By Ariel Repotula
Just because tilapia is one of the most hardy species in aquaculture, many if not all farmers, neglect to provide the best culture conditions for the fish in the ponds. In the Philippines where more than 60% of tilapia production comes from fishponds, today’s losses from fishkills are often blamed to extreme weather situation due to climate change. Sometimes, blame is laid on the brand of commercial feed used. It is very easy to point fingers at the weather or the feed, but the real situation can only be determined through frequent farm visits and observations and talking with farmers.
In Pampanga as in most of Central Luzon, tilapia production shifted from the eastern part where salinity and high temperature are a perennial issue to the western area. These brackishwater ponds are often wide with a single pond ranging from 3 to 15 hectares and were used for milkfish and shrimp production at an extensive level. When these were used for tilapia culture, no significant modifications were made. For example, most ponds remain shallow which results in higher temperature during summer.
High temperature means higher rate of evaporation, thus, increasing the salinity and accumulation of decomposing organic matter. High salinity ponds are always deadly to tilapia especially during stocking periods. Another factor is the inefficient supply and drainage system of these very big ponds. Water gets in and out through the same opening. Often, the portion close to the gate has the best water quality, while about a half or two-thirds of the pond has foul water filled with dead algae, feces uneaten feeds and even dead fish. Needless to say, this condition is not conducive to growth and survival.
There are practical ways to minimize mortalities and potential losses. The following measures are doable and can be done using locally available resources:
1. Deepen ponds to a minimum of 1.5 to 2 meters. It will make water more stable in terms of temperature, salinity, dissolved oxygen and will take longer time for the fish wastes to accumulate and result in negative effect on the stock. Although it may be expensive in the short term, it is the long term solution to mortalities and is best for tilapia business as the ponds become more productive.
2. Provide a separate drainage gate opposite the supply gate if possible. A one-hectare-1.5-meter-deep pond should have a gate that can drain 750 cubic meters per hour so that a maximum of 30% water volume can be changed in one tide cycle when water quality outside is good.
3. If an additional drainage gate is not possible, interconnect two adjacent ponds by installing two gates along the dike separating the two ponds: one installed at one end of the dike and the other at the other end. The dike between serves as a baffle. In one of the gates, install a paddle wheel, propeller or pump to move the pond waters. This has proven very helpful especially when water from the outside is of poor quality and addition of new water is not possible. The water gets more oxygen while releasing ammonia and toxic gases during the process. The decomposition rate of organic matter increases without competing for oxygen with the fish stock.
4. Subdivide very large ponds to result in a maximum of 5 to 7 hectares per pond. The smaller the ponds, the easier to manage water quality. It may entail some cost to do, but again, the result can more than compensate for it.
5. Provide emergency aerators, reservoir or back up deepwell pumps. While it may be expensive, it is an efficient measure to manage water quality when there is no immediate source of clean water not only during grower or finisher stages but also during emergency situations. And again the cost can be easily recovered in just one harvest.
6. Construct and provide acclimation ponds with a source of freshwater. Smaller ponds must be provided with aeration system. This is one of the most critical problems in brackishwater ponds that needs to be addressed. Hatcheries are not built to provide fingerlings for saline waters. In real practice, the pond owner provides his or her pond’s salinity level to the hatchery and the latter will acclimate the fingerlings overnight or in two nights before delivery. This is an ineffective practice and results in poor survival of only 10 to 50%. Acclimation should be done slowly at a maximum rate of 0.5-1 ppt per day. This also serves as a measure to avoid predation from birds and extraneous fish during the fingerling stage.
Although tilapia culture started out in freshwater ponds, many farmers are not well informed on the principles of pond construction. The following are several things that need to be reconfigured in order to mitigate the risks of extreme weather:
1. Deepen ponds to a minimum of 1.5 meters. Water temperature is more stable during hot or cold season in deep ponds. Water deterioration is also slower as compared to shallow ponds. A depth of one meter can no longer be recommended. Most farms with one meter minimum depth suffer during hot or cold months. If a pond owner can manage to have 2.5 or 3 meters depth, this is even better. Production per hectare can be higher by up to 66%.
2. Hatcheries are advised to have an average depth of 1.2 meters for breeding ponds and not 0.6 meters as often recommended. This is a better option rather than providing artificial shades. Eggs can hardly hatch in shallow ponds during hot seasons because the breeders are too stressed. During the cold season, pond temperatures quickly go up or down which is again stressful to breeders. Breeder holding and condition ponds must be deeper at 1.5 to 2 meters. Many hatcheries lose or have delayed operations because the heart of their operations, which is the breeders, all perish. It takes 5 to 8 months for new breeders to be fully useful. Negligence or lack of knowledge is costly to farmers.
3. Provide emergency aerators, backup water sources such as deepwells, reservoirs, etc. Several freshwater ponds lay parallel to NIA canals. These canals usually shut down periodically often times during summer for repairs or because there is a water shortage. It is fatal when this happens when the ponds are in the grower or finisher stages requiring good water supply. Water quality deteriorates quickly as farmers tend not to control feeding in order to catch up with the target harvest time or avail of high market price. As a result, mortality occurs.
4. Provide nursery ponds.
5. Provide physical barriers against all extraneous animals and predators. Surrounding ponds with net fences has been proven effective against soft-shelled turtles that devours fishes of all stages. Finer mesh nets can be used as fence if ricefield-eels also occur in the area. Bird scares are recommended to ward off predatory birds. Livestock must not be allowed in ponds as they may bring diseases though their feces or saliva. They can also result in destruction of dikes.
All these recommendations have been proven effective. Take note that not all measures may be applicable to all areas. By following the suggested retrofitting of ponds and structures, there is no way a farm could be totally lost during extreme weather conditions. Production can only increase – so will profits. Farmers should always observe successful practices and act based on aquaculture principle.
This appeared in Agriculture Monthly’s August 2018 issue.