Historically, aquaculture nutrition research was primarily focused towards understanding the impact of dietary factors, which affect fish growth and health directly. However, the diet is also the principal source of waste in aquaculture. In recent years, waste management has emerged gained prominence due to increasing emphasis on sustainability and stricter implementation of regulations concerning aquaculture waste discharge. This has directed attention towards interventions which can either reduce the amount of waste generated or enhance its effective removal from production system (i.e., water).
Recirculating aquaculture system (RAS) offers opportunity to minimize impact of aquaculture on the surrounding environment either by effectively removing the waste from the water or by converting the waste into less harmful forms. In RAS, effective solid waste removal is the first and a critical step in waste management. If not removed promptly from the system, these solids contribute to the accumulation of suspended solids and dissolved waste, which can hamper fish health and welfare. Suspended solids can also hamper the functioning of biofilters and UV units in RAS. In RAS farms with optimal feed management, faeces is the major source of solid waste. Thus a key objective in a RAS system is to ensure rapid and efficient removal of faeces, minimizing the accumulation of non-removed faecal waste.
Diet plays a critical role in determining the amount of non-removed faecal waste accumulating in a system. This is mediated both by determining the amount of faeces produced through impact on dry matter digestibility of diet and by altering faecal characteristics that govern faecal removal efficiency (by sedimentation and/or screen filtration). Characteristics of faeces relevant in determining the rate of solid waste removal are size, stability and density of faeces. Alongside these factors, system design and its interaction with faecal characteristics ultimately determine the efficiency of solid waste removal. At present, knowledge on factors determining faecal characteristics remains limited, with several unanswered questions as: what is a natural faecal pellet and stability; do faecal characteristics differ across fish species; which dietary factors govern removal efficiency; can dietary interventions improve faecal stability (i.e., reduce disintegration of faecal pellets); are faecal waste characteristic determined by ingredients or nutrients within a diet; are dietary factors determining faecal removal similar across fish species? Addressing these questions is essential to formulate optimal feeds for RAS. In recent years, several studies investigating the link between dietary factors and faecal waste management in RAS have been conducted at our group (Wageningen University, The Netherlands). This presentation will provide an overview of published and in-house data, highlighting the role of dietary interventions in faecal waste management. The impact of dietary interventions on factors such as size, stability and removal efficiency of faeces (Table 1) will be discussed for various fish species, like salmon, trout, tilapia, common carp, sea bass and striped catfish. Specific dietary factors that will be addressed include feeding level; dietary ingredient composition; type of carbohydrates; type of protein rich ingredients; dietary starch level; dietary viscosity etc.