Abstract. Oreochromis niloticus (L.) young of two size groups, 170‐213 mg (±7) and 1‐017‐1‐072 g (±0‐04), were reared under seven different dietary schedules for 70 days. The two control diets had 18% and 30% protein content; the other five schedules were randomly determined, where the feeding of the low‐protein diet alternated with the high‐protein diet, for differing periods. On the basis of the observed rate of growth, food conversion ratio, protein efficiency ratio and percentage protein retained in both size groups of young O. nitoticus, those reared on a dietary schedule of 2A/3B performed best followed by those reared on schedule 1A/3B; where the numerical value refers to the number of days a particular diet was offered continuously. The feed cost‐saving factor, by adopting alternate feeding schedules as compared to presenting a high‐protein diet continuously, is shown to be significant.

The digestibility of the aquatic macrophyte Hydrillu verticellatu by the Asian cichlid, Etroplus suratensis was estimated using three indigenous markers viz. hydrolysis resistant organic matter (HROM), crude fibre (CF) and hydrolysis resistant ash (HRA). In the fodder HROM showed the least variation and HRA the highest. The recovery of HRA in the faeces was always higher than 100% whereas HROM was the nearest to 100%. The total digestibility varied between 34.6% and 5 1.9%, and the digestibility estimates were higher when HRA was considered as the marker. The overall protein and lipid digestibility varied between 59~2%70~9% and 65~0%70~1% respectively. It is suggested that HROM is a better reference marker in comparison C F and HRA. Daily variation in protein digestibility ranged from 5 1.2% to 8 1.8%.

The biology of young grey mullet, Mugil cephalus L. populations of a coastal lagoon-the Negombo Lagoon on the West Coast of Sri Lanka was studied from October 1976 to March 1978. The relative abundance of fry were found to vary from month to month, with peaks occurring in December-January, May and September-October, coinciding with the end of rainy seasons. It was found that the young fry tended to migrate into the lagoon at a size of about 10-20 mm, most frequently between 15-20 mm in length, by which time they were fully scaled. The young fry after a period of growth in the shallow areas of the lagoon, when they reached a size of 30-35 mm, emigrated into the deeper waters of the lagoon.The rate of growth, as computed from length frequency distribution studies, was found to be around 0,24 mm day+. Results of diurnal surveys showed a movement of the larger size fry into shallower waters during the hours of darkness and this movement appears to be independent of the tidal cycle. It is postulated that the cause of this migration is to escape from predators, chiefly piscine. Investigations of the meristic characters indicated that the young fry which migrated into the lagoon at different times of the year probably originated from a single spawning stock.