Nutrient loading and effluents of intensive and semi-intensive milkfish (Chanos chanos Forsskal) ponds, and the environment capacity of receiving water / by Neila S. Chavoso.

Thesis (PhD Marine Science) University of the Philippines

Includes bibliographical references.

Laboratory and pond experiments, and survey of the environmental condition of a receiving water were conducted to determine the amount, forms, and flow of N and P wastage from milkfish to the receiving environment. Laboratory results showed that the total nutrient wastage from feces and excretion vary with the type of feed used and size of fish (31.2 — 263.0 grams). Nutrient losses were lowest in fish fed /ub-/ab (natural food-based diet) (0.31 g N and 0.14 g P (kg fish)') and in very big fish (0.30 g N and 0.11 g P (kg fishy"). Total N and P losses of fish fed commercial diet (0.48 g N (kg fishy', 0.17 g P (kg fish)! or 27.7 g N (kg feed)', 9.8 g P (kg feed)') were comparable to fish fed SEAFDEC formulated diet (0.47 gN (kg fish)', 0.19 g P (kg fish)! or 28.8 g N (kg feed)", 11.7 g P (kg feed)"). Uneaten feed (SEAFDEC formulated diet) was calculated to contribute about 10 — 12% of total N and 11 —
22% of total P input to the pond. Approximately 5 — 7% of total N and 17 — 24% of total P input to the pond came from fecal production, and 4 - 7% of total N and 15 — 21% of total P from fish excretion. Fecal matter and uneaten feed entered the pond mostly as organic N (88 — 95%) and unavailable P (96 — 97%, bound and residual P), The N and P input from fish wastes and uneaten feed undergo nutrient transformation in the pond at biomass of 1419 kg ha! and below. Nutrient build-up was apparent at biomass of 1610 kg ha‘ and above.

The holding capacity of un-aerated, semi-intensive pond was safely set slightly below 1348 kg ha! (~1300 kg ha’') or 54 kg feed ha! day! (~50 kg feed ha" day") based on D.O concentration of less than 1.0 mg L"'. The holding capacity of intensive pond was determined to be slightly below 5107 kg ha (~5000 kg ha") or 110 kg feed ha’! day! (~100 kg feed ha” day’) based on maximum chl a concentration of 0.37 mg L"' and average effluent criterion of
13.4 mg NO3-N L".

Upon undergoing some internal processes within the pond, organic N and P wastes were mostly transformed, discharged, and finally reached the receiving water as dissolved inorganic N (ammonia + nitrite + nitrate) and PO,-P. The major N and P output in semi intensive pond (N, 93%; P, 54%) and N output in intensive pond (63%) were in the water discharge. In the culture period of 162 days for intensive pond and 118 — 120 days for semi- intensive pond, the total N losses to the environment represented 76% (106 kg N (ton fish)') and 88% (192 kg N (ton fish)') of N input in intensive and semi-intensive ponds, respectively.

Percentage P losses were slightly lower compared with N but still higher in semi-intensive
pond (83%, 20 kg P (ton fish)') than in intensive pond (73%, 12 kg P (ton fish)').
These results revealed that medium-scale production (semi-intensive system) has higher nutrient loss per kg of fish produced, but high-scale production (intensive system) has greater impact in the receiving environment because of the higher total amount of nutrient loss.

The receiving environment is characterized as net heterotrophic and denitrifying, and
its environmental capacity has been reached during the dry months. Positive net DIP during the dry months and DIN during the rainy months revealed that the system is a source of DIN and DIP for fish farms operating around the area. Based on N and P discharges of semi-intensive and intensive milkfish ponds, the environmental capacity in terms of the maximum DIN or DIP input was set at 13.3 x 10° mole DIN per day” and 3.3 x 10? mole DIP day. At holding capacity (intensive, 5000 kg ha’'; semi-intensive, 1300 kg ha’), it is recommended that the maximum operational area should not exceed 73 ha for intensive pond and 82 ha for semi-intensive pond if the maximum DIP criterion of 0.045 mg L" is followed. These are equivalent to total feed input of 2.9 tons for intensive pond or 3.3 tons for semi-intensive ponds at a feeding rate of 4% of biomass daily. Production could be affected through reduction in fish growth, occurrence of diseases, and fish mortalities if this environmental capacity is exceeded.

Submitted to the University Library 08/04/2020 T-5941