Fish as Feed: Using the nutrient Fish In: Fish Out ratio (nFIFO) to enhance nutrient retention in aquaculture

Abstract

The aquaculture industry is often criticised for its use of “fish as feed”, due to the limited stocks of fish used as marine ingredients (MIs). Several efforts to measure the efficiency of wild marine fish inputs into aquaculture have used versions of the Fish In: Fish Out metric (FIFO), the ratio between fish biomass included in aquafeeds compared to the biomass of fish produced from aquaculture. However, FIFO metrics overlook the retention of nutrition from those resources to the final farmed product, particularly omega-3 long chain polyunsaturated fatty acids (LC-PUFAs) that are concentrated in MIs and a central reason for their continued use in aquafeed. We introduce the nFIFO ratio to measure nutrient flows from marine resources and to aid improved retention of nutrients in aquaculture. Using literature values for LC-PUFA content in different feed fish species, MIs and farmed salmon, we quantify the retention of LC-PUFAs. We found that the mean retention of LC-PUFAs from feed fish to salmon was 37.4 %, giving a nFIFO of 2.17. Applying economic allocation principles as used in Life Cycle Assessment resulted in fish oil having a higher nFIFO burden but favoured the use of processing by-products as raw materials in MIs. We tested these principles using sensitivity analysis by 1) incrementally replacing whole fish meals and oils for their by-product counterparts, and 2) raising the price of fish oil in relation to meal. nFIFO was much improved by substituting whole fish derived MIs with those from processing by-products and highly correlated to rising fish oil prices. nFIFO is dependent on high-resolution feed data, as our study revealed high variation in feed fish nutritional content related to various seasonal and environmental factors that affect the outcome of the calculations. We provide an Excel file to aid stakeholders calculate nFIFO and other FIFO metrics.