PART II
CULTIVATION OF LIVE FOOD AND WATER
QUALITY CONTROL TECHNIQUES
CHAPTER I
CULTIVATION OF LIVE FOOD
1.1 Collection of benthic diatoms
Benthic diatoms are found in abundance along the seashore. The major species used as basic feed for abalone in Korea belong to the Navicula genus. These diatoms grow on the surface of stones and sand in shallow seashores, as well as plastic plates and other suitable objects. When benthic diatoms aggregate they tend to form colonies of a yellow-green color which gradually turns into yellow-brown as they grow. They are abundant in waters approximately 0.5 m deep. A variety of small diatom species tend to be a suitable food for young abalone. The ideal sites for collecting these diatoms are usually near the abalone culture areas.
1.1.1 From polyethylene platos or plastic floats
Unwanted foreign substances encrusted on the plates and floats should be removed while leaving only the diatom layer. The diatoms are then carefully scraped off and place in a clean glass container or in a plastic bag filled with clean filtered seawater.
1.1.2 From sand
About 0.5 cm of bottom sand layer should be carefully collected from the sea bottom. The sand is then washed several times with a strong jet of seawater in order to detach the benthic diatoms. The diatoms are then collected in a fluid form.
1.1.3 From gravel or bivalve shells
The surface of gravel or bivalve shells are rubbed with gauze or padding. The diatom fluid is collected by washing the gauze or padding with a strong jet of clean seawater. The diatom fluid is then filtered through a 99 um-mesh size sieve, then through a 60 um-sieve and finally through a 20–30 um-sieve. Should the series of sieves not be available, the same result can be obtained by filtering the seawater initially through a 2-folded nylon cloth and then through a 20–30 um perforated nylon cloth.
1.2 Diatom settlement on feeding plates
Transparent polyethylene sheets measuring 30×30 cm are used as the settling structures for the collected diatoms. These plates are grouped into bundles of 10–20 sheets arranged at 3–5 cm intervals.
Such a structure is convenient to handle as well as for studying the sticking behaviour of the diatoms. In static water the diatoms sink and rapidly stick to the plates. As a result the plates should be placed into the diatom rearing tank at a sloping angle. The diatom rearing tank is then filled with clean filtered seawater into which the diatom starter is eventually poured. The diatoms have usually a density of more than 4000 cells/l, and 1 m3 of seawater should be inoculated with 20 litres of the diatom fluid. Within 24 hours the diatoms will have settled evenly over the plates, and if they do not fall apart when disturbed by slight splashing, the operation may be considered successful. At this stage the tank should be emptied and re-filled with clean seawater which will allow good light penetration and add the necessary nutritive salts. Following the above procedures the diatom culture is left to expand.
1.3 Management of diatom rearing conditions
1.3.1 Water temperature
Benthic diatoms grow best at water temperatures between 21–25 °C, and can survive at temperatures as high as 30 °C. The low water temperature in early spring or autumn is sufficient for their growth in the rearing tanks, however propagation is very slow. When culturing diatoms in tanks, the water temperature should be 3 °C higher after changing the water so that propagation process is not affected.
1.3.2 Light
Indoor diatom culture requires enough light to activate photosynthesis. Glass roofs and/or big side-wall windows fitted with curtains are needed in the culture rooms. The curtains are to prevent direct sunlight and to adjust the intensity as desired. Navicula spp. propagate fast at relatively strong light levels (1500–3500 lux). Artificial illumination (about 1500 lux) is necessary in the rainy season when natural sunlight is dim. The illuminating photoperiod time is usually 10–12 hours a day.
1.3.3 Nutritive salts
The salinity of seawater used in the cultivation of diatoms is around 30 °/oo. Inorganic salts, micro-elements and organic substances should be added into the rearing seawater at a certain proportion to accelerate growth. The concentration proportion of nutritive salts to be added is as follows:
N : P : Fe : Si = 10–25 : 1–2.5 : 0.1 : 1 ppm.
The nutrient salts used are NH4NO3, KH2PO4.3H2O, FeC6H5O3 3H2O, Na2SiO3.9H2O. Vitamin B12 is usually added at a concentration of 0.25 ug/l of seawater. The nutrients are added after changing of water in the early morning or in divided amounts 2–3 times throughout the morning. Following the addition of the nutritive liquid, the rearing seawater should be softly agitated to evenly disperse the nutrients.
1.3.4 Water quality control
Culture of diatoms demands regular changes of enriched seawater which is carried out once every 2–3 days. At high water temperatures, changes should be more frequent; once or twice daily. Diatoms reared under such condition can be fed to the young abalone after one week. Species with a comparatively fast growth rate can be fed to the juvenile abalone after 5 days, when cultured under appropriate conditions.
1.4 Growth and culture of diatoms
Naked-eye observation and microscopic examination of the cultured diatoms should be carried out daily. The distribution over the plates should be observed whether it is uniform or not, from the different colour shades.
The growth status of diatoms can also be judged from the change in cell colouration, cell density, degree of stickiness, etc. Healthy and normal cells are uniform in pigment, gradually turning from light yellow to dark yellow-brown and do not fall apart when the rearing water is agitated. Tiny air bubbles floating up the water column during midday hours is an indication of good health.
Lengthy culture periods are undesirable as the aging diatoms tend to fall apart in pieces. The colour gradually turns from yellow-brown to purplish-blue and large air bubbles tend to be trapped under the diatom layer floating up individually from time to time.
1.5 Diatoms as young abalone feed
The diatom plates are hung in the upper layer of abalone culture tank on the second day of free-swimming stage after spawning. On the third day the larvae will settle on the plates and begin the benthic creeping stage, feeding on the diatoms. Optimum diatom density on the plates for larval growth is 3000 cells/mm2. If the density is low, more diatoms should be added into the rearing tank in order to increase the food level. Prior to adding diatoms, they should be decontaminated through filtration. In addition, sufficient light and the right amount of nutrient salts should be made available to enhance diatom growth.