Sena S. De Silva

DIGESTIBILITY IN SAROTHERODON NILOTICUS FRY: EFFECT OF DIETARY PROTEIN LEVEL AND SALINITY WITH FURTHER OBSERVATIONS ON VARIABILITY IN DAILY DIGESTIBILITY

Abstract Dry matter and apparent protein digestibility coefficients were determined in Sarotherodon niloticus fry of mean weight 21 mg maintained on four artificial diets with protein content ranging from 9.6% (Diet 1) to 30.4% (Diet 4) at salinities of 0‰, 5‰ and 10‰. The total digestibility estimates made using faecal material accumulated through the night did not significantly differ ( P > 0.05) from those estimates based on faecal material collected during the day time, between feedings. The mean apparent total, protein and energy digestibility of the different groups of fry ranged from 56.7% to 69.1%, 71.7% to 87.1% and 73.6% to 83.9%, respectively. The dry matter digestibility, in all experimental groups of fry, showed a significant decrease when the dietary protein content changed from 9.6% to 30.4%. Salinity had no significant effect on the digestibility of any of the four experimental diets. In S. niloticus maintained on Diets 3 and 4 at salinities of 5‰ and 10‰ the total and protein digestibility showed daily variation. Generally a day or two of high digestibility was followed by a day of low digestibility. The possible utilization of this rhythmicity, whether apparent or real, in aquacultural practices to reduce the feed costs is suggested.

The dietary protein requirements of young tilapia and an evaluation of the least cost dietary protein levels

Abstract In this study our goal was to establish the most economical dietary protein content for tilapia culture. To this end we assessed the relationship of growth, measured as percent average daily gain (% ADG), food conversion ratio (FCR) and protein efficiency ratio (PER) of young of four tilapiine species, Oreochromis mossambicus, O. niloticus, O. aureus and Tilapia zillii , for which experimental growth data were available, in relation to body weight and dietary protein content (% protein). The relationship of % ADG to % protein in young tilapia weighing less than 1 g or 1–5 g was found to be a second order polynomial quadratic function whereas FCR and PER were linearly related to % protein. % ADG, FCR and PER were correlated better to body weight (curvilinearly) than % protein in both size groups, and multiple regressions were derived between the above parameters. The most economical dietary protein content was evaluated from the polynomial quadratic function utilizing 95% confidence limits and also from the multiple regressions incorporating FCR and % ADG for a set of nearly isocaloric diets (gross energy) of different protein content ranging from 12% to 44%. Data derived from the foregoing approaches showed that young tilapia weighing between 1 and 5 g require 28% of the diet as protein. This dietary protein content, however, is considerably less than the protein level which supports maximum growth, namely 34%.

Effects of Dietary Protein Level on Growth, Food Conversion, and Protein Use in Young Tilapia nilotica at Four Salinities

Abstract Effect of seven artificial diets, having protein contents ranging from 10 to 48%, on the growth and food conversion in young Tilapia nilotica of mean weight 24 mg were tested in 0‰, 5 ‰, 10‰, and 15‰ salinities. Growth of young T. nilotica was consistently better on diets containing 28–30% protein. For diets up to 30% protein, the fish grew best at 10‰, and for diets of higher protein contents, fish grew best in fresh water. Efficiency of food conversion increased with dietary protein content up to 30% protein level and then decreased at higher contents. Food conversion efficiency was greater for young fish raised in 10‰ than in other salinities.

The composition, nutritional status and digestibility of the diets of Sarotherodon mossambicus from nine man-made lakes in Sri Lanka

SynopsisThe dietary composition and the nutritional status and the digestibility of the diets of Sarotherodon mossambicus from nine reservoirs in Sri Lanka were evaluated. The feeding habits of S. mossambicus were variable from reservoir to reservoir; they ranged from herbivory to total carnivory. The protein, total lipid, carbohydrate and total organic matter content of the ingested material were related to the dietary composition and ranged from 18.53% to 35.15% (x−24.18%), 5.94% to 9.84% (x−7.91%), 11.6% to 34.7% (x−22.34%) and 34.4% to 64.4% (x−45.71%), respectively. Irrespective of the feeding habits, the diet contained a significant proportion of organic material which cannot be accounted for by protein, total lipid and carbohydrate. As much as the ingested material was related to the feeding habit, the digestibility of the nutrient components was related to the food material devoured. For example, the mean digestibility of the total organic matter in S. mossambicus feeding on detritus, plants and animal were 36.85, 33.5 and 29.5 respectively, and compared well with observations from elsewhere. It is hypothesised that the favourable nutrient quality of the available dietary material in the reservoirs of Sri Lanka, which could be and is effectively utilized by S. mossambicus, may have been, at least partially, responsible for its almost unprecedented success in Sri Lanka.

Reproductive biology of six Barbus species indigenous to Sri Lanka

SynopsisSix Barbus species were sampled monthly in two river systems of SW-Sri Lanka, over a 15 month period. Sex ratio, size at maturity, maximum and modal sizes, distribution of egg sizes in mature ovaries and individual fecundities were measured. Seasonal changes in size distributions, sex ratio, frequency of female gonadal stages and the gonado-somatic index (GSI) were analysed on the basis of the monthly samples. Seasonal cycles in reproductive activity were assessed, using mainly the two last-mentioned parameters and the appearance of young in the populations. It was found that three of the species (B. bimaculatus, B. cumingi, B. vittatus) tend to concentrate their reproductive activities in certain months of the year, roughly coinciding with the seasonal peaks of precipitation. The other three (B. nigrofasciatus, B. dorsalis, B. titteya) spread their reproductive efforts more evenly through the year. Sexual dimorphism with females bigger, relatively early maturation of males, high maximum GSI values, a distinct ‘final mode’ of large mature ova in the egg-diameter distribution, and a comparatively steep increase of fecundity with body weight or gonad weight were found to be more or less consistently associated with seasonal reproduction.

Reproductive biology ofSarotherodon mossambicus, an introduced species, in an ancient man-made lake in Sri Lanka

SynopsisThe reproductive biology ofSarotherodon mossambicus (Peters), a species exotic to Sri Lanka, was studied in Parakrama Samudra — an ancient man-made lake. Females outnumbered males by approximately 2 to 1, but the predominanace of females tended to decrease with increasing size. Males mature at a length of 27.5 cm and females at a length less than 15.0 cm.S. mossambicus breeds throughout the year with four possible peak periods, which coincide with the tailend of the monsoon and intermonsoon rains. The egg diameter distribution indicates the presence of reserve oocytes and yolked oocytes, the latter falling into a single mode between 1.2 to 3.6 mm. Fecundity varied between 360 and 1775 for fish ranging in length from 20.0 to 31.9 cm and 145 to 538 g in weight. The diameter of nests increased with depth up to 60–65 cm. Nests were not found in depths over 90 cm.

Effect of dietary protein level and amount of plant ingredient (Phaseolus aureus) incorporated into the diets on consumption, growth performance and carcass composition in Oreochromis niloticus (L.) fry

Abstract Feeding trials were conducted for a 10-week period on young O. niloticus of mean weight 2.92±0.3 g maintained on 15 isocalorific diets with protein contents of 20%, 25% and 30%, and with progressively increasing substitution levels (13% to 50%) of the legume green gram ( Phaseolus aureus : synonym Vigna radiata ). Daily food consumption was variable. The mean consumption, expressed either as g/g fish day −1 or g/fish day −1 , did not vary significantly between plant-protein-substituted diets at any one protein level, except in one instance. The overall consumption was independent of the dietary protein content at lower protein levels, and of the amount of plant ingredient in the diets. Growth (percent average daily gain in weight (%ADG)), food conversion ratio (FCR) and protein efficiency ratio (PER) ranged between 3.13 (G 50 P 20 ) and 7.79 (CP 25 ), 3.21 (G 50 P 20 ) and 1.53 (G 25 P 30 ), and 1.51 (G 50 P 20 ) and 2.99 (G 37 P 20 ), respectively. In parentheses are the dietary codes where the subscript of P indicates the dietary protein level, that of G the level of incorporation of the plant ingredient and C the control diets. The best %ADG and FCR were observed at the 25% level of plant-ingredient substitution at all three dietary protein levels. Carcass composition indicated that protein deposition was higher with high-protein diets.

New fish resources of reservoirs in Sri Lanka. II. Further studies on a gillnet fishery for minor cyprinids

Abstract This paper presents a further evaluation of the multimesh gillnet experimental trials carried out in five reservoirs in Sri Lanka with a view to introducing a small meshed gillnet fishery for the commercial exploitation of indigenous minor cyprinid species. In all reservoirs the minor cyprinid catches accounted for more than 50% of the total but the extent of those catches differed from one reservoir to another depending on the individual species composition. It is suggested that, based on the species composition and relative abundance, subsidiary gillnet fisheries of 15, 25 or 30 mm stretched mesh size could be introduced. Also, because the minor cyprinid catches exceed the existing Oreochromis mossambicus commercial catches by 10-fold in certain reservoirs, it is suggested that small meshed gillnet fisheries in Sri Lankan reservoirs are commercially viable.

New fish resources of reservoirs in Sri Lanka. 3. Results of commercial-scale trials and yield estimates of a gill-net fishery for minor cyprinids

Abstract It has been established that the indigenous cyprinids, which are all minor carps, which have successfully colonized Sri Lankan reservoirs could be exploited by the introduction of a subsidiary gill-net fishery without apparent detrimental effects, due to the capture of juvenile fish, on the existing gill-net fishery dominated by the exotic Oreochromis mossambicus (Peters). In this paper, the results of commercial-scale trials, using 15- and 28-mm mesh-size multi-filament gill nets carried out simultaneously by the survey team and commercial fishermen in six reservoirs are presented. The results of the survey team and the commercial fishermen were almost identical with respect to landings of cyprinids, O. mossambicus and other forage species. The results confirm the feasibility of a subsidiary gillnet fishery for cyprinids in Sri Lankan reservoirs. The potential yield from such a fishery was estimated using an indirect approach in which the species composition during the experimental gill-net surveys and the existing commercial fishery was taken into consideration; the ratio of minor cyprinids to O. mossambicus landings per unit of netting was used as an index of the relative abundance of the two groups. On the basis of this computation, the yield is estimated to range from 35 to 2191 kg ha−1 year−1 in reservoirs of widely different hydrological and hydrobiological characteristics, with a mean of 907 kg ha−1 year−1. It is estimated that a well managed subsidiary gill-net fishery in the major perennial reservoirs of Sri Lanka could yield 90 700 ton year−1.

Aspects of the biology of the euryhaline Asian cichlid, Etroplus suratensis

SynopsisAspects of the biology of the euryhaline Asian cichlid Etroplus suratensis in a coastal lagoon of Sri Lanka were studied for a period of 15 months. Comparisons are made from the catches from the fishery of manmade, inland freshwater reservoirs. Changes in the stock structure in the sublittoral region of the lagoon indicate that recruitment takes place twice a year, during the high rainfall/low salinity periods. The feeding habits of E. suratensis were different in the two habitats. In the lagoon it fed mainly on molluscs and in the freshwater reservoirs on macrophytes. Feeding chronology based on diurnal surveys indicate that it feeds mainly during the daylight hours. The dentition of E. suratensis is adapted for both tearing and crushing (pharyngeal teeth). Distinct differences in the mean relative intestinal length between populations from the coastal lagoon and inland reservoirs were evident and these differences are correlated to their respective feeding habits. Diurnal changes in feeding activity are associated with changes in the stomach pH. The egg-diameter distribution of mature fish indicate that E. suratensis is a single spawner, that it sheds its mature eggs all at once. It is inferred that E. suratensis breeds twice a year but that an individual female is capable of spawning only once a year.