Guidelines for Monitoring of Productivity in Fish Ponds

by
Regional Lead Centre in India
Freshwater Aquaculture Research and Training Centre
of the Indian Council of Agricultural Research
Dhauli, Bhubaneswar

NETWORK OF AQUACULTURE CENTRES IN ASIA
Bangkok, Thailand
December 1985

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GUIDELINES FOR MONITORING OF PRODUCTIVITY IN FISH PONDS

1. Among the various types of freshwater bodies available in the region for fish culture, the rural undrainable ponds are an important resource and present an entirely different ecosystem. The tropics having definite monsoons, are characterised with periods of ‘heavy rainfall’ and ‘no rain’. While excess rainfall leads to floods during the monsoons, summer months result in severe drought conditions. These rural pereunial ponds serve to collect a part of the precipitation and meet requirements during the dry periods. In fact, they are used for many purposes such as drinking water for human and cattle use, irrigation, washing, etc. and fish culture too. In the recent years, the ponds are being increasingly put to fish culture use. The inherent productivity and external organic enrichment make the ponds more productive, at times leading even to eutrophication. There are a variety of sources of organic enrichment, ranging from fertilizers used in agriculture to domestic sewage. Coupled with these, because of perennial retention of water, the pond bottoms are never exposed to sunlight, rendering them more anaerobic. And in this context, it is essential to develop a reasonable rapid environmental monitoring system for surveying the basic architecture and production processes in this particular type of fish pond ecosystem so as to manage them successfully for optimization of fish production. The system needs to be simple as regards the instrumentation, working time and worker skill, yet offering sufficient information to know the nutrient and productive status of the pond sediment and water. On the basis of studies conducted at the Regional Lead Centre in India, it has been decided to include 31 parameters in this monitoring system. Most of the parameters can easily be measured on the pond site except six chemical compoments of the monitoring system. Quantification of some of the above parameters may at times be made by on-the-site visual estimations. Fig. 1 indicates design of the environment monitoring system and Table 1 shows the evaluation sheet. A brief description follows about the relevance, importance and methodology of all the parameters selected for this environmental monitoring system.

1.1 Pond Number and Name

The exact geographic locations of the ponds along with coded names are essential to compare the productivity levels in different agro-climatic zones.

1.2 Water Area

A knowledge of the water area is essential for quantifying the production processes as also deciding on the management measures. It may be desirable to measure both the maximum and minimum water areas during the year simple measuring techniques may be used to know the water volume.

1.3 Age

The age of a water body has direct bearing on the intensity of ecological processes. Ageing of a water body might cause increased nutrient levels, as also have deleterious effects like accumulation of organic matter, posing hazards for fish life. The evolution of the aquatic system from oligotrophy to eutrophy and finally into dystrophic condition with age is well known. With the accumulation of organic matter over years, the sediment-water interactions tend to decrease. A knowledge of the age helps in planning suitable measures for improved nutrient recycling like raking of pond bottom, aeration or replacement of water in the pond, in the event of water becoming ‘biologically old’.

1.4 Management

The management status must also include information on the natural influences already being exerted on the water body such as those from human population, livestock, agriculture, etc. This would indicate the biogenic nutrient load being received by the water body. The species of fish present, stocking structure and density, fertilization, feeding, harvesting etc. are to be known.

1.5 Visual Colour

The visual colour of ponds is a simple, but important measure, reflecting the basic production processes. While the inherent true colour of water results from colloidal substances or those in solution, the apparent colour is due to the suspended matter and extrinsic conditions (Welch, 1952). The nature of the bottom sediment, quality and quantity of plankton influence the visual colour. Slight greenish to green colour of the water are indicative of a productive system.

1.6 Water Transparency

Water transparency measured with secchi disc or the scalepin method is intended to quantify the result of those processes which determine and modify the visual colour. Giving an idea of the depth to which sunlight penetrates, it indicates the extent of productive zone of the water sheet. A low transparency may result either from high turbidity or dense algal population and thus cannot reflect the correct conditions of the water. At the same time, the quantitative nature of secchi disc transparency together with visual colour has a high practical value in fish pond management since the primary production depends on the incidence of sunlight.

1.7 Water Depth

It is an important factor on which depend the temperature and circulation patterns and the extent of photosynthetic activity. The water depth is measured using a 4–6 m long bamboo pole with a wooden disc of 25 cm diameter fixed at its end. Not only the surface water area, but the volume available during peak summer months should also be taken into consideration for a rational stocking programme. Maintenance of a particular depth of water to provide required water volume is important, particularly towards the end of the culture period when the fish would have attained big size and the water volume reduced.

1.8 Soft Sediment Depth

Depending on the texture and chemistry of the bottom soil and on the quantity of settling organic matter and inorganic parti= cles, a soft sediment layer is present in majority of the rural ponds. It is a suitable medium for the growth of microorganisms and bottom biota, providing food for bottom feeding fish, as well as a site where important chemical reactions take place influencing the sediment-water interactions. It is measured using a 4–6 m long bamboo pole with a wooden disc of 10 cm diameter fixed at its end.

1.9 Solid sediment layer

In older ponds, in addition to the soft sediment layer, a solid sediment layer with a low water content is also present. The thickness of this layer can be measured with a 6–8 m long bamboo pole with a pointed end. The total thickness of the soft and solid sediment layers depends on the age of the ponds and at times, measures more than 2 m. Such thick sediments, rich in nutrients, show low intensities of bacterial decomposition and mineral cycling due to the prevailing anaerobic conditions.

1.10 Sediment Gases

The thick, organic-rich, anaerobic sediment layers of most ponds contain a large amount of several gases, viz. methane, nitrogen, hydrogen, carbon dioxide, nitrous oxide, etc. While having a retarding effect on the rates of chemical reactions the accumulated gases also pose health hazards to fishes. It may be quantified using a standard sediment stirrer-grid, superimposed on a funnel mounting, connected to a gas collector through a rubber tube. This device can be attached to a bamboo pole for gas measurements at greater depths.

1.11 Sediment Organic Carbon

The deposition and accumulation of organic matter and silt result in several sediment layers. The anaerobic conditions in the sediment and reduced interaction with the overlying water layers lead to reduced decomposition rates, increasing the organic carbon content. The prevailing anaerobic conditions coupled with reduced substances favour the development of Microcystis blooms that have an adverse effect on the pond conditions (Olah, 1983). The recycling of organic matter accumulated in the sediment through an accelerated aerobic decomposition is one of the promising tools to increase fish production in these ponds, without additional inorganic fertilization or artificial feeding. The Hargrave sampler is the most suitable tool to collect sediment samples and the simple ignition procedure is sufficient to measure the sediment organic carbon.

1.12 Sediment Detritus

It consists mainly of decomposing aquatic plants and animals. In addition to the autochthonous organic matter, the detritus in ponds also contain considerable amounts of allochthonous organic particles, as the ponds are used for washing of human and livestock populations. This important nutrient resource can be quantified using a sieve of mesh size 400 um. The residue may simply be taken as the quantity of sediment detritus, but care is to be taken to remove the inorganic particles. Detritus undergoes different stages of decomposition. Leaching of soluble substances from freshly dead organisms, colonization by microorganisms and stabilization are the main phases during detritus formation also changing its nutritive value from time to time (Olah, 1972). The colonization period is characterise by high nutritive value as the organic particles are surrounded by bacterial populations, offering protein for the aquatic animals. During stabilization, the nutritive value decreases gradually. Filter feeding fish as also bottom feeders are able to utilize the detritus and therefore a knowledge of its magnitude helps in deciding the percentages of surface and bottom feeding fish.

1.13–17 Chemical Environment in the Water Column

The water is chemically characterized by pH. alkalinity, NH₄-N, NO₃-N and PO₄-P measurements using standard methods. Normally, the variations in pH and alkalinity values between ponds on the same maternal soil are minimal. The nitrogen and phosphorus levels indicate the basic inorganic nutrient status of the ponds, necessary for formulating a sound fertilization programme.

1.18 Dawn Oxygen

Generally, fish ponds exhibit wide diurnal fluctuations in the oxygen content. These help in quantifying the production and respiration processes and arrive at the community metabolism of the ecosystem. A variety of factors influence the oxygen budget, that is often difficult to quantity. However, even a single measurement at the end of the dark period indicates the minimal levels of oxygen present in the pond. As is well known, an optimum oxygen level is essential for a proper feed assimilation and growth of the fish. The oxygen measurement can be done by oxygen electrode in situ.

1.19 Bacterioplankton

The structural and functional role of total bacterioplankton in energy flow and mineral cycling of fish pond ecosystems is usually underestimated. The character and intensity of bacterial metabolism are the basis of nutrient cycles which occur in water bodies (Rodina, 1972). The classical practice of counting the bacterial colonies appearing on the agar surface is not recommended, since the number of bacteria growing on any of nutrient media respresents only a small portion of the total bacterial community present in natural waters. Enumeration of bacterioplankton collected by membrane filters (0.2 um) provides a better picture of the conditions. The dried filters are erythrosin-stained and counted for the total bacterioplankton (Razumov, 1947). For qualitative assessment of bacterial communities, however, special nutrient media are provided. The variations in the bacterioplankton are expected to remain within narrow ranges under similar agroclimatic conditions, varying with management measures and trophic conditions.

1.20–25 Phytoplankton and Seston Size Fractions

Plankton analysis has been an important aspect of most hydrobiological investigations. Plankton forms the basic natural fish food resource, indicating the primary productivity status of the ponds as well as the potentials at higher trophic levels. Collection of two seston samples using mesh sizes of 60 um for plankton and detritus and 150 μm for organisms large enough for rough filter feeders, would be adequate for quantifying natural food resources in the water column. After centrifugation, the volume and wet weight of the seston are measured. Formalin preserved samples can be analysed for dominant species of phyto-and zooplankton, using Sedgewick-Rafter plankton counting cell.

1.26–29 Zoobenthos and Zootecton

The growth of some important fish species like Cirrhinus mrigala and Cyprinus carpio depends on the abundance of the bottom biota. Most of the undrainable ponds with a thick anaerobic sediment layer do not support large animal populations. Even certain chironomids and oligochaetes adapted to the sediment environment are affected due to the total absence of oxygen and high concentrations of reduced compounds. If, however, the ponds have a good macrophyte cover, crustaceans, insects and molluscs establish a flourishing population of fish food organisms. The presence and the number of benthic animals in the sediment and of zoobiotecton living among the macrophytes can be detected and counted after rapid sieving with a mesh size of around 400 μm. The samples for zoobiotecton counting may be taken by a sieving tray, placing and retrieving it carefully beneath the macrophyte cover. After sieving and washing the sediment and macrophyte samples with 400 um pore size, all the washing remains may be poured into a large white tray and the animals analysed quantitatively and qualitatively.

1.30–31 The percentage cover_of macrophytes

The macrophytes deplete nutrients in the ecosystem, block sunlight, impede face movements of the fishes, provide shelter for fish enemies, pose problems for fish harvest and cause oxygen depletion in the waters at times of death and decay. At the same time, they harbour fish food organisms and add to the nutrient budget on complete decomposition, Visual examination and estimation of the percentage cover of macrophytes are sufficient for quantitative and qualitative estimations.

2. GENERAL MORPHOMETRY OF UNDRAINABLE RURAL FISH PONDS

A total of 32 representative rural undrainable ponds in Orissa was surveyed using the pond environmental monitoring scheme evolved. Most of them are old, excavated for house building activities or constructed for multipurpose use (Table 2).

The water area is usually less than 2 ha and in case of small ponds with high embankments, wind action is limited. Oxygen and nutrients in the water column and at the sediment-water interface are transported by the limited convection currents. The water depth in most ponds remained less than 2 m. In shallower waters, the soft sediment layers are often disturbed making the water turbid, and consequent to this blanketing effect, adequate plankton is not produced. The photosynthetic processes are light-limited in these ponds and oxygen levels may not be sufficient for fish life, as the sediment oxygen consumption and total community respiration consume considerable amounts. Very deep ponds are also not suitable, where the dark tropholytic layers exceed the photosynthetic trophogenic zone.

The sediment depths in the undrainable ponds are often more than a metre and there exists a close relation between age of the ponds and the sediment deposits. Macrophytes cover only a small percentage of surface area in most ponds. They are able to infest the water column of ponds with abundant filter feeders and the dominant weeds included Eichhornia and Pistia. The water column in majority of the ponds surveyed was slight greenish to brown, corresponding to the nature of maternal soil and reflecting the overall low phytoplankton density. Ponds were found to be infested with Microcystis Oscillatoria and Euglena.

3. CHEMICAL ENVIRONMENT AND NUTRIENT STATUS OF UNDRAINABLE RURAL FISH PONDS

The slightly alkaline waters (pH 7.0–8.8) show total alkalinity values of 52–244 ppm (Table 3). The ranges of NH₄-N, NO₃-N and PO₄-P concentrations remai low. The sediment nutrient status was completely different from that of the overlying waters. The sei was characterised by high organic content. The level of most basic nutrients in the sediment interstitial water were thousand times higher than that of water column, that are mostly being used by Microcystis bloom. This blue-green algal group is able to utilize the sediment nutrients during early developmental stages as well as during their diurnal vertical migrations.

4. NATURAL FISH FOOD RESOURCES IN UNDRAINABLE RURAL FISH PONDS

It was seen that ponds with floating macrophytes had less bacterial populations than those ponds with emergent or submerged macrophytes. In phytoplankton dominated fish ponds, the daily release of organic nutrients during the photosynthetic activity of algal species is more important and the decaying algal populations also become an important autochthonous nutrient source for the bacterial populations. Ponds with less than 3 million bacterioplankton per ml of water were seen to be nutrient-deficient (Table 4).

The planktonic detritus suspended in water column was observed to show the same pattern of distribution as those of bacterioplankton. Their numbers were low in macrophyte-infested ponds and high in ponds with algal blooms. While the phytoplankton mainly comprised Microcystis, Oscillatoria and Euglena, zooplankton comprised Ceriodaphnia, Diaptomus, Cyclops, Keratella and rachionus (Table 5).

In general, the bottom faunistic content remain low (Tables 5 & 6). In most ponds, the benthic animal communities were dominated by chironomid larvae and oligochaetes, indicating the general oxygen deficiency, while a few ponds had significant gastropod populations. Ponds with greater macrophyte cover had diverse animal communities - insects, gastropods and shrimps. It was observed that this community formed an important food resource for fish species, as the other niches were generally poor.

5. PRIMARY PRODUCTION AND RELATED FISH YIELDS

The diurnal variations of dissolved oxygen were monitored in the undrainable rural fish ponds with three measuring points and the McConnel (1962) equation used to calculate the primary production. The total annual fish production was found and the unit of g C/m²/d was chosen for comparison. The conversion efficiencies from primary production to fish production in terms of carbon were calculated as ratios and presented as percentages. The gross primary production values ranged from 1.76 to 4.79 g C/m²/d and the average fish production values from 2.8 to 15.8 kg/ha/d at stocking densities varying from 2800 to 7000/ha. The fish production efficiencies ranged from 0.65 to 6.75.

6. COMMUNITY METABOLISM AND SEDIMENT OXYGEN CONSUMPTION

The quantification of community metabolism including primary production and community respiration in these ponds was also attempted. The total community respiration values were close to the production values, being 1.66 to 4.69 g c/m²/d. The sediment the benthic community respiration values measured 0.5 to 1.27 g c/m²/d. While low values of positive net primary production of 0.03 to 1.35 g c/m²/d were observed in some ponds, negative values of -1.29 to -0.30 g C/m²/d were recorded in the others. This indicates the signi- operating in these organic enriched systems.

The quantification of sediment oxygen consumption and further partitioning into chemical, bacterial and macro-invertebrate respiration were carried out, along with studies on the influencing factors like oxygen levels, mechanical disturbances, bioturbulance, etc. The sediment oxygen uptake values varied between 0.39 and 3.39 g O₂/m²/d, being in the low ranges of 5.14–29.74% of gross primary production and 5.34–29.95% of total community respiration.

The chemical and bacterial oxygen uptake formed the major portions of total sediment consumption, the respective percentages being 15.53–100.00 and 0–77.27 and the animal uptake was negligible. The vertical distribution of potential sediment oxygen uptake did not present steep gradients, suggesting the dominance of chemical oxidation (Table 7). A direct relation between the uptake rates and oxygen concentrations was observed (Table 8) and the latter was a controlling factor in these ponds. The effects of mechanical disturbances and bioturbation by chironomid larvae on the uptake rate were considerable, the absence of which had caused large organic accumulation in the pond bottoms.

7. GENERAL REMARKS

The studies have shown that the deep sediment layers rich in organic matter are playing a limited role in the community metabolism of the undrainable rural fish ponds. They are acting as energy traps with reduced substances and anaerobic conditions, aggravated by low benthic fauna and wind driven turbulance in these systems. Certain measures like mechanical raking of the pond beds, broadcasting of bottom sediment on the pond surface, introduction of bottom-feeding fish and enriching the benthic community are suggested for accelerating transport mechanisms. These could enhance the nutrient recycling processes and also prevent hazards like fish kills at times of occasional disturbances.

8. REFERENCES

Hutchinson, G.E. , 1957. A Treatise on Limnology. Vol II. John Wiley and Sons, Inc. p. 1115.

McConnel, W.J., 1962. Productivity relations in carboy microcosm. Limnol. Oceanogr., 7: 335–343.

Olah, J., 1972. Leaching, colonization and stabilization during detritus formation.
Mem. 1st. Ital. Idrobiol., 29: 105–127

Olah, J., 1983. A programme of investigations on the hydrobiology of fish ponds. FAO Field Document 6, FI. DP/IND/75/031, 43 p.

Razumov, A.S., 1947. Methods of Microbiological Studies of Water. Ed. of the Inst. WODGEO, Moscow.

Rodina, A.G., 1972. Methods in Aquatic Microbiology Ed. Tr. Rev. R. R. Colwell and M.S. Zambruski, University Park Press, Baltimore and Butterworth Co., London.

Welch, P.S., 1948. Limnological methods. McGraw Hill Book Co., Inc. Philadelphia, 381 p.

Welch, P.S., 1952. Limnology, Mc-Graw Hill Book Co., Inc., 538 p.

Table 1 DATA EVALUATION SHEET FOR PERENNIAL POND

1. Pond code
2. Water area, ha
3. Age, year
4. Management
5. Visual colour
6. Transparency, cm
7. Water depth, cm
8. Soft sediment depth, cm
9. Solid sediment depth, cm
10. Sediment gases, dm³m^-2
11. Sediment organic -C, mg g^-1
12. Sediment detritus, gm^-2
13. pH
14. Alkalinity, mg dm^-3
15. NH₄-N, ug dm-3
16. No₃-N, ug dm^-3
17. PO₄-P, ug dm^-3
18. Dawn oxygen, mg dm^-3
19. Bacterioplankton, 10⁶ cm^-3
20. Phytoplankton, cell dm^-3
21. Seston detritus, particles cm^-3
22. Seston 60 um, wet weight
23. Dominant species 60 um
24. Seston 150 um, wet weight
25. Dominant species 150 um
26. Macrozoobenthos 400 um, m^-2
27. Dominant species 400 um
28. Macroteeton 400 um, m 2
29. Dominant species 400 um
30. Macrophyte cover, %
31. Dominant species

Table 2 MAIN CHARACTERISTICS OF THE SURVEYED RURAL PERENNIAL PONDS USED FOR FISH CULTURE

Table 3 CHEMICAL ENVIRONMENT AND INORCANIC PLANT NUTRIENTS IN THE WATER OF UNDRAINABLE PONDS DURING JANUARY 1983

Table 4 PLANKTONIC FISH FOOD COMPARTMENTS

Table 6 SEDIMENT FISH FOOD COMPARTMENTS

Table 7. Partitioning of benthic community respiration in sediments of undrainable rural fish ponds, gO₂/m²/d

Table 8 Effect of oxygen concentration on the bacterial and chemical oxygen consumption of the sedimentwater interface of one pond, gO₂/ sq.m./d

Fig. 1 Design of the environmental monitoring system to survey undrainable rural fishponds