Effect of dietary protein source and ingredient grinding size on fish performance, faecal waste production and characteristics of yellowtail kingfish (Seriola lalandi) fed restrictively and to apparent satiation

Recently, yellowtail kingfish (Seriola lalandi) is being cultured in recirculating aquaculture systems (RAS). Yellowtail kingfish have a poor faecal integrity, which makes the removal of faeces by traditional RAS technology difficult. Reducing the faecal waste load in RAS can be achieved by reducing the amount of faeces produced (e.g., increasing digestibility) and/or increasing the removal of faeces. This study assessed the effect of partial fish meal replacement by plant ingredients and the effect of ingredient grinding size on the amount of faecal waste produced and faecal characteristics, like faecal removal efficiency and particle size distribution (PSD), in yellowtail kingfish. This was investigated during two 35-d experiments, where fish were fed restrictively (experiment R) or to apparent satiation (experiment S). For each experiment, individual batches of four experimental diets were produced according to a 2 × 2 factorial design (protein source × ingredient grinding size). The formulas used were identical for both experiments. FM100 diets contained only fish meal as protein source, whilst at FM30-P70 diets approximately 70% of the fish meal were replaced by plant protein ingredients. The effect of ingredient grinding size was tested by including 40% of either a fine or coarse grinding mixture. Tanks were stocked with 20 fish and 27 fish for experiment R and experiment S, respectively. For each tank, fish performance, faecal waste production, faecal removal efficiency and faecal PSD were measured. During both experiments, ingredient grinding size did not affect the faecal removal efficiency or PSD, whilst fish fed the fine FM30-P70 diets restrictively showed a lower faecal waste production. The inclusion of plant ingredients resulted in a lower absolute growth and higher FCR. Furthermore, fish fed the FM30-P70 diets showed a higher faecal waste production, a smaller PSD and a lower faecal removal efficiency. This ultimately resulted in a higher amount of non-removed faeces by 58.3% and 37.1% compared to FM100 diets for the experiment R and experiment S, respectively. In conclusion, the replacement of fish meal with plant ingredients in yellowtail kingfish diets is challenging due to the adverse effects on fish performance, faecal waste production and faecal characteristics. However, feeding yellowtail kingfish to apparent satiation partly reduced these adverse effects of plant ingredient inclusion in terms of faecal waste production and faecal cha