Culture of Natural Food For The Larval Rearing Of Freshwater Fishes
The culture of natural food for the hatchery of marine fishes is well established. The protocol for the culture of phytoplankton like Chaetoceros and Skeletonema for the larval rearing of tiger and white shrimps and the culture of the rotifer as first food for most marine fishes – like milkfish, seabass, snappers, groupers, and siganids – have been an integral part of their hatchery operations. Feeding the newly hatched shrimps and fish larvae with these natural food has been shown to be the only method necessary to achieve good survival rates of shrimp postlarvae and fish fry.
It cannot be said however, that the same is true for freshwater fishes. At present, no commercial freshwater fish hatchery in the country cultures natural food. The reason perhaps is because tilapia, the most popular cultured freshwater fish in the Philippines, can be successfully larval reared using commercial fry feed due to the big size of its hatchlings. On the other hand, other fishes like carps, aquarium fishes and even catfishes are stocked directly to nursery ponds a few days after hatching. With this method, it is considered fortunate to have recovery rates of 20 percent because on the average, only about 5 percent can be achieved.
Despite the breakthrough in the induced spawning technology, the lack of proper larval rearing techniques could be the main reason why the freshwater aqua-culture industry, including the freshwater aquarium fish industry in the Philippines, has lagged behind.
The larval rearing of freshwater fishes if at all done uses brine shrimps or Artemia. These are almost instant feed because it comes in cans and all one has to do is to decapsulate the cysts in brine water. The use of brine shrimps in freshwater larval rearing operations, however, poses several problems.
First, newly hatched brine shrimp nauplii are considered big for most newly hatched freshwater fish to ingest. Therefore, a smaller sized zooplankton is still needed to serve as the first food of most fishes. Second, because it is a marine species, brine shrimps will survive only for about 15-20 minutes under freshwater conditions. When they are left unconsumed after this time, they die, decompose, and eventually pollute the rearing water. And third, brine shrimps are expensive.
Almost all of the newly hatched fish fry, whether marine of freshwater, are carnivorous in nature. Most of them are even cannibalistic. Even milkfish that are herbivorous during their fingerlings and adult stages are carnivorous during their early stages of development. Hence, fish larvae require suitable zoo-plankton for their first food. It is important that the food given is of the right size and quantity.
To date, the most suitable first food for fish larvae that are less than 1 cm long is the freshwater rotifer, Brachionus calycorus. After 10 days or so, or when the larvae are big enough, they are then fed with a bigger zoolankton, Moina. Moreover, these natural food must be available in adequate quantities to ensure that the fry have equal opportunities to take in sufficient food for their growth and survival.
Feeding freshwater fish larvae with Brachionus calyciflorus and Moina have advantages. One, they are live feed and are inherently freshwater species. Two, they provide continuous supply of food to the larvae such that whenever the larvae gets hungry, food is always available. This is important especially among cannibalistic fish larvae, the continuous availability of natural food will prevent them from eating each other. Three, they are most preferred food by all freshwater fish larvae. And four, they are relatively easy and economical to produce.
CULTURE OF THE FRESHWATER ROTIFER, BRACHIONUS CALYCIFLORUS
Brachionus calyciflorus is the most suitable freshwater rotifer for the larval rearing of most freshwater fish and shrimps. They are ideal as the first fish food because of its very small size, slow motility, and its ability to stay at the water column.
To culture Brachionus, a starter culture of the freshwater Chlorella, a green alga and Brachionus is necessary. These can be obtained from the Binangonan Freshwater Station of the Southeast Asian Fisheries Development Center (SEAFDEC, BFS).
Chlorella, the food for Brachionus should be cultured first. To grow Chlorella, the starter culture is scaled up in increasing volumes as follows: 6 liters used mineral bottle container, 200-1i plastic drum liner (translucent type) and eventually to the tanks. The tanks can be any container that will hold water similar to that used for Moina culture. The tanks should be placed in 70 percent to 80 percent filtered light to enhance the growth Chlorella and are provided with vigorous aeration.
For all the containers, the following fertilizers are added per 1,000 liters of water:
Ammonium Sulfate 100g
When Chlorella blooms or when it becomes dark green in color, the Brachionus starter is added. Harvesting of Brachionus is usually done after 3-5 days or when the culture turns light green in color using a silkscreen net sewn to form a 10 cm x 50 cm bag. The bag is attached to a rubber hose that drains the Chlorella and Brachionus from the tank by siphoning.
About 10 cm of the culture is left in the tank. This will serve as starter for the next batch culture. Usually, several tanks of Chlorella are made. These are transferred to the previously harvested
tank with the Brachionus to start a new batch culture.
CULTURE OF MOINA
Moina are small freshwater cladoceran crustaceans and are commonly called “water fleas” because of their small size and their short jerking movement in, water. Moina are also sometimes called Daphnia. They are closely related, but these two belongs to different genus.
Adult Moina (700-1,000 gm) are bigger than newly hatched brine shrimp (500 p.m) and approximately two to three times the length of adult rotifers.
Moina starters can be obtained from ornamental fish pet shops or they can also be collected from stagnant open canals. These are the reddish flea-like organisms that can be seen at the water surface between 5:30 am-7:00 am. After this time, Moina sinks at the bottom and will be difficult to see. When collecting from canals, a scoop net made up of silk-screen net should be made. The collected Moina can be cleaned by passing them through a fine meshed mosquito net to remove the bigger particle sized insects and other debris.
Any container that will hold water can be used for Moina culture. The size of the container will depend on the population of fry to be fed. The ideal depth of the container is between 40-60 cm. The culture should be placed in 50 percent-60 percent filtered light. A cover is necessary when it rains so as not to dilute the fertilizer that will cause Moina to die.
To prepare the medium for Moina culture, for every ton of water, one sack of dried chicken or hog manure is added. The manure should not be removed from the sack so as not to collect the debris when harvesting the Moina. Allow the manure to soak for one day prior to the addition of the Moina starter. About two cups of concentrated Moina should be used as starter in order to hasten the increase in their population. The first harvest can be done on the third day and every day thereafter. Total harvest is done on the tenth day. After the seventh to the tenth day, a new tank should be prepared to ensure the continuous supply of Moina to the fish fry. The starter can be collected from the old culture tank.
Moina is sensitive to chlorine, detergent, and pesticides. It is, therefore, important that the water to be used is free from such agents.
By J.P. Baldia