Longevity of simultaneous operation of aquaculture and mangrove forestry as explained in terms of water and sediment qualities

Abstract

A silvofishery system (SFS) of 5.2 ha, simultaneously combining aquaculture (shrimp, crab, and fish) and forestry, was studied to understand how the water/sediment qualities had remained viable for 30 years. The long life of this SFS pond contrasts sharply with a short life of many conventional, intensively managed shrimp ponds (5 years on average). Total ammonia nitrogen in the SFS water (0.06 mg l−1) was much lower than the Thai environmental safety standard for shrimp ponds (1.0 mg l−1) and approximately 0.05 % of an average conventional, intensively managed shrimp pond. Total organic nitrogen of the pond sediment was 1.47 mg g−1 which was almost half of conventional, intensively managed shrimp ponds. The flux study revealed that NH4–N was the dominant form of nitrogen, with lesser amounts of NO2–N and NO3–N, and that NH4–N was being released from the sediment into the water. Nitrogen loss from the pond, which was regarded as the denitrification rate, was estimated to be 71.5 mgN m−2 d−1, corresponded to 55 % of the total nitrogen input. As the average denitrification rate in a conventional, intensively managed shrimp pond is 13.4 %, the SFS was shown to be relatively efficient in removing accumulated nitrogen from the pond. Assuming accepted feed conversion rates, 3,340 kg of feed would have been necessary for the amount of fishery production recorded during 5 May 2005 and 22 March 2006. However, only 380 kg of trash fish was added, representing a saving of 2,960 kg of feed. Such a saving could be attributed to detritus from the mangrove trees that have been growing within the pond and algae encouraged to bloom by the shallow water depth. Therefore, it is suggested that the efficient nitrogen removal due to the high denitrification rate as well as the reduced feed input from mangrove detritus substitution, have contributed to maintaining favourable water and sediment qualities, resulting in the longevity of SFS pond.