Aquaculture of Bluefin tuna in the Mediterranean: evaluation of organic particulate wastes

This study shows that alternatives for fishmeal in a fish diet affect not only fish growth but also faeces stability and nitrogen (N) and phosphorus (P) waste production. Wheat gluten diet (WGD), soybean meal extract diet (SBE), soybean meal diet (SBM), duckweed diet (DWD) and single-cell protein diet (SCP) were evaluated as a fishmeal replacement on a 15% weight weight-1 basis in tilapia diets. Fishmeal replacement affected dry matter (dm), protein, ash and P digestibility significantly. Faeces recovery (6.8–11.2%) was not significantly affected, although the amount of non-recovered faeces and total faeces showed significant differences. Duckweed diet and SCP resulted in the largest amounts of non-recovered and total faeces (199–210, 224–225 g dm kg^-1 feed dm). Compared with fishmeal diet (FMD), the WGD and SBE resulted in similar growth, but higher non-faecal N losses (471–495 vs. 416 g N kg^-1 N). Soybean meal diet, DWD and SCP resulted in lower growth but less nonfaecal loss (409–450 g N kg^-1 N). The DWD and FMD had the highest N retention (480 g N kg^-1 N) compared with the other diets (431–451 g N kg^-1 N). Carbon retention, faecal and non-faecal losses and P retention were similar for all diets (302–358, 142–176 and 489–523g C kg^1 C, 606–704 g P kg^-1 P). Phosphorus faecal loss was lower for all diets (329–381 g P kg^-1 P) than for the FMD (401 g P kg^-1 P).