Tilapia culture in trinidad and tobago: yet another update

Figure 3. Showing Simplified food chain showing pathways through which fertilizer nutrients are turned into fish flesh. (Bocek, 2009)

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Figure 3. Showing Simplified food chain showing pathways through which fertilizer nutrients are turned into fish flesh. (Bocek, 2009)

During work on the growth of algae in experimental tubs, it was found that when certain small planktonic animals became numerous, their feeding had very striking effects on the numbers of algae and on the general conditions in the tub. Similar effects were later observed in ponds. The importance of the phytoplankton, including the nannoplankton, as a source of food for rotifers and Cladocera, is generally recognized, but it is perhaps not so widely realized how seriously these small animals can reduce the numbers of the phytoplankton.

Dieffenbach &Sachse (1912), working on the biology of rotifers in ponds, noted that a rich growth of planktonic algae was frequently followed by a great increase in the number of rotifers, which fed on the algae and rapidly reduced their numbers. When the food supply was exhausted, the number of rotifers decreased.
The plankton-feeding animals Brachionuspala and Daphnia pulex, when present in sufficient numbers, can reduce the numbers of the phytoplankton very rapidly. In all cases observed, such a rapid reduction of the phytoplankton was accompanied by almost complete oxygen depletion, and death of the animals, after which the numbers of algae again increased. This cycle of events, first observed in experimental tubs, has been found to occur in ponds. It is suggested that in addition to such 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 (Pennington,1941).

The dominant algae of the plankton were nearly always small members of the Chlorococcales-Chlorella, Scenedesmus, or a minute alga which has been described (Pennington, 1941), under the name of Diogenes rotundus, and which, apart from its method of reproduction, resembles a small Chlorella.

At the time when the population of a tub had reached a high, more or less constant, level, Diogenes rotundus almost invariably formed the bulk of the phytoplankton, and in bright summer weather its numbers often exceeded 20,000 per cu. mm., when the water would be bright green and almost opaque (Pennington,1941).

In such a tub, it was frequently observed that in the course of a few days the colour changed from bright green to a dull olive green, and then to black, and at the same time became sufficiently clear to show the bottom of the tub.

Counts of the algae showed that their numbers had decreased very rapidly, and on examination, the water was found to contain enormous numbers of small animals-in every case either the rotifer, Brachionuspala, or the crustacean, Daphnia pulex. This sudden destruction of the algae by small invertebrate animals is here termed a 'crash'(Pennington,1941).

When the significance of the 'crash' phenomenon was appreciated, further investigations of the feeding habits of small animals from the tubs were carried out. The gut contents were examined, and those species which appeared to feed on plankton algae were kept and observed in cultures in beakers. Then closer investigations were made of their feeding habits in the tubs, and the course of a crash followed in detail.

Gut contents

Of all the small animals whose gut contents were investigated, it appeared that only rotifers and Daphnia were important in reducing the numbers of plankton algae. Brachionuspala and Daphnia pulex both had large numbers of the smaller plankton algae from the tubs in their stomachs-in fact, these algae appeared to be their main diet in the tub environment. Live individuals of Brachionuspala in a culture of Diogenes were observed to take in large numbers of the algae by the action of the cilia on their trochal disks. Once eaten by a rotifer, the algae fairly rapidly became unrecognizable, only the somewhat misshapen cell wall surviving digestion. In the gut of Daphnia, the algae retained their shape over a longer period. Neither of these animals appeared to show any selectivity in feeding, apart from that imposed by the relative sizes of animals and algae. Brachionus ate Chlorella as well as Diogenes rotundus, when both were present, but nothing larger. Daphnia ate any alga occurring in the cultures in which it was grown, up to the size of Pediastrum, Boryanum, small individuals of which were found in its gut (Pennington,1941).

The other animals commonly present in the tubs were not important in reducing the numbers of plankton algae. The only other plankton feeder was the larva of Culex sp., which was frequent in the summer. The guts of these were full of plankton algae, but the larvae did not occur in sufficiently large numbers to cause an appreciable reduction in the numbers of algae in the tub (Pennington,1941).

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