Nutrient availability drives production in pond-based aquaculture systems . The principal nutrients considered in this study are carbon (C), nitrogen (N) and phosphorous (P) . T he challenge is to make these nutrients available in the right amounts and ratios to the farmed species . The nutrient use efficiencies , measured as the fraction of available nutrients retained in de novo fish production , provide insight in the total amounts of nutrients accumulating, volatilizing or discharged from the pond. The e nvironmental sustainability of pond farming depends on how well the nutrients that were not retained in fish biomass can be recycled.
Carbohydrates (CHO) are the most abundant type of molecules available in nature, the majority of which are non-starch polysaccharides (NSP) (Fig.1). Higher animals, including fish and shrimp, cannot digest NSPs, but bacteria and fungi in ponds can. Globally, inland surface waters emit annually 6 times more CO2 by degrading NSPs than the burning of fossil fuels (doi:10.3389/fenvs.2022.904955) . In nature, t hese CHOs are the fuel used to break down organic matter and provide the N and P that drive natural food production in aquatic ecosystems.
With pond feeds we do the opposite . Compared to terrestrial NSP-rich wastes, aquaculture feeds are super-rich sources of N and P, but also a poor energy (C) source. Post-feeding aquaculture waste contains insufficient energy to recycle the N and P waste through the food web. I n outdoor ponds, extra carbon is added through algae production, but in intensive systems the algae production is too small to recycl e all the post-feeding N and P waste. For this reason, nutrients are removed, ofte n by discharging to surrounding surface waters, or extra CHO is applied, as in biofloc systems, to partially recycle the waste and to maintain water quality.
Recent research results will be shared showing that the type of CHO administrated affects production in ponds and biofloc systems . In extensive and semi-intensive systems , including up to 40 – 45 % of feed ingredients like wheat bran or rice bran rich in lignocellulosic and hemicellulosic compounds (Fig. 1) in pelleted feed did not lower production compared to conventional feed in tilapia ponds, and resulted in slightly less production in whiteleg shrimp biofloc systems.