Practical implications of lowering dietary starch content on waste management in recirculating aquaculture systems operated with drum filtration or sedimentation in yellowtail kingfish (Seriola lalandi)

Resúmen

Dietary starch negatively affects faecal removal efficiency by sedimentation in yellowtail kingfish (Seriola lalandi). In general, research on waste management in recirculating aquaculture systems (RAS) is conducted on solid removal by sedimentation. However, most commercial farms operate with screen filtration. Compared to sedimentation, where the efficiency of removed faeces depends on faecal particle size and density, faecal removal by screen filtration is based on particle size and its stability. Accordingly, this study investigated the effect of dietary starch level on solid waste loading and faecal removal efficiency by different solid removal techniques, namely drum filtration and sedimentation. This was tested in six individual operating RAS over an experimental period of 47 days. The experimental period was divided into two consecutive phases (Phase 1 and Phase 2). During Phase 1 (42 days), the six independent RAS were operated with drum filtration, while during Phase 2 (5 days), the same RAS were operated with sedimentation. Two experimental diets, one with low starch (LS; 0% gelatinized wheat flour inclusion) and one with high starch (HS; 20% gelatinized wheat flour inclusion) levels, were formulated to test the effect of dietary starch level. HS diet negatively affected the apparent faecal waste production, apparent faecal removal efficiency by sedimentation, and the apparent amount of non-removed faeces. No dietary treatment effect was observed when RAS were operated with drum filtration. However, to achieve similar apparent faecal removal efficiency by drum filtration, the backwash frequency was on average 110% higher at HS RAS compared to LS RAS. This is likely to be related to both an increased apparent faecal waste production and the tendency for increased clogging of the drum filter screen at HS RAS throughout the experimental period. Overall, lowering dietary starch level has the potential to reduce solid loading (sedimentation) or improve drum filter performance (water usage) in RAS for yellowtail kingfish.