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Episodes of low concentrations of dissolved oxygen and high concentrations of ammonia are major causes of fish stress, which in turn, reduces growth and increases mortality rates in aquaculture ponds. This study measured the effects of water de-stratification on dissolved oxygen and ammonia concentrations in tilapia ponds in Northern Thailand. Fifteen ponds in five provinces in Northern Thailand were sampled on multiple dates in the hot, wet, and dry seasons. Thermal water stratification peaked around 14:00-16:00 h each day; wherein the differences between surface and lower water temperatures in the 0.8-2.0 m deep ponds reached 1.3-4.0C. Thermal de-stratification in the hot season and dry season usually occurred late at night; in the wet season however, it occurred early in the evening due to the cooling effects of rain. The mixing of surface and bottom waters decreased dissolved oxygen levels in water near the surface, and, increased concentration levels in the bottom water layer. Mean DO concentrations of integrated and commercial ponds were lower than 1 mg/L from 02:00 to 06:00 h, in 68 % of the observations. Repeated measures ANOVA analysis showed that water depth and fish culture systems have significant effects on total ammonia nitrogen (TAN). TAN concentrations near the surface were lower than at the bottom, and increased after water destratification. TAN in the integrated culture system was significantly higher than in commercial and subsistence systems. Toxic, un-ionized ammonia (NH3) concentrations in all culture systems, however, were still lower than standard toxicity level for tilapia. The improved understanding of the seasonal and culture-system specific effects of de-stratification on pond water quality provided by this study, are useful for improving pond management practices to reduce the risks of exceeding DO thresholds. In particular, fish farmers should: (1) take care to avoid over-feeding and manage water and sediments to prevent excessive accumulation of organic matter and waste at the bottom of ponds, which can influence other water quality parameters; and, (2) use aeration and mechanical mixing interventions at critical times to reduce stress on fish from low DO concentrations, and thus avoid risks of mass mortality events. [ABSTRACT FROM AUTHOR]

A survey of integrated agriculture–aquaculture systems (IAAS) was carried out in four different agroecologies, an irrigated and three rainfed agroecologies (drought-prone, rainfed lowland and rolling land), in Northeast region of Thailand, of tropical wet:dry climate. Fish culture system of IAAS varied with agroecology but not with the length of farmers' experience in IAAS farming. The size of pond holdings in the rainfed lowland agroecology was estimated to be significantly smaller ( P≤0.05) than in the three other agroecologies. The highest average extrapolated gross fish yield of 2.3 tonnes ha−1 was recorded in the irrigated agroecology and this was significantly higher ( P≤0.01) than in the three rainfed agroecologies: drought-prone (1.1 tonnes ha−1), rainfed lowland (0.5 tonnes ha−1) and rolling land (0.4 tonnes ha−1). Higher fish yield in the irrigated agroecology was associated with longer stocking period and higher amount of pond inputs. Average income of IAAS households from fish also varied across agroecology in a fashion that was noted for yields. However, the farm pond played a pivotal role in the diversification of the farming system in rainfed agroecologies, as it was essentially the only source of water supply for fruit and vegetables production. While fish culture in all agroecologies was a low-input system, intensification through fertilization of ponds with chemical fertilizers can be envisaged to increase fish production in IAAS in Northeast Thailand. [ABSTRACT FROM AUTHOR]