Feeding value of low opportunity cost biomasses (agri-food by-products) for development of circular pond fish feeds: An evaluation with common carp (Cyprinus carpio)

In many cases, prior to their inclusion in feed, food system by-products need some sort of refinement. Many options are being explored on how these by-products could aid in direct re-generation of food with the shortest value chain. In this study, we hypothesized that common carp raised semi-intensively in European fishponds could convert waste resources to food directly. The 30 % ingredient replacement method was used to evaluate the digestibility of 12 local waste resources of plant origin as candidates for ‘circular feed ingredients’ for feeding pond fish. It was evident from the results that protein digestion (>70 %) is not an issue, even showing similarities between pre-treated resources (brewery wastes) and non-pre-treated resources (oilseed expellers or legumes) – but lysine or methionine decide from 20 % – 49 % utilization of digested protein from those ingredients. It is therefore, recommended that complementary combination of ingredients be explored to overcome limiting DIAAS in natural food base in ponds. In this case, rapeseed expeller and barley distillers’ grain seem to be closest candidates to fix the lysine and methionine limitation in natural pond food base. Microbial/enzymatic/mechanical pre-processed ingredients such as extract of sugar beet, brewery wastes, bakery or extruded breakfast cereals have low pollution potential (P bioavailability 50–80 %) – contrary to proteinaceous resources (oilseed expellers or broken legumes) which had P bioavailability ≤ 25 % (except pea). Bakery or breakfast cereals discards are novel sources of digestible energy for pond fish (∼400 kcal 100 g−1). Given that NDF:ADF ratio (>2:1) allow for easy fermentation in gut, besides possibly laced with microbial enzymes or some pre-hydrolysis, the undigested fibers in sugar beet, legumes, or oilseed expeller (AD carbohydrate 1–20 %) could boost pond food web through microbial upcycling. This study highlights the possibility of developing circular, less-refined semi-intensive pond fish feeds “commercially”. Such feed could enhance resource use efficiency at food-system level. • Human/agricultural food wastes can be used in aquafeed with high digestibilities. • Lysine and methionine are the first limiting amino acids of the protein ingredients. • Brewery/bakery/sugar factories/refined carbohydrates bioavailable P can be 50–74 %. • Undigested fibres may be energy sources for pond microbes for nutrient upcycling.