The dominance of algal-based food webs in fish ponds receiving chemical fertilizers plus organic manures

Abstract

Daily organic manuring plus chemical fertilization of three earthen, 400-m 2 ponds produced fish yields which were not statistically different from the yields produced in three similarly stocked ponds to which only nitrogen and phosphorus chemical fertilizers were added daily. Averaged over the entire 98-day grow-out period, daily yields were 29.5 kg/ha in the organically manured ponds and 27.2 kg/ha in the chemically fertilized ponds. The fish polyculture was composed of common carp (CC), silver carp (SC), tilapia hybrids (TH) and grass carp (GC), with a total stocking density of 20 000 fish/ha. Chemical fertilization provided 0.4 g N and 0.2 g P/m 2, 6 days/week. Organic manuring provided an average of 3 g carbon and 0.3 g nitrogen and approximately 0.3 g phosphorus/m 2 per day. Twenty-four-hour net primary production added an average of 4.0 algal carbon/m 2. Fish yield data, based on comparisons of stable carbon isotopes in the fish muscle and in the natural foods of the ponds and observations of stomach and intestinal contents showed that more than 90% of the fish yield in the organically manured ponds was based on food webs originating with algal carbon. This was harvested either as live algae (by the SC and possibly by the TH) or as microbially processed detritus (by the CC, TH and GC). Organic matter of the manure contributed only marginally to fish growth. These findings indicate a need to re-evaluate, with regard to the specific species of fish and type of organic manure, the relative contributions to fish growth of the mineral and organic fractions of organic manures and composts used in fish ponds.