This extraordinary book is the result of over three decades of Dr. Krisna Rungruangsak-Torrissens career at the Institute of Marine Research in Norway. The book provides new insights into a series of growth mechanisms in aquatic living resources through the digestion and utilization of dietary protein for growth and maturation. Section One shows the initial success of the relationships between genetic variations in trypsin phenotypes, growth, and feed efficiency; additionally, the isoelectric focusing technique to differentiate trypsin phenotypes has been developed. Section Two shows the other successes concerning the effects of temperatures and consumption rates on trypsin phenotypes, growth, and feed efficiency, wherein the first evidence of temperature preferences on feed efficiency and growth dependent on trypsin phenotypes of individuals has been observed. The unique studies of digestive efficiency and growth status have been developed through the activity ratio of trypsin to chymotrypsin (T/C ratio) for growth potential, with the new knowledge of chymotrypsin involving limited and reduced growth rates. Section Three shows insight into the utilization of dietary protein through absorption and transport of free amino acids (FAA), indicating that the levels of plasma FAA and white muscle FAA are dependent on trypsin phenotypes and dietary protein quality. The new buffers of the HPLC system for differentiating more than 40 physiological FAA in biological tissues have been developed. A possibility of white muscle free-hydroxyproline levels related to growth rate has been observed. Section Four explains protein growth efficiency dependent on the genetics of growth capacity and dietary protein levels, whereas a higher level can increase skeletal growth (length) resulting in lower condition factors. The new determinations of RNA and protein by single separation have been developed. Section Five shows the first success on studies of maturation rate in females through active oocyte protein breakdown, using the activity ratio of trypsin-like to chymotrypsin-like (T/C ratio) oocytes. Sections Six and Seven show the in vitro digestibility techniques using dialyzed crude digestive enzyme extracts for quality assessments of dietary protein and carbohydrates, uniquely standardized with respect to the activities of trypsin and amylase, respectively, for comparisons among different enzyme extracts. It is evident that dietary protein is the primary important nutrient while dietary carbohydrates are the secondary important nutrients, regardless of animal feeding habits (carnivores, omnivores, herbivores). Section Eight illustrates the uniqueness of the different biochemical techniques for implementations in natural marine ecosystems of the Northeastern Atlantic Ocean and the Barents Sea, including the development of a neural computational model through digestive efficiency for future studies of aquatic living resources without knowing their histories concerning food availability and growth. Section Nine concludes the importance and usefulness of the biochemical techniques, and describes how to collect the samples. The knowledge from this book can be beneficial for lecturers, researchers, undergraduate and graduate students, and any readers who are interested in nutritional biochemistry. It will provide new perspectives, ideas, and inspiration for finding a new way to make a difference in doing research.