The early life stage is a critical period in fish species since complicated physiological and morphological changes and often massive mortalities occur at this stage. Studies of the growth patterns and physiological characteristics during the early life stage often uncover lethal factors in this stage, and the information gained can lead to improvements in survival rates during larval production. Allometric growth can be used as an indicator for larval production, and fish experience a change in shape in relation to increases in their ability to perform vital biological functions needed for survival during their early life stage. The RNA/DNA ratio is a sensitive indicator of growth rates, which can be used to evaluate growth potential and nutritional condition, as well as to determine critical periods in the larval stage. Specifically, studies on the development of the digestive tract and digestive capability of the organism can be used as an indicator of nutritional status at an early life stage, thereby providing information useful either for improving feeding protocols in larviculture, suggesting more suitable food items, or designing feasible larval rearing procedures. Through a review of previous studies, this article summarizes information on allometric growth and ontogenetic changes in nucleic acids and digestive enzymes during the early life stage of fish species. It is hoped that this review might revive interest in conducting investigations of the early life stage of fish species, and accordingly we provide some basic information meant for future studies.