Beyond feeding and weighing: utilizing physiological approaches in fish nutrition research  

Research aimed at reducing the percentages of fishmeal and fish oil in feeds has been underway for decades, but the urgency to make progress has increased greatly over the past eight years. Conventional nutritional evaluation of feed ingredients and feed formulations has involved measuring apparent digestibility coefficients followed by assessment of fish growth and feed efficiency as test ingredients are increased in feed formulations at the expense of fishmeal or fish oil. As a result of this research, fishmeal and fish oil levels in feeds have been reduced by over 50%. However, in the case of fishmeal substitution, further reductions generally lead to decreased fish performance, even when feeds are fortified with amino acids, minerals and other essential nutrients. Researchers have successfully utilized molecular tools to gain insight into metabolic or physiological changes associated with higher use levels of alternative protein and oil sources. This has increased our understanding of possible mechanisms associated with reduced fish growth or feed efficiency but has not led to further reductions in feed fishmeal levels. To make further advances, researchers must use physiological approaches that measure the effects of feed formulation changes on growth and fish health at the tissue and whole animal levels, especially over time. Fish have demonstrated a capacity to adjust to feed formulation changes over short-term feeding trials, but over longer periods, metabolic and morphological problems develop that impair fish growth. Measuring apparent digestibility coefficients is essential to formulate feeds, but more information is necessary on temporal aspects of nutrient absorption and utilization to truly understand why feeding alternate proteins or protein blends reduces fish growth and protein retention. To develop useful information and transfer knowledge to the aquaculture industry, fish nutrition research should utilize more comprehensive, holistic approaches to measure downstream effects of feed ingredient changes on fish physiology. This will help 'connect the dots' and avoid concentrating on single metabolic pathways that heretofore have not led to meaningful breakthroughs in development of sustainable fish feeds.