The water quality in relation to the well-being and growth of fish

Water quality is an essential parameter for effective and disease-free aquaculture systems (Ye, 2001). The quality is influenced by the ambient water and soils, geological and climatic properties of the watershed as well as the pond and farm management practices.

At least for filling ponds and balancing water losses through leaching (seepage) and evaporation, water is required. In the sloping study area, pond water is derived from precipitation and water run-off as well as from springs, irrigation canals or streams. Whereas ground water from (deep) springs is commonly considered to be constant in quality and free of toxic pollutants as well as contamination with predators or parasitic living organisms (Appleford et al., 2003; Summerfelt, 2008), the water quality of watershed ponds is strongly influenced by the land use and household activities. Adverse effects from farm and household

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activities on the well-being of fish may be associated with the entry of sediments, pesticides and detergents, which will be discussed later (7.3.7). Contamination might be even more severe in ponds located downstream compared to upstream ponds. However, the close linkage between ponds and fields may also have positive external factors, since nutrients from the ponds in the outflow can be reused in the paddy fields. The use of the effluents from a hybrid catfish culture yielded in rice production that was comparable to that which received a regular fertilization regime (Lin and Yi, 2003). The flow of nutrients within the watershed in the Chieng Khoi commune is further investigated within the framework of the “Uplands program”.

The presence of water emersion points on the pond bottom is a special feature in some ponds and may be associated with the karst character of the region (2.1.3). These ponds are not completely drainable, which may hamper pond management in such factors as the control of pathogenic agents by drying out the pond floor.

In general, Yen Chau ponds are constructed either consecutively or parallel and water flows by means of gravity. In the case that they are constructed in consecutive order, water flows through each pond before it is discharged. This may lead to water pollution in the ponds located at lower positions, and this layout also has the disadvantage that the decoupling of disease-affected ponds becomes difficult or even impossible. In the case of a parallel pond layout, different ponds receive and discharge water from and to the same canal or stream. Also in this pond layout, ponds at lower locations may be affected by waste-water derived from ponds at higher locations.

Limited water availability for the ponds was a severe problem for a number of fish farmers during the study period, which was very severe at the times of rice transfer. In one of the investigated ponds, for example, the water level dropped from almost 120 cm to only 55 cm (Figure 40). Shallow water has highly fluctuating water temperatures and DO levels.

For ectothermal animals such as fish, the water temperature is critical and influences the growth and well-being of the fish. Usually, the growth of the fish increases with a higher temperature; then, it passes an optimum peak and falls quickly once the temperature approaches the upper lethal limit (Black, 1998). The average temperatures measured over a year ranged between 22 to 24°C in the morning and 23 to 26°C in the afternoon. The lowest water temperature sampled was 13°C and the highest was 33°C over the entire course of the study period. Grass carp can tolerate a wide range of temperatures from 0 to 38°C (Fishbase, 2006b). The preferred temperature is around 29°C, and the superior incipient lethal temperature is 39.5°C (Alcaraz et al., 1993). The ideal temperature range for fish culture is

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generally above 25°C for most warm-water Asian fish (Cagauan, 2001) and feeding activity tends to decrease or stop at temperatures below 20°C (Ling, 1977).

Whereas the water temperatures in the study area were close to the optimum during the hot summer months, they probably did not satisfactorily support fish growth in the wintertime. The relatively cold winters in the study area limit fish production and may even lead to mortalities of the tropical fish species such as pirapitinga and tilapia.

While tilapia can tolerate high temperatures, up to 42°C in the case of O. niloticus (Fryer and Iles, 1972), at temperatures below 20°C they generally reduce feeding and other activity, which stops completely at temperatures around 16°C (Chervinski, 1982). It has been reported that tilapia begin to die when the water temperature drops to 11°C (Sifa et al., 2002) or 13.6°C (Charo-Karisa et al., 2005). The relatively cold winter in the study region is therefore not suitable for the year-round production of tilapia on a large scale. The lowest water temperature measured was 13.4°C (6.2.1). However, it is likely that the temperatures dropped even lower than this, since the measurements usually took place after sunrise and the temperature was not monitored permanently over the entire study period. The observed tilapia mortalities are further indices of this.

Another factor associated with temperature that probably led to stressed fish and even fish mortalities in the study area are temperature shocks. This occurs when fish are transferred to a new pond without first letting them adapt to the new environment. Furthermore, heavy rainfalls and/or hailstorms can also lead to sudden temperature changes. Temperature changes (e.g. water colder by 8°C) that occur shortly after feed application may stop or slow down the digestive processes with the result that food remains undigested or half-digested in the digestive tract. This may lead to gassy and bloated fish, which could lose their balance and die (Svobodova et al., 1993). However, while some temperature-related mortalities may be avoided through better management such as a proper tempering of fish, not much can subtend the mortalities in the case of sudden environmental changes. However, if farmers avoid feeding fish when heavy rains are announced, it may at least mitigate the adverse effects on the fish.

The sudden fish deaths that occur after heavy rainfall may also be related to high (oxygen requiring) organic loads on the pond bottom. Especially after some days of windless cloudy weather and a consequently low DO availability caused by the low photosynthetic activity of aquatic plants, sudden rains may lead to a depletion of the available DO. When the surface temperature is lowered as a result of the rain, warm water from the bottom rises and the pond bottom may be turned upside down within only a few hours (Ling, 1977). After a

7 Discussion ₁₃₅

hailstorm in April 2006, for example, the DO level was recorded to be near zero and the SDD below 10 cm in two of the case study ponds (Table 23).

As the temperature increases, the DO content of the water decreases; however, the DO requirements of the fish increase. Oxygen is the first limiting variable in the aquatic environment; the food intake of fish may be suppressed with limited oxygen supply (e.g. Black, 1998; Ross, 2000). Absolute lethal limits are species-specific, e.g. < 0.5 mg l-1 in the case of grass carp (Fishbase, 2006b). Tilapia is a representative for fish tolerant to low DO concentrations and may survive short-term exposures to 0.1 mg l-1 DO. Nevertheless, tilapia will not tolerate low DO in the long-term, nor will they grow, feed, digest or reproduce in a typical way under these conditions (see Ross, 2000).

In general, DO levels above 5 mg l-1 are usually recommended for warm-water aquaculture (review of Boyd, 1982; Cagauan, 2001; Summerfelt, 2008). The average DO levels at 8 a.m. were typically between 2 and 4 mg l-1 _{in the case study ponds. Much lower}

DO concentrations occurred at dawn, which was indicated by fish gasping for air at the pond surface. Hypoxic events usually occur in the morning after high oxygen consumption by aquatic organisms in the night (e.g. Black, 1998). In the example of SV1 in Figure 35, the DO levels at dawn were close to zero. The chronic exposure to lower oxygen concentrations may have adverse effects on feeding and growth and may lead to a higher susceptibility to diseases among stressed fish (Summerfelt, 2008).

Oxygen comes from the photosynthesis of aquatic plants, mixing of air with water as well as from the inflowing water. The more or less frequent water-flow is a particular feature of the ponds in the study area. In intensive farms, a high water exchange is required in order to wash out the excretory products from the fish and to maintain an adequate DO level (Appleford et al., 2003). In the case study ponds, the amounts of water flowing through the ponds varied enormously among the measurements and the average water exchange rate was much lower compared to intensive trout farms with hourly or twice-hourly water exchanges, for example (Scheffer and Marriage, 1975).

The inflowing water carried DO into the system, although only relatively low amounts due to the low amounts of water. In the case of very low oxygen availability in the ponds, the inflow might mitigate the problem of DO insufficiency, especially in the vicinity of the water inlets. However, the water flow does not result in overall DO levels in the ponds considered suitable for fish culture.

In ponds with higher water flow, there tended to be a lower presence of plankton availability, which is discussed later in this book (7.3.4). Furthermore, the sediments entering

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the pond with the water flow regularly led to turbid water in the hot and rainy season. Also, some of the stocked fish species tend to burrow in pond mud in search for food, which may also contribute to the sediment-caused turbidity. Water is considered turbid with a SDD lower than 30 cm (Sevilleja et al., 2001). The turbidity limits photosynthesis due to impaired sunlight penetration and thereby reduces the production of DO.

Pond SV1 demonstrated the highest amplitude of DO between the morning and afternoon measurements as well as the highest phytoplankton concentration (e.g. of Chlorophyceae) among the case study ponds. The high fluctuations of DO can probably be related to relatively high photosynthetic activity during the day and a relatively high consumption of DO by these organisms at night. In the respective pond, the water source was almost free of sediments, since the water originated from a spring nearby and was not polluted from erosion.

Oxygen is consumed through respiration, decomposition and mineralization of organic material and lost to the atmosphere as well as with the water outflow. In the study area, the oxygen-requiring microbial degradation of the feed leftovers on the pond bottom is assumed to have a major impact on the low DO availability. Farmers tended to apply huge amounts of slowly degrading (fibre-rich) plant material, which is discussed in chapter 7.3.5.

Whereas the temperature and DO levels are often not suitable for fish culture, the slightly alkaline pH is considered to be favourable (Svobodova et al., 1993). In contrast, pH values above 10.8 and below 5.0 may be dangerous for cyprinids (Svobodova et al., 1993), values that are very much different to those values measured in the case study ponds.