Tilapia culture in trinidad and tobago: yet another update


Table 1 showing the types of plankton found in tanks



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Table 1 showing the types of plankton found in tanks


Tanks #

Photoplanktons

Zooplanktons

1

Chlorella

Cladocers (Daphnia pulex ) Rotifers (Brachionuspala),and copepods

2

Chlorella

Mosquito larvae (Aedes or Culex)

3

Blue green algae

Cladocers (Daphnia pulex ) Rotifers (Brachionuspala),and copepods

4

Chlorella

Cladocers (Daphnia pulex ) Rotifers (Brachionuspala),and copepods




No feeding

Organic fertilizer

Inorganic fertilizer

Combined organic and inorganic fertilizer

Final average weight (grams)

38.9

42

45.8

51.2

Initial average weight (grams)

22.8

22.6

22.4

22.6

Change in weight (grams)

16.1

19.4

23.4

28.6
Table 2 Showing the weight of the fishes that consumed the plankton

Figure 4. Showing the growth rates of the fishes



Figure 5. Showing the water quality reading



Discussion

Table 1, shows the planktons found in each of the tanks. Tank # 1 which was not fertilized had a bloom of both phytoplankton and zooplankton. There was also blue green alga present. The plankton blooms could have resulted from the fecal contents produced by the fishes.

Tank # 2 which was fertilized with the organic manure maintained the chlorella algae alone. There was no zooplankton or other algae. This could have resulted from the properties of the cow manure. Organic fertilizers have small amounts of nitrogen.

Tank # 3 was fertilized with the inorganic fertilizer, Triple superphosphate and Urea, this resulted in a bloom in zooplanktons, Cladocers (Daphnia pulex) Rotifers (Brachionuspala),and copepods. The copepods however, were not in significant amounts as compared to the cladocers and rotifers. There were also small concentrations of blue-green algae. An inorganic fertilizer such as Triple superphosphate is a fertilizer produced by the action of concentrated phosphoric acid on ground phosphate rock.

Ca3 (PO4)2(s) + 4 H3PO4 (aq) → 3 Ca2+(aq) + 6 H2PO41-(aq) → 3 Ca(H2PO4)2(aq)

The active ingredient of the product, monocalcium phosphate, is identical to that of superphosphate, but without the presence of calcium sulfate that is formed if sulfuric acid is used instead of phosphoric acid. The phosphorus content of triple superphosphate (17 - 23% P; 44 to 52% P2O5) is therefore greater than that of superphosphate (7 - 9.5% P; 16 to 22% P2O5).

Urea fertilizer, also known as carbamide, is the most important nitrogenous fertilizer. It is a white crystalline organic chemical compound containing about 46 percent nitrogen. It is a waste product formed naturally by metabolizing protein in humans as well as other mammals, amphibians and some fish. Synthetic urea is produced commercially from ammonia and carbon dioxide.

The blue-green algae produced could have resulted from too much nitrogen in the tank since the fishes also excrete ammonia into the water. The combinations of the inorganic fertilizer, urea with the ammonia may cause the undesirable growth of the blue green algae.

Tank #4 had planktons Chlorella which was inoculated into the tank and the same zooplanktons as found in tank # 3, which were Cladocers (Daphnia pulex ) Rotifers (Brachionuspala),and copepods.

Table # 2 and Figure # 4 shows the weight gain and the growth rates of the fishes. The highest increase in weight and growth rate came from tank # 4 probably due to the phytoplankton and zooplanktons that were consumed by the fishes. The minimum growth rate was from tank # 1 while the second highest growth rate was from tank # 3, which contained the zooplanktons. This maybe due to the fact that zooplanktons would have a higher amount of protein as compared to the phytoplankton is a tiny aquatic plant, which comprises of more water and less protein. Tank # 2 growth rates were higher than tank 1 but less than tank 3 and tank 4.For centuries fish farmers have increased fish yields in ponds by using inorganic or chemical fertilizers and organic fertilizers or "manures." (Bocek, 2009).

Figure 5. Shows the water quality readings for pH, dissolved oxygen, temperature and transparency. The pH was significantly different in tank # 1 as compared to the other tanks where there was no significant difference. However, tank # 2 had the highest dissolved oxygen (DO), which was significantly different from tanks # 3 and 4. When tank # 1 was compared to tank # 2 there was no significant difference, where as, there were significant differences when compared to tank # 3 and 4. We hypothesized that tank # 2 had the highest DO because there was a higher amount of phytoplankton in that tank. The zooplankton population was higher in tanks # 3 and # 4, which consumed the phytoplankton, hence a lower DO. Bocek, 2009 had observed this action.

There was no significant difference in temperature between all the tanks. There was significant difference in tank # 3 as it related to the transparency, which was measured using a Secchi disc where as there were no significant differences in the other tanks. There reasons why tank # 2 had the lowest transparency is probably because the zooplankton population was higher in the tanks, which consumed the phytoplankton. This was similar to what Bocek, 2009 had observed.

Phytoplankton populations, or blooms, can grow rapidly, particularly on sunny days when the water is warm and nutrients are available. Alternatively, they can die-off quickly, especially in the spring and fall as water temperatures change rapidly with weather fronts. However, a bloom die-off can occur at any time of the year with little or no warning.

Typically during a bloom die-off, the color of the water will start to change. Leading up to a bloom die-off, the pond water may have a “streaky” appearance. Streaks of brown or gray-black through the otherwise green water of the pond is an indication that the algae are starting to die. As the die-off progresses, the whole pond will turn from green to gray, brown, or clear. The pond water will typically clear after a die-off as the dead algae settle to the bottom.

Plankton die-offs cause rapid oxygen depletions for two reasons: 1) the remaining dissolved oxygen is consumed by aerobic bacteria and fungi in the process of decaying the dead algae and 2) few live phytoplankton’s remain to produce more oxygen. Secchi disks can be used to monitor bloom densities. Any bloom that reduces visibility in the pond to 25 cm or less may cause oxygen problems. Plankton-feeding animals control the numbers of the phytoplankton and have an impact on the numbers of the phytoplankton found in the tanks. This depends on the numbers of animals and algae present. When the numbers of algae are tiny, a small number of animals may prevent any increase in algal numbers. It is believed that this occurred in the tanks in which the algal inoculum failed to grow and disappeared.

Once the algae started to increase, the sequence of events described a crash. Initially, there is a steady increase in the numbers of algae, the reproduction rate is sufficient to compensate for the numbers eaten by animals. However, as the algae reproduction rate begins to slow down, a critical stage is reached when the reproduction rate, where the numbers balances the daily increase in numbers consumed by animals. Further, an increase in the number of animals at this stage resulted in a crash, whereby the numbers of algae were rapidly reduced, until nearly all was destroyed. This produced important changes in water composition, notably almost complete oxygen depletion, as a result of this the animals are frequently destroyed. The few remaining algae are not destroyed, and after the death of the animals begin to multiply rapidly once more. This may have been the reason why tanks # 3 and #4 had the low oxygen level and tank # 3 had the lowest Secchi reading. The time at which the critical stage is reached is not the same. This is may be due to differences from one tank to another, in the respective reproduction rates of animals and algae. The factors controlling these reproduction rates are still unknown. Sometimes the critical stage was reached soon after inoculation, before the numbers of algae were very high, and there was no sudden asphyxiation, but only a gradual disappearance, apparently from starvation, following the disappearance of the algae. While no direct proof can be offered, it seems likely that the differences were due to variation in the animal population, arising from chance inoculation with animals from previous experiments.

Pennington, (1941) found a phenomenon that was similar to what was described from the experimental tubs. In the rich culture solution of the tubs, both animals and algae were present in greater concentration than is found in ponds, but there was no reason why similar crashs should not occur in eutrophic ponds. A crash was observed in a pond near Burghfield Common, Reading, in the autumn of 1938. A rich growth of algae, comprising mainly of flagellates, developed in the water, and then suddenly disappeared, the disappearance coincided with the appearance of large numbers of Cladoceran (probably Daphnia sp.) and a Copepod. The water became black and acquired a foul smell, which was typical of anaerobic waters. The late phase of a crash, in which the zooplankton is concentrated in the upper layers of the water, which are more oxygenated, and algae have practically disappeared from the water, is common in farm ponds. This was also observed in the experiment conducted. Under the light microscope the gut contents indicated that the zooplanktons consumed the phytoplankton that were in tanks# 1, #3 and #4.

The differences in the growth of algae in similarly treated tanks may be due to the chance of variation in the number of plankton-feeding animals. In tanks, the effects of plankton-feeding animals on the phytoplankton showed no relation to season.



Conclusion

It was found that when organic fertilizers are used there is a higher phytoplankton bloom and higher oxygen level in the tanks where as when inorganic fertilizers are used there is a greater zooplankton population.

When organic and inorganic fertilizers are combined it provides food for fishes and the fishes in the combined tank had the highest weight gained. Obtaining maximum fish production, it is necessary to maintain the nutrient status of the pond to an optimum range. (Brunson et al, 1999). Brachionuspala and Daphnia pulex which are plankton-feeding animals, will decrease the numbers of the phytoplankton very rapidly when present in high numbers these were observed in# 1, #3 and #4.

It was observed that a rapid reduction of the phytoplankton was accompanied by almost complete oxygen depletion, and death of the animals, after which the algae population increased again. This cycle of events observed in experimental tubs, has been found to occur in ponds, Pennington, (1941). In addition to rapid and sudden reduction in numbers of algae, plankton-feeding animals may have important effects on the rate of increase in numbers of algae at any stage of the annual cycle.



References

Bocek, A. 2009.Water Harvesting and Aquaculture for Rural Development. International Center for Aquaculture and Aquatic Environments, Swingle Hall. Auburn University, Alabama 36849-5419 USA.

Brunson, W., Stone, N. and Hargreaves, J. 1999. Fertilization of Fish Ponds. PR SRAC Publication No. 471 SRAC Publication No. 471.

Dieffenbach, H. &Sachse, R. (1912). BiologischeUntersuchungenanRadertieren in Teichgewassern. Internat. Rev. Hydrobiol., Biol. Supp. 4.

Pennington, Winifred (1941). Diogenes rotundus, gen. et sp. nov.-a new alga from experimental tubs. J. Bot. 1941 (in the press).

Winifred Pennington (Aug., 1941) The Control of the Numbers of Freshwater Phytoplankton by Small Invertebrate Animals, Journal of Ecology, Vol. 29, No. 2 (Aug., 1941), pp. 204-211 Published by: British Ecological Society



ANNEXES

Annex 1 How they use fertilizers to increase the production of natural food for fish



Annex 2

Item


Organic fertilizers


Inorganic fertilizers


Storage

Difficult, only short time

Easy, possibly for long time

Distribution

Difficult, esp. on larger scale

Easy

Mineral content

Variable, low

Consistent, high to very high

Organic matter

Present

Absent

Effect on soil structure

Improvement

No

Direct food for fish

Yes

No

Decomposition process

Yes, with oxygen consumption

No

Price

Low to medium

High to very high

Cost per nutrient unit

Higher

Lower

Availability

Possibly in neighborhood or even on own farm

Commercial suppliers only; sometimes imported

Direct pond fertilization

Possible by raising animals on or near the pond

Not feasible






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