Method and systems for measuring growth rate in plant or aquatic animal species such as embryonic or adult fish. The methods and systems utilize the measurement of NADH2 production by detecting a coiorimetric and fluorescent shift when a redox indicator such as resazurin is added to a sample. The colorimetric/fluorescent shift is indicative of the reduction of the redox indicator by NADH2. The methods and systems of the present invention may be used to predict growth potential of a plant or animal, and measuring the growth rate of said plant or animal may be helpful for identifying and selecting individuals within a group that have greater growth potential. The methods and systems of the present invention may help eliminate the need for special equipment (e.g., for measuring oxygen consumption), decrease variability of measures, and minimize the effects of external factors (feeding/hormonal status).