Water supply is the first and most important factor to consider in the suitability of a fishpond site. Usually, water supply comes from a river, a creek or from the sea. It must meet the quality and quantity requirement of the pond system throughout the year.
Water quality is affected by the physical, the chemical, and the biological parameters. Such parameters are affected by the 1) by-products and wastes resulting from urbanization, 2) agricultural pollutants such as pesticides and fertilizers, 3) industrial wastes from pulp mills, sugar, oil refineries, and textile plants, 4) radio-active wastes, 5) oil pollution arising navigational activities, uncontrolled spillage, and oil exploration. Some of these parameters are discussed in detail under fishpond management.
Poor quality water sometimes causes the fouling of gates, screens or metal pipes. This happens when heavy dredging is being conducted in an area. Heavy dredging increases turbidity and causes the release of organic substances embedded in the soil. Once these organic substances are released, they use up oxygen causing high biological oxygen demand (BOD). Higher BOD causes oxygen depletion which in turn makes the water foul. Similar conditions also occur during floods.
Water supply in tide-fed farms must be adequate especially during some months of the year when the height of high water is at minimum. This problem can be solved by proper gate design and by the use of pumps.
The rate of volume flow of nearby tidal stream needs also to be considered; measurement is made during the dry stream flow and during floods. The data obtained give the developer the minimum and maximum rates of discharge. These are important requirements in fish farm design. For details, refer to Annex I.
The suitability of a tide-fed area for a “bangus” fishpond project depends on the relationship between the tidal characteristic of the area and its ground elevation.
The only free source of energy that could be tapped for flooding a brackishwater coastal pond is tidal energy which is available once or twice a day depending on geographical location. Five reference stations in the Philippines exhibit five peculiarly different patterns during some months of the year. Figure 1 shows in a graphical form the relationship of natural ground elevation to tidal characteristic. Tables 1 and 2 show such relationships as they are applicable to the six stations of reference.
Figure 1 - Suitability of Proposed Fishpond Site Based on Tidal Characteristic and Ground Elevation.
|LOCALITY||Elevations in Meters Above Mean Lower Low H20|
|Mean High Water (MHW)||Mean Sea Level (MSL)||Mean Low Water (MLW)|
|Pier 13, South Harbor, Manila||0.872||0.479||0.104|
|Pier 2, Cebu City||1.250||0.722||0.183|
|Legaspi Port, Legaspi City||1.329||0.744||0.165|
|Sta. Ana Port Davao City||1.405||0.753||0.101|
|Port of Poro, San Fernando, La Union||-||0.372||-|
|Jolo Wharf Jolo, Sulu||0.631||0.338||0.034|
Table 1. List of Primary Tide Stations and Datum Planes
|Highest recorded tide (m)||Lowest recorded tide (m)||Absolute annual range (m)||Normal daily fluctuation low/high(range) (m)||R E M A R K S|
San Fernando, La Union
|1.04||(-)0.21||1.25||(-)0.03/0.61(0.64)||Tidal fluctuation too narrow for proper fishpond management|
|Manila City||1.46||(-)0.34||1.8||0.14/1.05(0.91)||Tidal fluctuation slightly narrow for proper fishpond management|
|Legaspi City||1.83||(-)0.4||2.23||1.09/1.40(1.49)||Tidal fluctuation favorable for proper fishpond management|
|Jolo, Sulu||1.19||(-)0.12||1.31||(-)0.03/0.98(1.01)||Tidal fluctuation slightly narrow for proper fishpond management|
Table 2. Suitability of Six Tidal Stations of Reference for Fish Farms
Areas reached only by the high spring tides should be ruled out as it is costly to move large quantities of soil during the process of excavation. There is that other problem of where to place the excess materials. While these can be solved by constructing high and wide perimeter dikes, putting up more dikes will create narrow compartments resulting in less area intended for fish production.
Low areas on the other hand will require higher and more formidable dikes which may mean that earth will have to be moved long distances. The pond bottom should not be so low that drainage will be a problem.
The best elevation for a pond bottom therefore, would at least be 0.2 meter from the datum plane or at an elevation where you can maintain at least 0.6 meter depth of water inside a pond during ordinary tides. This index should satisfy the requirements of both fish and natural fish food.
The attractive forces of both the moon and the sun on the earth surface which changes according to the position of the two planets bring about the occurrence of tides. Tides recur with great regularity and uniformity, although tidal characteristic vary in different areas all over the world. The principal variations are in the frequency of fluctuation and in the time and height of high and low waters.
When the sun, the moon and the earth are in a straight line, greater tidal amplitudes are produced. These are called spring tides. Tides of smaller amplitudes are produced when the sun and the moon form the extremes of a right triangle with the earth at the apex. These are called neap tides. When high and low waters occur twice a day it is called a semi-diurnal tide. When the high and the low occur once a day it is called a diurnal tide.
The moon passes through a given meridian at a mean interval of 24 hours and 50 minutes. We call this interval one lunar day. Observations reveal that the mean interval between two successive high (or low) waters is 12 hours and 25 minutes. Thus, if there is a high water at 11:00 A.M. today, the next high water will take place 12 hours and 25 minutes later, i.e., 11:25 P.M. and the next will be at 11:50 A.M. of the following day. Each day the time of tide changes an average of 50 minutes.
The difference in the sea water level between successive high and low waters is called the range. Generally, the range becomes maximum during the new and full moon and minimum during the first and last quarter of the moon. The difference in the height between the mean higher high and the mean lower low waters is called the diurnal range.
The difference in the tide intervals observed in the morning and afternoon is called diurnal inequality. At Jolo, for instance, the inequality is mainly in the high waters while at Cebu and Manila it is in the low waters as well as in the high waters. The average height of all the lower of low waters is the mean lower low (MLLW), or (0.00) elevations. This is the datum plane of reference for land elevation of fish farms.
Prediction of tides for several places throughout the Philippines can be obtained from Tide and Current Tables published annually by the Bureau of Coast and Geodetic Survey (BCGS). These tables give the time and height of high and low water. The actual tidal fluctuation on the farm however, deviates to some extent from that obtained from the table. The deviation is corrected by observing the time and height of tidal fluctuation at the river adjacent to the farm, and from this, the ratio of the tidal range can be computed. From the corrected data obtained, bench marks scattered in strategic places can be established. These bench marks will serve later on as starting point in determining elevations of a particular area.
There are six tide stations in the Philippines, namely: San Fernando, Manila, Legaspi, Cebu, Jolo and Davao stations. Reference stations for other places are listed under the “Tidal Differences” and “Constants” of the Tide and Current Tables.
The predicted time and height of high and low waters each day for the six tide stations can be read directly from the table. Tide predictions for other places are obtained by applying tidal differences and ratios to the daily predictions. Tidal differences and ratios are also found in the Tide and Current Tables.
Let us take for example, the tidal predictions for Iloilo on 23 Sept. 1979. Looking through the tidal differences and constants of the Tide Tables, you will find that reference station for Iloilo is Cebu. The predicted time and height of tides for Cebu obtained from the tide tables on 23 Sept. 1979 are as follows:
|0004||:||1.43 m||0606||:||0.14 m|
|1216||1.52 m||1822||0.18 m|
(The heights are in meters and reckoned from mean lower low water (MLLW); 0000 is midnight and 1200 is noon).
Again, from the table on Tidal Differences and Constants, the corrections on the time and height of high and low waters for Iloilo are as follows:
|Time||Height of High Water||Height of Low Water|
|+ 0 hr. 05 min.||+ 0.09||+ 0.03|
Thus, the corrected time and heights of high and low waters for Iloilo are:
|0009||:||1.52 m||0611||:||0.17 m|
|1221||:||1.61 m||1827||:||0.21 m|
The height of the tide at any given time of the day may be determined graphically by plotting the tide curve. This can be done if one needs to know the height of the tide at a certain time. The procedure is as follows:
On a cross-section paper, plot the high (H) and the low (L) water points between which the given time lines (see Fig. 2). Join H and L by a straight line and divide it into four equal parts. Name the points as Q1, M and Q2 with M as the center point. Locate point P1 vertically above Q1 and P2 vertically below Q2 at a distance equal to one tenth of the range of the tide. Draw a sine curve through points H, P1, M, P2 and L. This curve closely approximates the actual tide curve, and heights for any time may be readily scaled from it.
Figure 2 shows the curve on 23 Sept. 1979 for Iloilo. H is 1.61 m at 12:21 hr and L is 0.21 m at 18:27 hr. Since the range is 1.40 m, P1 is located 0.14 units above Q1 and P2 is located 0.14 units below Q2. The height of the tide at 14:30 hr is given by point T to be 1.22 m.
Figure 2. Height of Tide at any Given Time for Iloilo on 23 Sept. 1979.
Most of our fishponds are constructed on tidal lands consisting of alluvial soils which are adjacent to rivers or creeks near the coastal shores and estuaries at or near sea level elevation. If you pick up a handful of soil and examine it closely, you will find that it is made up of mineral and organic particles of varying sizes. The mineral particles are the clay, silt, and sand while the organic particles are plant and animal matter at various stages of decomposition. Soils are assigned with textural classes depending on their relative proportion of sand, silt and clay. Each textural class exhibits varying colors which are based on their chemical composition, amount of organic matter and the degree of decomposition. U.S. Department of Agriculture Classification System has classified soil as:
|Common Names||Texture||Basic Soil Textural Class Names|
|2.||Loamy Soils||Moderately Coarse||Sandy Loam|
|Fine sandy Loam|
|Medium||Very fine Sandy Loam|
|3.||Clayey Soils||Fine||Sandy Clay||Clay Loam|
|Silty Clay||Sandy Clay Loam|
|Clay||Silty Clay Loam|
Many properties of soil, which are related to its texture, determine how well suited it is for fishpond purposes. A sandy loam, for instance, is more porous than silty loam and the latter will hold more nutrients than the former. Clay or sandy clay may be the best for dike construction but not as good as clay loam or silty clay loam in terms of growing natural food. So, in general, finer textured soils are superior for fishpond purposes because of their good water retention properties.
Each soil texture exhibits different workability as soil construction material. Studies conducted show that clayey soil is preferred for diking purposes. Suitability of a soil class as dike material decreases with decreasing percentage of clay present in the mixture (see Table 3).
|CLASS||RELATIVE CHARACTERISTIC||COMPACTION CHARACTERISTIC||SUITABILITY FOR DIKE MATERIAL|
|Clay||impervious||medium||fair to good||excellent|
|fair to very|
|good to very|
Table 3. Relationship of Soil Classes and Suitability for dike material
Sediments are a dominant and observable characteristic in lower areas of brackishwater swamplands. Field observations and laboratory analysis of soil samples taken reveal that the majority have a thick layer of loose organic sediments which make them unsuitable for fishpond development and other infrastructures. Engineering and other technical considerations indicate that areas having this type of soil are rather difficult to develop because it is directly related to future land development problems such as (1) subsidence and related flood hazards, (2) unavailability of stable and indigenous soil materials for diking, and (3) unavailability of land with adequate load bearing capacity for future infrastructures such as buildings for storage and production facilities.
Areas dominated by organic and undecomposed sediments are expected to experience considerable subsidence which eventually result to loss in effective elevation of the land after development as a result of drainage or controlled water table. Since elevation of most tidal lands converted to brackishwater fishponds are generally one meter above MLLW, any future loss of elevation due to subsidence shall predispose the area to severe drainage and flooding problems due to blocking effect of seawater during high tides.
Organic and undecomposed sediments are not a good foundation for dikes nor for diking material. Fishpond areas dominated by this type of soil will mean that there is an inadequacy of indigenous soil materials for diking or filling of lower areas. In the absence of good soil materials, the site under consideration will require importing of soils from the adjoining areas which will make the system of development a very expensive process, or considerable excavation for diking will cause (1) unnecessary exposure of acid organic layers, (2) difficulty in leveling, (3) high cost of dike maintenance and (4) technical problems on seepage losses which will cause difficulty in maintaining water levels in the pond.
Sand - Soil has granular appearance. It is free-flowing when in a dry state. A handful of air-dry soil when pressed will fall apart when released. It will form a ball which will crumble when lightly touched. It cannot be ribboned between thumb and finger when moist.
Sandy Loam - Essentially a granular soil with sufficient silt and clay to make it somewhat coherent. Sand characteristic predominate. It forms a ball which readily falls apart when lightly touched when air-dry. It forms a ball which bears careful handling without breaking. It cannot be ribboned.
Loam - A uniform mixture of sand, silt, and clay. Grading of sand fraction is quite uniform from coarse to fine. It is soft and has somewhat gritty feel, yet is fairly smooth and slightly plastic. When squeezed in hand and pressure is released, it will form a ball which can be handled freely without breaking. It cannot be ribboned between thumb and finger when moist.
Silty Loam - It contains a moderate amount of finer grades of sand and only a small amount of clay; over half of the particles are silt. When dry, it may appear quite cloddy; it can be readily broken and pulverized to a powder. When air-dry, it forms a ball which can be freely handled. When wet, soil runs together and puddles. It will not ribbon but has a broken appearance; it feels smooth and may be slightly plastic.
Silt - It contains over 80% of silt particles with very little fine sand and clay. When dry, it may be cloddy; it is readily pulverized to powder with a soft flour-like feel. When air-dry, it forms a ball which can be handled without breaking. When moist, it forms a cast which can freely be handled. When wet, it readily puddles. It has a tendency to ribbon with a broken appearance; it feels smooth.
Clay Loam - Fine texture soils break into lumps when dry. It contains more clay than silt loam. It resembles clay in a dry condition. Identification is made on physical behaviour of moist soil. When air-dry, it forms a ball which can be freely handled without breaking. It can be worked into a dense mass. It forms a thin ribbon which readily breaks.
Clay - Fine texture soils break into very hard lumps when dry. It is difficult to pulverize into a soft flour-like powder when dry. Identification is based on cohesive properties of the moist soil. When air-dry, it forms long thin flexible ribbons. It can be worked into a dense compact mass. It has considerable plasticity, and can be moulded.
Organic Soil - Identification is based on its high organic content. Much consists of thoroughly decomposed organic materials with considerable amount of mineral soil finely divided with some fibrous remains. When considerable fibrous material is present, it may be classified as peat. Soil color ranges from brown to black. It has high shrinkage upon drying.
A watershed is a ridge of high land draining into a river, river system or body of water. It is the region facing or sloping towards the lower lands and is the source of run-off water. The bigger the area of the watershed, the greater the volume of run-off water that will drain to the rivers, creeks, swamps, lakes or ocean.
Precipitation from a watershed does not totally drain down as run-off water. A portion of the total rainfall moving down the watershed's surface is used by the vegetation and becomes a part of the deep ground water supply or seeps slowly to a stream and to the sea.
The factor affecting the run-off may be divided into factors associated with the watershed. Precipitation factors include rainfall duration, intensity and distribution of rainfall in the area. Watershed factors affecting run-off include size and shape of watershed, retention of the watershed, topography and geology of the watershed.
The volume of run-off from a watershed may be expressed as the average depth of water that would cover the entire watershed. The depth is usually expressed in centimeters. One day or 24-hours rainfall depth is used for estimating peak discharge rate, thus:
Volume of Flood Run-off (Q) + S1
1 Engineering Field Manual For Conservation Practices, 1969, pp 2–5 to 2–6
|where||Q||=||accumulated volume of run-off in centimeters depth over the drainage area|
|P||=||accumulated rainfall in cm depth over the drainage area|
|Ia||=||initial obstruction including surface storage, interception by vegetation and infiltration prior to run-off in cm depth over the drainage area|
|s||=||potential maximum retention of water by the soil equivalent in cm depth over the drainage area|
Floods are common in the Philippines due to overflowing of rivers triggered by typhoons and the southwest monsoon rain prevailing over the islands during the rainy season. Overflow of the rivers is largely attributable to the bad channel characteristic such as steep slopes as well as meandering at the lower reach of the river. The network of the tidal streams in some delta areas has been rendered ineffective in conveying the flood-water to the sea due to fishpond construction. Flooding is common in this country and is considered the most destructive enemy of the fishpond industry. The floods of 1972 and 1974 greatly affected the fishpond industry in Central Luzon causing damage amounting to millions of pesos. Because of the floods, fishponds became idle during the time necessary for operators to make repairs and improvements. Floods cannot be controlled, but what is important is to know how a fishpond can be free to some extent from flood hazard.
In order to prevent frequent flooding, it is necessary to know the weather conditions in the area where the fishpond project is located. The highest flood occuring in an area can be determined by proper gathering of information. In big rivers, the Ministry of Public Works (MPW) records the height of flood waters during rainy seasons. However, in areas where the MPW has no record, the best way is by gathering information from the people who have stayed in the area for many years. The size of the creek, river and drainage canal should also be determined to find out whether it can accommodate the run-off water or flood water that drains in the area once the fishpond project is developed.
Records of the highest flood in the site, especially during high tide, is very important. It will be the basis in providing allowance for the drainage of flood water coming from the watershed.
Climate has been described in terms of distribution of rainfall recorded in a locality during the different months of the year. In the Philippines, it is classified into four climatic zones preferably called weather types, namely:
|Type I||-||Two pronounced seasons; dry from November to April and wet during the rest of the year.|
|Type II||-||No dry season with very pronounced maximum rainfall from November to January.|
|Type III||-||Season not very pronounced; relatively dry from November to April and wet during the rest of the year.|
|Type IV||-||Rainfall more or less evenly distributed throughout the year.|
The elements that make up the climate of a region are the same as those that make up the weather, the distinction being one mainly of time. But the elements that concern most fishpond operators are the rainfall, temperature and the prevailing wind direction because they greatly affect fish production directly or indirectly.
Data on rainfall and wind direction are very necessary in planning the layout and design of pond system. Knowing past rainfall records, you can more or less decide whether it will be necessary to include a drainage canal in the layout, and how large it will be when constructed. Knowing past rainfall records will also be necessary in computing the height of the secondary and tertiary dikes.
Wind on the other hand, plays a role in fishpond design. Strong wind generates wave actions that destroy sides of the dike. This causes great expense in the construction and maintenance. However, this problem can be minimized with proper planning and design. For instance, longer pond dimension should be positioned somewhat parallel to the direction of the prevailing wind (see Fig. 3). This will lessen the side length of the dike exposed to wave action. This orientation of pond compartments will also have some advantageous effects in the management aspect.
Figure 3. Layout of Pond Compartments Oriented to the Prevailing Wind Direction
Nearly every location is subject to what is called the prevailing wind, or the wind blowing in one direction for a major portion of the year. Monsoons are prevailing winds which are seasonal, blowing from one direction over part of the year and from the opposite direction over the remaining part of the year. Trade winds, which generally come from the east, prevail during the rest of the year when the monsoons are weak.
Figure 4. Wind Directions
Wave action in ponds is caused by wind blowing across the surface. One cannot totally control wave action in ponds although it can be minimized. In typhoon belt areas or in areas where a strong wind blows predominantly, it is better to include wind breakers in planning the layout of ponds.
Mangrove swamps occur in abundance on tidal zones along the coasts of the Philippines which are being converted into fishponds for fish production, but not all mangrove swamps are suitable for fishpond purposes. Some are elevated and are not economically feasible for development; others have too low an elevation to develop.
The distribution of mangrove species in tropical estuaries depend primarily on the land elevation, soil types, water salinity and current. It has been observed that “api-api” and “pagat-pat” trees (Avicennia) abound in elevated areas while “bakawan” trees (Rhizophora) are mostly found in low areas. It has also been observed that nipa and high tannin trees have a long-lasting low pH effect on newly constructed ponds. Presence of certain shrubs and ferns indicate the elevation and frequency of tide water overrunning the area. Certain aquatic plants such as water lily, eel grass and chara sp. indicate low water salinities.
The type and density of vegetation, the size, wood density and root system of individual trees greatly affect the method of clearing, procedure of farm development and construction cost. Thickly vegetated areas, for instance, will take a long time to clear of stumps.
Density of vegetation is classified according to kind, size and quantity per unit area. This is done to determine the cost of land clearing and uprooting of stumps. One method used is by random sampling. The process requires at least five or more samples taken at random, regardless of size, and vegetation is classified according to kind, size and number. Then the findings are tabulated and the average of the samples is determined. However, vegetation of less than 3 cm in diameter is not included. The total vegetation of the area is determined as follows:
Table 4. Classification of Vegetation According to Kind, Size & Quantity
Aside from the above factors in the selection of a good fishpond site the following factors should also be considered: