# From Feed to Fish--Nutrients' Fate in Aquaculture Systems
## 3 Results
### Nutrient Output in Fishponds
#### 3.4.2. Outlet Water  
Draining the ponds is the first step of fish harvesting, and depending on the pond size, depth, and shape, the process can be complex, requiring more than a hundred days to be completed in larger ponds [39]. Considering that about 90 to 92% of the pond water is flushed out [13; 18] and that the generally high nutrient concentrations in the pond water further increase during the emptying and harvesting process as a result of resuspension [40], this flux can be an important component of the overall nutrient budget. Thus, the nutrient in outlet water was accounted for in the budget of 17 of the 23 assessed papers. Overall, the contribution of outlet water to the total output of C was low, accounting for no more than 1.5%, as observed in different studies (Table 7). However, higher values were observed in integrated (IMTA) fishponds, where the outlet water contributed 7% to the outputs [25]. In other integrated cultivation systems (IAA) with high-water-exchange ponds, the contribution reached up to 16% of the outputs [13]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Reference** & **Methods** & **C (\%)** & **kg ha\({}^{-1}\) d\({}^{-1}\)** & **N (\%)** & **kg ha\({}^{-1}\) d\({}^{-1}\)** & **P (\%)** & **kg ha\({}^{-1}\) d\({}^{-1}\)** \\ \hline \multirow{2}{*}{[16]} & Same concentration as & & & & & & \\  & pond water & 0.01 & 0.03 & n.r. & n.r. & n.r. & n.r. \\  & Sediment cores & n.r. & n.r. & 0.5 & 0.01 & n.r. & n.r. \\  & Same concentration as & n.r. & n.r. & 3 & 1.7 & 14 & 0.9 \\  & pond water & & & & & & \\  & Sediment cores & 0.2 & 0.2 & 0.4 & 0.04 & 0.5 & 0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Contribution of infiltration to the outputs of carbon, nitrogen, and phosphorus and its rates (kg ha\({}^{-1}\) d\({}^{-1}\)). Values shown in the table are the mean of the values reported in the assessed papers. n.r. : not reported. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Reference** &
\begin{tabular}{c} **Pond** \\ **Hydrology** \\ \end{tabular} & **C (\%)** & **kg ha\({}^{-1}\) d\({}^{-1}\)** & **N (\%)** & **kg ha\({}^{-1}\) d\({}^{-1}\)** & **P (\%)** & **kg ha\({}^{-1}\) d\({}^{-1}\)** \\ \hline \multirow{2}{*}{[13]} & Flushed & 16 & 6 & 61 & 7 & 38 & 0.8 \\  & Not flushed & n.r. & n.r. & n.r. & n.r. & 4 & 0.08 \\  & Not flushed & 0.1 & 0.3 & n.r. & n.r. & n.r. & n.r. \\  & Not flushed & 1.2 & 0.14 & 1 & 0.01 & 0.6 & 0.002 \\  & Not flushed & 1 & 0.15 & 0.9 & 0.01 & 0.6 & 0.003 \\  & Not flushed & n.r. & n.r. & 5 & 0.2 & n.r. & n.r. \\  & Not flushed & n.r. & n.r. & n.r. & 8 & 0.07 \\  & Not flushed & n.r. & n.r. & 12 & 0.5 & n.r. & n.r. \\  & Not flushed & 7 & 2 & n.r. & n.r. & n.r. & n.r. \\  & Not flushed & n.r. & n.r. & n.r. & n.r. & 9.5 & 0.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Contribution of outlet water to the total output of carbon, nitrogen, and phosphorus and its rates (kg ha\({}^{-1}\) d\({}^{-1}\)). Values shown in the table are the mean of the values reported in the assessed papers. n.r. : not reported. The contribution of outlet water to the total N output varies widely, with contributions lower than 15% found in several studies. The contribution of outlet water to the total N output is highest (61 and 76%) in ponds in which the water is flushed out (i.e., replaced by inlet water) during the production cycle [13; 36]. As for P, outlet water accounted for less than 1% of the total outputs in studies conducted in polyculture carp ponds (_Catla cath_, _Labco rohita_, _Cirrhinus mrigala_, and _Hyndulmithths nobilis_) [17; 18; 30] and in _Osphronemus goramy_ ponds [27]. In general, the water outlet was responsible for less than 10%, whereas the highest contributions of outlet water to the total P output were within the range of 33 to 55%. The highest contribution of outlet to the total output of P and N was found in a system where effluents from other fish production (nutrient-rich inlet water) in _Cyprinus carpio_ cultivation were used [33].