significantly affected this relationship. Consequently, fish with unlimited feeding opportunity invested more energy into somatic growth during vitellogenesis compared to those held under a restricted ration. This indicated that once Atlantic cod had made the decision to invest in first reproduction, they allocated a certain amount of energy relative to their size into egg production and any surplus was invested into somatic growth. Low temperature led to an arrest in the onset of vitellogenesis and significantly affected the number of females that matured.