CHAPTER VIIILARVAE CULTURE8.1 Free-swimming larvae cultureWhen the larvae begin to swim, their metamorphosis and growth should be closely followed and given more care. Actively swimming larvae are transferred from the upper water layer of the spawning tank to the rearing tank.Should the larvae be reared in the spawning tank, due to the unavailability of other tanks, the discharged egg membranes, detritus and abnormal larvae should be removed from the bottom of the tank. Sterilized and filtered seawater should also be added into the tank after hatching. At this stage, it is important to provide optimum conditions for larval growth. The optimum water temperature is about 20 °C; water temperatures below 17 °C and above 24 °C are not desirable for larval growth. It is possible to use aeration in the rearing water for evenly dispersing the larvae in the water column. The ideal larval density is around 300 specimen per litre.8.2 Creeping larvae cultureThe planktonic larvae search for a suitable ground for their creeping life style as a result of physiological and morphological changes: the foot begins to grow and the velum disappears within 4 days. When a suitable substratum is found they change shape several times and soon start to take food actively.At this stage it is most important to provide the larvae with a biologically, chemically and physically suitable ground as well as other environmental requirements. When larvae enter the creeping stage, board cultches should be placed into the rearing tank. The colour of the boards should not allow light penetration and their surfaces toxic-free as well as suitable for creeping.Normally, the boards are corrugated and made of a plastic material, measuring 30–60 cm × 30–60 cm. From 100,000 to 1,000,000 larvae per ton of rearing water are placed into the tank and 80 setting boards are fixed on the water surface. Beforehand, micro-organisms such as bacteria, yeast, protozoa measuring 5 micrometer and Navicula sp. are cultured on the setting boards.The density of food organism below 3,000 cell mm2 is effective for the settling and survival of larvae. Larvae which set on the boards are able to feed on food organism measuring between 5–10 micrometer during the first 5–10 days. After 20 days they can take diatom of 20 um. At this stage it is important to supply the species and quantity of food organism according to the feeding habit of larvae.The basic biological conditions for rearing larvae are as follows:The optimum water temperature is about 20°C; above 25°C, the survival rate decreases.A constant temperature of about 20°C should be maintained for 60 days.The salinity should be above 30 parts per thousand and pH value between 8.0–8.3.The degree of oxygen saturation in the rearing water should be maintained at 70–100 %. The culture water requires periodic aeration.Much attention should be paid to the growth of food organism and other biological processes naturally occurring inside the tank.In many cases, the larvae mortality rate is high during the first 60 days; this is due to factors such as egg quality, contamination by microorganisms, water quality, and quality and quantity of feed on the boards.The water in the tank should be filtered and sterilized. Sometimes the mortality rate may be high due to the overgrowth of food organisms. Bearing in mind these considerations, a technical system to control the growth of diatom should be established.CHAPTER IXJUVENILE CULTURE9.1 Rearing conditionsAt this stage diatomaceae are cultured on the surface of the board which are fed upon by the juvenile abalones until they reach 5–6 mm in shell length.The optimum rearing conditions at this stage are as follows:Water temperature 20°CSalinity >30 ‰Do >4 ml/lLight intensity >3000 LuxNH4OH-N <5 ppbpH 8.0–8.3Juvenile density 400–500/m2In order to supply sufficient food to the juveniles, the rearing water must contain the correct content of nutrients (5 ug-atom NO3-N/1, 5 ug-atom PO4-P/1 and 1 ug-atom Si/l).When the juveniles reach 5–6 mm after 80 days, they are transferred from the setting boards to the intermediate rearing tanks. Separating abalones from the setting boards is not done by hand but with the use of anaesthetizing reagents such as carbonic acid gas, ether, ethanol, para-amino benzoic acid, etc.After anaesthetizing the juveniles they are sieved and selected according to size; abalones of 5–6 mm are transferred to the intermediate rearing tank.
การแปล กรุณารอสักครู่..
