Feeding

As previously indicated two feeding methods are most widely adopted: 1) a combination of algae and baker’s yeast and 2) a totally artificial diet. For its reliability and higher output, the latter is progressively replacing the first method.

Mass culture with algae/yeast as food

The initial method of mass culturing rotifers in Mediterranean hatcheries, makes use of a common and easily available food staple, the bakers’ yeast Saccaromyces cerevisiae. It is a labour and cost sparing food, which has no nutritional value for rotifers that feed on bacteria associated with the yeast.

Compared to the artificial diets, this method has a lower yield and requires more time, typically one extra day. Density at harvest rarely exceeds 450 rotifers/ml with an average daily increase ranging from 19 to 33%. In addition, rotifers should be enriched with high levels of (n-3) HUFA and vitamins. A major constraint of this method is the absolute necessity to improve the otherwise very poor nutritional quality of yeast-fed rotifers before their distribution to fish larvae. The enrichment procedure, which takes place the day before harvesting the rotifers is explained below.

Fig. 30.01 Large containers with hypochlorite solution are used to keep cleaned small equipment (photo STM Aquatrade)

Protocol:

1. fill the tank with sterilized sea water diluted with tap water to obtain a 20 ppt salinity; check the chlorine content of tap water and, if present, neutralize it with an excess of sodium thiosulphate (see Annex 7). Take care to leave enough space for the algal cultures to be supplied as food (about 30% of the tank volume).

2. place the air diffusers and switch on the aeration;

3. place the traps for ciliates and impurities;

4. select the most suitable rotifer bags to be used for inoculum, checking for contaminants and using only clean batches; 5. filter the selected batch or batches (see below for procedure);

6. inoculate the tank to achieve an initial density of 150-200 rotifers/ml. This is considered day 0;

7. add algal culture as 20% of tank volume to provide rotifers with their initial food; as usual, the algae should be in their log-phase and from non contaminated cultures, even if of different species.

8. the next day (day 1) fill the remaining 10% volume with algal culture;

9. on the tank file, record all information on the culture growth, food distributed and environmental parameters monitored (see below for details and Annex 9 for a file sample);

10. from day 1 on, feed with bakers’ yeast at the following rates, according to the recorded rotifer density.

The daily amount of fresh yeast is divided into 4 equal rations fed at 2 and 8 am, 2 and 10 pm (the last two rations are given by the night watchman). Each yeast ration is taken from weighed out of the yeast cake kept in the refrigerator and placed in a plastic bucket filled with tap water at a concentration of 100 g/l. A kitchen or better an industrial blender helps to separate yeast cells and keep them suspended in water. Feed immediately and discharge any leftover. Prepare fresh for every meal.

Mass culture with Culture Selco® as food

A different technique based on a compound feed has been developed a few years ago by the Belgian Company INVE SA. The product, named Culture Selco® (CS) is a dry and complete rotifer diet that does not require algae and is also effective as enrichment medium. Particle size (5 to 7 µm) and physical characteristics ensure an optimal uptake by rotifers. The feed composition includes proteins (>35%), lipids (>15%, of which 23% are PUFA), carbohydrates (30%), carotenoids and other micronutrients as minerals and vitamins A, D3, F and C.

The average daily production of rotifers fed on CS ranges consistently from 45 to 60% of the initial rotifer density. In addition rotifers are enriched with high levels of the essential (n-3) PUFA and vitamins.

Fig. 30.02-03 Rotifers mass culture performed in very large heated tanks at Ittica Mediterranea (photo STM Aquatrade)

This diet has made rotifer mass culturing reliable and predictable, and has consistently reduced the need for algal cultures and their associated labour and facilities requirements. New rotifer cultures can be easily started from old ones, thanks to their enhanced fertility.

Feeding

The following schedule gives the CS daily feeding rate (DFR), expressed as grams of CS per million rotifers. The day before harvest rotifers are enriched with other artificial diets made by INVE, Protein Selco® or DHA Protein Selco® (see Enrichment below for details).

a/Replace CS with Protein or DHA Protein Selco®

Fig. 30.04 An industrial blender for yeast or Culture Selco® suspension (photo STM Aquatrade)

To prepare CS suspend the amount required for a single meal in tap water, up to 50 g CS/l, and mix vigorously for 3 minutes (use a kitchen or better an industrial blender). Mixing or shaking by hand or using a magnetic stirrer is not sufficient to separate the CS cells. Remember that cell agglomerates left in the feed suspension cannot be ingested by rotifers because of their large size. Take care not to overfeed as uneaten feed can also quickly spoil water quality. Feed the daily amount in four to six meals evenly distributed over the 24-h period. In case it would be necessary, the feed suspension can be stored at a temperatures below 8°C, and the amount needed for the whole day can be prepared at one time. The feeding ration can thus be distributed from the stored suspension at each meal.

Fig. 30.05-6 CS preparation and distribution to the tanks (photo STM Aquatrade)

Before transferring rotifers to a new tank, place about 25% of the first day’s food ration in the tank, so that the feed would be already evenly spread when the rotifers enter the medium. Add a few drops of silicon based antifoam agent, such as Rhodorsil Antimousse AM 70414 by Caldic Belgium NV.

Aeration

Aerating the rotifer culture is essential to provide oxygen, to keep rotifers and food cells suspended and to optimise tank cleanliness. The aeration rate and the air diffusers positioning should be carefully adjusted. A reasonably strong aeration is detected by an evenly spread water turbulence at water surface without large air bubbles.

Number and positioning of air diffusers depend on tank shape and volume. In all cases, however, they should be suspended at about 15 cm above the tank bottom to prevent re-suspension of sediments. As to the number of diffusers, a reliable rule of thumb is to keep a distance of approximately 60 cm among them.

Fig. 30.07 CS distribution by gravity to the tanks at Ittica Ugento (photo STM Aquatrade)

Cleaning cultures

In round tanks with conical bottom, bottom sediments are removed by letting them first settle for 10 15 minutes in absence of aeration (take care to check DO levels during the operation). Then the bottom valve is opened for a few seconds and it is closed when the outflowing water is again clean. Repeat the operation twice a day, in the morning and evening.

Complete the cleaning procedure by removing any greasy layer that may be formed at the water meniscus with a sponge or a paper cloth. Never dip your hands in water.

The rotifer tank water can be rapidly polluted by particulate matter, faeces and flocks of uneaten food. Their removal may prevent an excessive bacterial development and will increase the oxygen available for rotifers. As the continuous aeration of the water volume prevents in part the settlement of particles, their removal is achieved by means of “particle traps”. These devices consist of floating mats of coarse sponge-like material, such as the housing polishing mat Scotch Briteä. In the case of a 3,000-l tank, three pieces measuring 15 x 100 cm are hung vertically in the water (by means for example of wooden stick laid across the tank top rim), and are kept vertical by a terminal weight. The water circulation pushes particulate matter against the trap, where they adhere and clog the mat. These mats are also an ideal substrate for Vorticella, a sessile ciliate that competes with rotifers for food. To be effective, the traps must be cleaned at least twice a day.

To clean particle traps proceed as follows:

1. take them out of the tank carefully, avoiding dripping of trapped material back into the water;

2. remove all particles with a high pressure jet of tap water;

3. dip the trap in a 500 ppm hypochlorite solution for one hour to disinfect;

4. rinse well with tap water, dip in a thiosulphate solution to neutralize residual chlorine and place again in the tank.

Fig. 31.01-2 Scotch Briteä after one night of work and during cleaning procedures (photo STM Aquatrade)

 

Amany Esmail

Manger GAFRD Web Site