Faecal waste characteristics of yellowtail kingfish (Seriola lalandi) fed with pelleted and natural feed

Yellowtail kingfish (Seriola lalandi) is a fast-growing fish species. One of the challenges of farming yellowtail kingfish in recirculating aquaculture systems is their poor faeces integrity, also referred to as ‘diarrhoea-like’ faeces. Whether diarrhoea-like faeces occur under conditions when feeding on its natural feed items or are diet induced, is unclear. This study assessed the effect of feed type (pelleted vs. natural feed) on the faecal characteristics and faecal waste production of yellowtail kingfish. Three dietary treatments were studied over a 35-d experimental period: a pelleted diet based on marine ingredients (Marine; open formula); an experimental pelleted feed based on the ingredient composition of a commercial kingfish feed (Commercial Dummy, CD; closed formula); and a diet composed of four, individually fed raw (unprocessed) natural ingredients and commercial dummy pellets at a ratio of 1:1 on dry matter basis (Natural and Commercial Dummy, NCD). The NCD treatment was intended to clarify whether diarrhoea-like faeces are naturally occurring in yellowtail kingfish. Each dietary treatment was tested in four tanks, which were stocked with 27 yellowtail kingfish (mean initial weight 39 g). Fish were fed to apparent satiation twice daily for 1 h. For each tank nutrient digestibility and faecal characteristics were measured. The inclusion of natural ingredients reduced the faecal waste production (p < 0.001). Furthermore, the faeces integrity of yellowtail kingfish fed with natural ingredients was not poor (not diarrhoea-like). At the natural treatment, fish excreted faecal pellets and short strings, which were not observed at the other treatments. Faecal waste collected from fish receiving only pelleted feed was classified as diarrhoea-like. The highest faeces removal efficiency by settling was observed at the NCD treatment compared to the other treatments (p < 0.001). Consequently, the lowest amount of non-removed faeces per feed intake (p < 0.001) was observed at the NCD treatment (62.9 g OM/kg OM FI), followed by the Marine (101.1 g OM/kg OM FI) and Commercial Dummy treatment (111.7 g OM/kg OM FI). In conclusion, this study shows the potential of dietary interventions to alter the amount and integrity of faecal waste. This offers possibilities to reduce the total suspended solid load for yellowtail kingfish farming in recirculating aquaculture systems.