With reference to the role of biofilters in Recirculating Aquaculture System, consider the following statements : 1. Biofilters provide waste treatment by removing uneaten fish feed. 2. Biofilters convert ammonia present in fish waste to nitrate. 3. Biofilters increase phosphorus as nutrient for fish in water. How many of the statements given above are correct?

Answer: B. Only TWO statements are correct (1 and 2); Statement 3 is wrong. Recirculating Aquaculture Systems (RAS) are tank-based fish farming systems where water is treated and recirculated to minimise water use. Statement 1 is CORRECT. Biofilters (along with mechanical filters and clarifiers) help remove uneaten fish feed, faecal matter and dissolved organic waste from the recirculating water. Statement 2 is CORRECT. The CORE function of a biofilter is NITRIFICATION: ammonia (NH3/NH4+) excreted by fish (highly toxic) is converted to NITRITE by Nitrosomonas bacteria, then NITRITE to NITRATE (less toxic) by Nitrobacter bacteria. Nitrate is then removed by water exchange or denitrification. Statement 3 is WRONG. Biofilters do NOT increase phosphorus as a nutrient. Phosphorus is a UNWANTED accumulating waste in RAS systems, generally removed via clarifiers and water exchange. Fish do not need phosphorus added through biofilter action. Source: FAO Aquaculture publications / standard aquaculture engineering texts.