CHAPTER IX
SOME ASPECTS OF WATER QUALITY MANAGEMENT FOR CAGE CULTURE OF SEABASS

Siri Tookwinas

Cage culture of seabass is one type of coastal aquaculture that can be undertaken in the sublittoral zone of the coastline. The coastal ecosystem, or estuarine ecosystem, is where freshwater, as run-off from the land, and sea water meet. Consequently, the estuarine environment is more extreme, and undergoes more violent fluctuations than the open sea or freshwater habitats. Therefore, coastal aquatic organisms have to tolerate variations in the physico-chemical properties of the habitat.

This lesson describes some ecological aspects of seabass which have been cultured in cages in Thailand. Certain bio-physico-chemical parameters of the estuarine ecosystem which influence cage culture of seabass are summarized. Some methods for water quality management are also explained.

1. Suitability of the Aquatic Environment

As mentioned previously, an estuarine ecosystem is subject to various changes in the aquatic environmental parameters. Those aquatic environmental factors that influence cage culture of seabass are summarized below:

a. Water Salinity

Water salinity tends to fall with increasing distance from the open sea but this is not always true. The water salinity gradient depends upon the relative balance of three factors: run-off from the land; rainfall; and evaporation from the estuary itself. The range of water salinity tends to be greater near the water surface than in the bottom. This is due to the specific gravity of sea water being greater than that of freshwater (Table 1).

When the water salinity decreases, aquatic organisms use up more dissolved oxygen for respiration indicating an increase in metabolic rate. Osmotic regulation has to occur, otherwise organisms would be killed by the reduced water salinity.

b. Dissolved Oxygen

Fundamentally, the concentration of oxygen dissolved in sea water is inversely dependent upon salinity and temperature, and is normally about 80 percent of the concentration in freshwater at the same temperature. Vertical gradients in oxygen concentration develop only in estuaries which have a vertical salinity stratification. In other words, the amount of oxygen at various water depths vary with the differences in salinity also at various water depths. Regeneration of oxygen in estuaries is brought about by mixing with well-oxygenated water from rivers or the sea, direct re-aeration from the air, and by the photosynthetic activity of plants. It should be noted that, in an unpolluted estuary, decaying organic matter can produce localized oxygen deficiencies in bottom water. However, in a polluted estuary, the oxygen concentration drops and, under extreme conditions, the water may become entirely lacking in dissolved oxygen (anoxic).

Figure 1. Suitable habitat for cage culture of seabass.

The desirable range of dissolved oxygen for aquatic organisms is 5.9 mg/l and higher. A range of 5.0 mg/l to 1.0 mg.l may have a pronounced but sublethal effect on growth rate to fish and other aquatic organisms. A value of less than 1.0 mg/l would have a direct harmful effect on fish (Figure 2).

c. pH

The pH of estuarine waters is more variable than that of the open sea. Under normal and unpolluted conditions, pH values from 6.8–9.25 have been recorded. In vertically stratified estuaries, the surface waters generally have a higher pH than the bottom waters. This is due to photosynthetic activities in the surface water taking up carbon compounds from the water column. The absorbed carbon compounds would affect pH balance in the surface water.

Figure 2. Effect of dissolved oxygen on aquatic organism (after Boyd, 1979).

The desirable range of pH for fish production is 6.5–9.0. Values lower than 6.5 and higher than 9.0 have direct effect on fish (Figure 3).

d. Turbidity

The penetration of light into estuarine waters depends largely on the turbidity which is much more in estuaries than in the open sea. This turbidity is due to sedimentary materials from three sources: the rivers flowing into the estuary, transport into the estuary from the open sea, and a reworking within the estuary.

In general, turbidity decreases, and therefore the depth of light penetration increases as one gets nearer the open sea. The effect of turbidity and rapid absorption of light upon the photosynthetic processes which can take place in an estuary is considerable. Clearly, the phytoplankton will receive sufficient light for this vital process in the surface layers only, and will generally contribute little to primary production in turbid estuaries. Under these conditions, the shore-dwelling plants, particularly the microflora, take on a real importance as the primary source of food.

Fig. 3. Desirable range of pH for aquatic organisms. (after Boyd, 1979)

2. Water Quality in Seabass Culture Area

The physico-chemical properties of coastal aquaculture and seabass cage culture in lower west south of Thailand (Krabe, Trang and Satul Provinces) were surveyed between July 1980 and August 1982 in 19 survey stations. The methodology for water analysis was set along the standard line of APHA (1975), Lind (1974) and Strickland and Parsons (1969), as temperature, visibility, dissolved oxygen, pH, salinity, ammonia-nitrogen, nitrate-nitrogen, phosphate and silicate.

The water quality then were as follows (Table 2):

Table 2. Water quality in coastal aquaculture area in Krabi, Trang and Satul Provinces (lower west south of Thailand, monthly average from 19 surveyed stations, July 1980 to August 1982.

3. Sudden Fish Kills in Songkhla Lake

a. Cage Culture of Seabass in Songkhla Lake

Songkhla Lake is the largest lagoon in Thailand and in Southeast Asia as well. It is located at Latiture 7 08 N-7 50 N and Longitude 100 07 E-100 37 E. Total area is approximately 98,680 hectares (616,370 rai). The eastern side of the lake (Thale Sap Tonnok) opens into the Gulf of Thailand).

The major aquaculture system in the lake is seabass culture in netcages. It was introduced 16 years ago (in 1972) by the Department of Fisheries. In 1986, seabass production from the 300 netcages of some 115 farmers in the lake was approximately 98.5 tons.

The annual average water values in the lake (Thale Sap Tonnok) are as follows: salinity, 13.68 ppt; dissolved oxygen, 7.17 mg/1 and pH, 7.89. More details are shown in Table 3.

Table 3 Annual average of water properties in gongkhla Lake (Thale Sap Tonnak) (1984–1985).

b. Cause of Sudden Fish Kills

The fish farmers in Songkhla outer lake always face the problem of fish kills in the summertime. Results of technical investigations have shown that the fish kills are caused by aquatic environmental pollution; the depth of water for cage culture was only 0.37–0.90 m (average 0.66 m). Dissolved oxygen was depleted to 1.30 mg/l at night time (Table 4 and Figure 4).

Table 4. Water properties at seabass culture in Songkhla Lake: comparing average water properties from every culture area to the area of sudden fish kills.

Remark The water samples were collected between 09.00–11.00

The dead fish were examined and a bacterial plate count was conducted at the disease laboratory. The morphology of the fish and the bacterial check on dead and live fish showed normal conditions.

Other culture areas were also investigated. The bottom sediment under the netcage contained a high value of waste organic matter as shown in chemical oxygen demand (C.O.D.) value. The benthic organism found was a polychaete which can bloom in polluted condition. It can be noted that the decomposition process occurs at the bottom sediment, which consumes a lot of dissolved oxygen in water column.

4. Aquatic Environmental Improvement

Seabass culture in Songkhla Outer Lake has been going on for 8 to 10 years. The waste material from cage culture is directly deposited at the bottom. The cages are also set very close to each other so that the number of culture cages could have been more than the carrying capacity of the area. The fish farmers also stock densely at 42.8 kgs/cubic meter. The technical investigations suggest that the aquatic environment at the culture area must be improved by following these measures:

1. The maximum stocking density of fish should be 300 fish per cage (cage size 7 × 8 × 2 m).

2. The dissolved oxygen can be increased by air pump, especially at night from 0200–0800 hours.

3. For a long-term improvement measure,

1. The culture cages should be moved further away from one another and away from the village (about 300 m). This would avoid the effect of domestic waste materials.

2. The bottom sediment should be dredged. This would directly decrease the decomposition of waste materials.

Figure 4. Dissolved oxygen in 24 hours of seabass culture in Songkhla Outer Lake (after Tookwinas, 1986).

REFERENCES

Boyd, C.E. 1979. Water quality in warmwater fish ponds. Auburn University, Alabama, 359p.

Perkins, E.J. 1974. The biology of estuaries and coastal waters. Academic Press Inc. Ltd., London, 667 p.

Remane, A. and C. Schlieper. 1971. Physiology of brackishwater. Wiley Interscience Div., New York. 365p.

Tookwinas, S. et. al. 1985. Investigation on water properties in coastal aquaculture areas in Krabe, Trang and Satul Provinces. Brackishwater Fisheries Division, Dept. of Fisheries. 35p.

Tookwinas, S. et. al. 1986. Study on aquatic environment of seabass cage culture at Songkhla Outer Lake: investigation on the cause of sudden fish mortality. Thai Fisheries Gazette, 39 (3): 255–263.

Tookwinas, S. 1986. A general review of the hydro-physico-chemical properties of Songkhla Lake, Southern Thailand. Songklanakarin J. Sci. Technology. 8(1):111–115.