# From Feed to Fish--Nutrients' Fate in Aquaculture Systems
## 4 Discussion
### Outputs  
Fish are the targeted endpoint for the nutrients added to the aquaculture ponds; hence, they are generally responsible for a considerable share of the output. Our review, however, shows that the share of nutrients ending up in the harvested fish varies considerably within and among cultivation systems. Harvested fish were reported in most of the assessed studies, indicating their importance in the overall studies on nutrient dynamics or growth performance. The contribution of harvested fish to the nutrient outputs in aquaculture systems in the assessed papers varied depending on factors such as pond management practices (e.g., feed type, species composition, and cultivation methods). In either monoculture, polyculture, or integrated systems, no more than 20% of the carbon was harvested as fish. Although nitrogen and phosphorus had higher percentages (>70%) in one study, the majority of the papers reported no more than 45% of the nitrogen as fish, and the phosphorus assimilated by the fish biomass was generally less than 45%.  
The pond's bottom was found to often be the second most important endpoint of nutrients, with up to >80% of C, N, and P ending up there. However, the variation among aquaculture systems was large, particularly in the high-water-exchange ponds where nutrient retention in the sediment was considerably lower. Only a few studies included the loss of nutrients from the sediment during water drawdown and fishing phases [47]. The nutrients that become mobilized and flushed out transform the fishponds into significant sources of nutrients for downstream aquatic systems [32].  
Particularly in ponds with a high water exchange rate, outflow was an important non-targeted endpoint of nutrients [28]. In cultivation systems where low or no water exchange is practiced, the loss of nutrients to downstream waters is low [18; 34]. Noteworthy is that even when water exchange during cultivation is low, nutrients may still be flushed out during the harvesting phase when ponds are drained. Part of the nutrients accumulated in the pond sediments throughout the whole cultivation period may then be flushed out due to resuspension [29; 32].  
Nutrient-rich sediments offer a viable option for growing crops within the ponds during the dry phase [14], used as fertilizer and soil conditioner in plant culture, and also in non-aquaculture agriculture activities [47]. This approach not only optimizes nutrient utilization for fish farmers who also engage in agriculture but also benefits the environment by minimizing the release of nutrients into water bodies and the atmosphere. New studies should incorporate sediment as indicators to assess the sustainability of fish farming, particularly in instances where there is a significant influx of nutrients from feed and fertilization methods.  
While infiltration may contribute to water and nutrient losses from fishponds, it was generally not quantified, and when it was, the loss of nutrients through leakage towards the groundwater had a minor contribution to the outputs.  
The reported denitrification and ammonia volatilization varied depending on pond management practices and species composition, but overall, their contribution to the total output was minor. N\({}_{2}\) efflux was measured only in a few studies [21; 24] and was found to be emitted more through ebullition rather than diffusion. Diffusive fluxes were also the main ways of CO\({}_{2}\) emission, but CH\({}_{4}\) was more emitted through ebullition. Both pathways had minor contribution to the output (<1.5%) [25]. Nevertheless, from an environmental sustainability perspective, these emissions are very important [9; 10]. It is also noteworthy that those values represent the emissions during the cultivation period, and the lack of investigation during other stages of the production cycle, i.e., pre-stocking phase fishless, after pond draining, and dry pond can lead to uncertainties on the overall greenhouse gas emissions [9].