HATCHERY PRODUCTION OF MARINE FISH WITH SPECIAL REFERENCE TO THE REPLACEMENT OF LIVE FOODS WITH MICROENCAPSULATED DIETS IN THE REARING OF THE LARVAE OF SEABASS, LATES CALCARIFER (BLOCH)

Abstract

To provide live food for fish larvae, rotifers (Brachionus plicatilis) were produced continuously through 15 cultures over a period of nine months using baker's yeast (Saccharomyces cerevisiae). The baker's yeast contained no highly unsaturated fatty acids of the n-3 series (n-3 HUFA). In the total lipids from samples of rotifers taken from several different cultures, the mean n-3 HUFA content was 2.8%. Secondary feeding with algal replacement microcapsules (5-10 um) was found to be an effective method for increasing the n-3 HUFA content in rotifers and Artemia nauplii and thereby improving their nutritional value for marine fish larvae. The level of n-3 HUFA in the lipids of rotifers was proportional to the level of n-3 HUFA in the lipids of the microcapsules. All-protein membrane microcapsules coloured with a fluorescent dye were fed to seabass (Lates calcarifer) larvae either alone or together with rotifers and the larvae were examined under a confocal scanning laser microscope or under a fluorescence microscope. Microcapsules were ingested by the larvae from first feeding; although microcapsules from 15 um to 150 um were provided, those ingested were 40-60 um. All larvae fed microcapsules alone had died by the tenth day after hatching and the protein wall of the microcapsules was not broken down in the larval gut. When rotifers were fed together with microcapsules, there was evidence that the protein wall of the microcapsules was broken down in the larval gut and that the protein membrane was absorbed. Cofeeding rotifers and microcapsules, both at high and at low salinities, enhanced the growth of seabass larvae compared to feeding them rotifers alone. Seabass larvae fed either microcapsules or live food alone or fed microcapsules together with live food were treated with thyroxine (T 4) or triiodothyronine (T 3) at high and low salinities. None of the combinations of food, thyroid hormone treatment and salinity tested produced a significant stimulation of growth (in terms of total length) in the treated larvae compared to the controls, The development of the digestive system and changes in activities of proteolytic enzymes were studied in seabass larvae and juveniles. A high level of pinocytotic activity was found in the rectal cells of 6 day-old and 14 day-old seabass larvae indicating that protein macromolecules were being absorbed by these cells and digested intracellularly. The stomach and pyloric sphincter did not start to form until day 13 and were not completely formed until day 17. The pH of the stomach changed from being initially alkaline (pH 7.7 on day 8); on day 17, the pH of the stomach had become acidic (pH 5.0) and pepsin-type enzyme activity had increased from an initial basal level. By day 22, the acidity of the stomach had become more pronounced (pH 3.7) and the pepsin-type enzyme activity had become well established. The mechanism of protein digestion in the larvae and the role of live food in the protein digestion process are discussed.