A study on an herbivorous fish, Oreochromis niloticus L., diet and its quality in two Ethiopian Rift Valley lakes, Awasa and Zwai

The natural food of Oreochromis niloticus from two Ethiopian Rift Valley lakes, Awasa and Zwai, was studied from the stomach contents. The food was analyzed to determine its chemical composition. The major consumed nutrient by the Awasa population was carbohydrate whereas in the Zwai population it was lipid. Microscopic identification revealed that Microcystis sp. and Botryococcus braunii are the dominant algae present in the food of the fishes from Lakes Zwai and Awasa, respectively. Assimilation efficiency of total organic matter is somewhat low, 28.3% for Awasa and 14.7% for Zwai populations. Both populations assimilated protein to a greater extent than other nutrients. In both lakes the quality of the food, expressed as a ratio of digestible protein to digestible energy, is adequate (>4 mg kJ ⁻¹) for growth, but the Awasa population appears to have higher growth and better condition.     

Growth, daily ration, and gastric evacuation rates of milkfish (Chanos chanos) fed supplemental diet and natural food

Growth, daily ration, and gastric evacuation rates of milkfish ( Chanos chanos ) that fed on natural food and supplement diet were evaluated. Milkfish fingerlings (5.5g) were stocked at 1.5 fish/m² in ten 12 m² concrete tanks layered with 15-cm thick earthen bottoms. All tanks were regularly fertilized (16–20–0 and chicken manure) to maintain natural food production; 4 of the tanks additionally received a supplemental diet containing 34.3% protein and 4290 kcal/kg gross energy. Estimates or daily ration (based on dry weight of stomach contents) were calculated using the E lliot and P erson (1978) and E ggers 1977) models. Gastric evacuation rate was lower in fish that fed on natural food (1.57) compared to fish fed a supplemental diet (1.79). Consequently, the lower rate resulted in lower food intake and slower fish growth. When fish were provided a high quality supplemental diet, daily rations for fingerlings (35 g) to marketable size (116 g) ranged approximately from 0.60 to 19.68 kcal/fish/day. The deviation in daily ration (kcal/fish/day) from the above estimates may indicate the insufficient quantity of dietary energy taken by fish from natural food alone, which could be provided by supplemental diet.     

The food habits of an herbivorous fish (Oreochromis niloticus Linn.) in Lake Awasa,Ethiopia

The natural food ofOreochromis niloticus in Lake Awasa was studied from the stomach contents of 10 fish measuring 18–32 cm (standard length) monthly from August 1984 to April 1986.Chroococcus, Oscillatoria andBotryococcus were found to be important food items in the diet. Animal foods were observed on rare occasions and these were mainly Rotifera. Blue-green algae as a group contribute about 28.1% of the ash free dry weight (AFDW) whereas diatoms and green algae (excludingBotryococcus) make 2.31% and 2.39% of AFDW, respectively.Botryococcus, detritus and other algae comprised 67.2% of AFDW and it was suspected that more than half of this was contributed byBotryococcus. Blue-green algae, which were especially abundant in the diet during the dry season, November–February, are nutritionally most important. Seasonal variation in algal species composition could influence the quality of food ingested by the fish.     

Food and growth of arctic char, Salvelinus alpinus (L.), in the Cumberland Sound area of Baffin Island

Arctic char, Salvelinus alpinus (L.), captured during the summer, 1972, in Cumberland Sound, Baffin Island, fed mainly on planktonic amphipods (Parathemisto libellula, Pseudalibrotus glacialis), copepods (Calanus hyperboreus), and fish (Boreogadus saida, Myoxocephalus sp.), but failed to utilize coelenterates, planktonic gastropods, and epi- and infauna. The considerable variation in the species composition of the diet of char of different lengths was due primarily to size selection. The average length of all individuals in stomach contents and of representatives of most food species increased with the length of fish. The minimum and maximum length of frequently ingested organisms increased four and 90 times, respectively, as char increased from 4 to 85 cm. Char less than 10 cm in length captured in rivers tributary to the Sound fed mainly on larval Chironomidae (Eukiefferiella bavaria) during the summer, whereas those longer than 10 cm fed predominantly on other char. During the winter the stomachs of the smaller individuals were always empty while the diet of the large char was restricted to other fish. Most species available to the char were consumed in proportion to their relative abundance in the rivers. The dry weight of stomach contents, when expressed on a unit weight basis, decreased with the wet weight of char in both fresh and salt water. Thus, fish 10 g in weight contained approximately 1.5 times more food in their stomachs than those weighing 1000 g. The stomach contents of char captured in salt water weighed approximately 11 times more than those of char of comparable size captured simultaneously in fresh water. The fish fed at random intervals during the day and ceased feeding at night. Arctic char, at all sampling areas, had reached a length of approximately 9.6 cm after four years. Upon migrating to salt water, their growth rate increased sharply with the result that after eight years they were 26.5 cm in length, reflecting food availability. The growth rate gradually decreased in fish older than nine years so that 20 year olds were approximately 70.0 cm in length.     

Effects of dietary fibre on the intestinal absorption of dextrin, blood sugar level and growth of tilapia, Oreochromis niloticm × O. aureus

The roles of five different dietary fibres (cellulose, agar, guar gum, carrageenan and carboxy-methylcellulose) each as 10% wt/wt added into the diet, affecting the intestinal sugar absorption, blood sugar level and utilization of dextrin were studied in hybrid tilapia, Oreochromis niloticus × O. aureus ; dextrin and glucose were also included in the study as controls for comparison. There were seven dietary groups: each diet was fed to three aquaria each containing 15 fish. The experiment was carried out in a closed circulation, filtered, rearing system for 2 months. The results indicated that the weight gain percentage and food conversion ratio were significantly (P<0.05) lower in tilapia fed fibre-containing diets than those of tilapia fed dextrin or glucose diet. The intestinal absorption of carbohydrate and the blood sugar content of tilapia were low when diets contained fibre, regardless of the source.     

Mullet grazing on surf diatom accumulations

Grazing by southern mullet, Liza richardsoni (Smith), on surf diatoms occurring in bloom concentrations off an ocean-exposed East Cape beach, South Africa, was investigated. Field observations and stomach content analysis demonstrated that surf diatoms taken from the air-water interface were a principal source of food. A qualitative examination of stomach contents revealed a feeding transition from planktonic carnivore in juveniles to a diet consisting entirely of surf diatoms in larger fishes. This change in diet commonly occurred at a standard length of 50–135 mm. Fish larger than 135 mm fed entirely on surf diatoms which were ingested together with large quantities of beach sediment. Grazing on surf diatoms only took place during daylight hours. Energy, ash, protein, fat and carbohydrate content determinations indicate a high food quality of surf diatoms. It is concluded that surf diatom accumulations form a richly concentrated and reliable food source of high nutritional quality for these fish. Possible widespread grazing on surf diatoms by mullets is considered.     

Nutrient digestibility and growth responses of rainbow trout (Salmo gairdneri) fed different levels of cassava and rice

Seven groups of fingerling rainbow trout (Salmo gairdneri) were fed for 10 weeks on 0%, 10%, 20% and 30% of cassava or rice in isonitrogenous diets.Optimum growth and food utilisation was at 20% dietary cassava. High fibre content of the control diet did not suppress protein digestibility in this group. Rather, at all levels, protein digestibility was good and remained between 84.4% and 87.5%. However, in the control group, carbohydrate digestibility was very poor. The cassava diet which had the highest digestible energy as carbohydrate produced the best growth performance, food utilization and protein sparing. At the levels studied, the dietary carbohydrates produced no hyperglycaemic effect on the fish. There was no evidence of drastic adverse effects on the tissue and liver composition of the fish receiving these carbohydrates.     

Quality and digestibility of food ingested by various trophic fish groups in the Upper Paraná River floodplain

Determining quality of food ingested and digestibility can be helpful in understanding the mechanisms that determine trophic plasticity, which is the ability of a given species to use a more advantageous food source at a given time. In this study, the chemical composition and digestibility of food ingested by various trophic fish groups at different sites of the Upper River Paraná floodplain are evaluated. Specimens of Pseudoplatystoma corruscans, Prochilodus lineatus, Leporinus friderici, Pterodoras granulosus and Schizodon borellii from the Baía, Ivinheima and Paraná Rivers and from Fechada and Ventura Lagoons were taken as samples (3-16 cm-mesh net). Volume participation analyses of food items were determined and contents from the stomach and the intestine's latter quarter were removed for bromatological analysis (energy, crude protein, ash and dry matter). Internal marker acid-insoluble ash was employed for apparent digestibility coefficients. P. lineatus and P. corruscans had an intake with lower and higher energy and crude protein contents, respectively. P. corruscans had slight variations in food items and composition, whereas P. granulosus had the greatest variation. Whereas P. lineatus had the highest apparent digestibility coefficients in energy, S. borellii showed least variation in diet. P. granulosus and P. lineatus had the higher percentages of dry matter in the stomach in the Paraná River, whereas P. corruscans had similar use in the Baía Rivers and Ventura Lagoon. S. borellii showed low utilization of germanous energy. It may be concluded that the site determined the variation in quality and use of diet by L. friderici, P. granulosus and S. borellii. The generalist species L. friderici demonstrated a good use of different food items ingested at the site; likewise, the generalist species P. corruscans had a similar diet and the same use of food.     

The diet of Brycinus nurse (Pisces: Characidae) from Asa reservoir,Ilorin, Nigeria

From November 1991 to October 1993, 980 specimens of the characid Brycinus nurse were collected from Asa reservoir to examine its diet. The diet was analyzed using the frequency of occurrence, numerical and gravimetric methods. Two hundred and sixty nine (27.45%) of the stomachs examined were empty. The fish was an omnivore feeding extensively on a wide array of plant and animal food items. These consisted of 9 families, 10 genera and 10 species. The most extensively consumed plant food item was aquatic plant parts which occurred in 63.88% of the stomachs, and accounted for 6.06% by number and 12.10% by weight while the ephemeropteran, Povilla adusta was the most dominant animal food item, occurring in 50.92% of the stomachs, and accounting for 11.98% by number and 11.86% by weight. Conversely, the least consumed plant food item was Volvox occurring in 4.49% of the stomachs and accounting for 0.18% by number and 0.35% by weight, while the fish Barbus sp. was the least consumed animal food item occurring in 0.51% of the stomachs, accounting for 0.03% by number and 1.62% by weight. New food items not previously recorded such as a watermite. Aspatharia sinuata and Barbus callipterus were found in the stomach contents. The nonspecific feeding regime of the fish and its ability to utilize different food items effectively was what accounted for the prominence and wide distribution of the fish in the lake.     

Aspects of the feeding ecology of Sargochromis codringtonii in Lake Kariba, Zimbabwe

Sargochromis codringtonii has always been considered a good candidate for the biological control of snails. However, there is lack of data on the feeding ecology of this species. The diet of S. codringtonii was examined by an analysis of stomach contents. The food of adult S. codringtonii in Lake Kariba is dominated by prosobranch snails. Pulmonate snails form a small insignificant component of the diet of S. codringtonii , and the reasons for this are discussed. Electivity indices were used to investigate prey selection. S. codringtonii does not show a particular preference for any snail species. It feeds on any snail species that is readily available. A morphometrical analysis of the feeding structures of S. codringtonii showed that the fish is well adapted to handling a molluscivorous diet. The daily food consumption of S. codringtonii was estimated over different seasons using the Elliot & Persson (1978 ) model [ J. Anim. Ecol. 47. 977–991]. Estimates of food consumption indicate that the fish consumed 14.0% of their dry body weight per day in summer and this falls to 4.5% in winter. The low consumption of pulmonates means that S. codringtonii cannot be a successful biological control agent against the vectors of schistosomiasis.     

Gastrointestinal responses of rainbow trout to dry pellet and low-fat herring diets

Long-term voluntary-feeding experiments were carried out on farmed, 2-year-old rainbow trout offered a commercial dry feed, or chopped low-fat Baltic herring. Despite large differences in dietary water, protein and lipid content, the trout adjusted their intake to consume similar amounts of dry matter. After an 18-week trial, the stomach volumes of the herring-fed trout were significantly larger (30–35%) than those fed on the dry diet. Greatest differences were observed when fish were fed one meal per day; increasing the number of daily feeding opportunities reduced these expected stomach volumes on each diet by 15–20%. The relative increase in stomach volume was shown to be due to growth of the cardiac stomach region (corpus) rather than the pyloric region, and not to muscle relaxation; the change was completed within 10 weeks. Data were collected in a separate study to investigate stomach size in fish (age 0+, 1+, 2+) of similar genetic backgrounds which had been grown using dry pelleted diets. Despite considerable variation between populations, stomach volume to body weight relationship was allometric ( S = a W ^b) with the exponent in the range of 0.3–0.4.     

Evacuation of pelleted feed and the suitability of titanium(IV) oxide as a feed marker for gut kinetics in Nile tilapia

The present study assessed the suitability of titanium(IV) oxide, TiO₂, as a digesta passage marker in Nile tilapia Oreochromis niloticus and studied the shape of the evacuation curve in this species. In three separate trials, fish were given one dose of either 0·5, 0·25 or 0·1% of their body mass (% BME) of feed marked with 1% TiO₂ or 0·5% BME of the same feed without marker. The fish were serially slaughtered at intervals after feeding and the stomach contents analysed for dry mass and marker content. The data for individual trials were analysed with the linear, square root, surface area and exponential evacuation models and parameter comparisons showed that, although the marker interfered slightly with the evacuation process, true meal size could be predicted more accurately from the marker data. The results of an analysis of the combined data sets suggested that stomach evacuation in this species is dependent more on food particle surface area (surface area model) than on stomach content mass (exponential model) as is generally assumed. On the basis of these results, it was concluded that TiO₂ at an inclusion level of 1% is an acceptable marker for quantifying evacuation with a view to predicting food consumption but should be used with caution in digestibility studies.     

Digestion of duckweed (Lemna spp.) by the grass carp (Ctenopharyngodon idella)

A feeding trial was conducted in 55-1 aquaria indoors with the grass carp ( Ctenopharyngodon idella Val.) using fresh duckweed ( Lemna spp.) as food. Faeces dry weight was closely correlated with dry weight of duckweed consumed based upon a linear regression. From this regression, the estimated true digestibility of dry matter was 65%. Samples of duckweed and fish faeces were analysed for crude protein and gross energy. Van Soest's Analysis was conducted on composite samples of duckweed and faeces. Apparent digestibilities were 80% for crude protein, 61% for gross energy, 72% for organic cell contents, and 30% for organic cell wall constituents.     

Growth and mortality of zebra fish, Brachydanio rerio (Hamilton Buchanan), maintained at two temperatures and on two diets

One hundred zebra fish per tank were maintained for 112 days at 24°C or 28°C in glass aquaria and fed a diet of flaked food made without cellulose (13.45 kJ g^?1, metabolizable energy, Type A) or with cellulose (8.71 kJ g^?1, metabolizable energy, Type B). Each experimental condition was repeated in triplicate (12 tanks). The weight of food given daily to the fish was based on daily records of survivors (from which mortality rates were calculated) and wet wt of fish (measured every 14 days) in each tank. All fish were fed with the same weight of food per day and the quantity of energy in the food in excess of standard metabolism (as a proportion of SM) was approximately 0–5 for fish maintained at 28°C and fed food B, 1–0 for fish maintained at 24°C and fed food B, 1–5 for fish maintained at 28°C and fed food A, and 2.2 for fish maintained at 24°C and fed food A. Non-ionized ammonia, nitrite and nitrate nitrogen in the tanks did not reach toxic levels although there was an increase in total ammonium nitrogen in one tank and a subsequent heavy mortality. It was assumed that this was caused by the build up of pathogenic bacteria. Apart from this tank, mortality was highest in tanks at 28°C with fish fed food A and second highest in tanks at 24°C with fish fed on the same diet. Growth was measured in units of length, wet and dry weights, carbon and energy. There was a good correlation (P < 0.001) between carbon (mgC mg^?1) and calorific (J mg^?1) values and a conversion factor of 46.2 J (mgC)^?1 was derived. Fish maintained at 24°C and fed food A had the highest rates of growth both in weight and in energy value per unit weight. Fish fed the same diet but kept at 28°C had the lowest growth rates. Both these groups of fish had the highest coefficients of variation in wet weights which increased during the experiment, indicating an increase in interaction within the tanks. There was agreement between the energy value of fish sampled for growth and a condition factor based on the length-weight relationships of fish remaining in the tanks. A correlation (P < 0.05) was found between instantaneous mortality and growth rates for fish between tanks when those maintained at 28°C and fed on food A were ignored.     

Enhancement of carbohydrates in a methylotrophic yeast

The use of methylotrophic yeasts has been suggested for recycling CO₂ to human food in extended space missions. Since the human diet requires higher carbohydrate levels than those normally found in microbes, attempts were made to increase the levels of storage carbohydrates, principally glycogen, through cultural and genetic methods. The effect of defining cultural conditions for the methylotrophic yeast Hansenula polymorpha resulted in increasing the storage carbohydrate content of the dry weight of the cells from 30 to 46%. During this analysis, a growth requirement for potassium was discovered. Several mutant strains were selected for high glycogen storage on plates and analysed for storage carbohydrate levels in submerged culture. These strains exhibited an additional 4–16% increase in the levels of storage carbohydrates over the parent strain in stationary phase. One strain was also able to store excess carbohydrate during exponential growth at levels 10% above the parent strain. Through a combination of cultural control and genetic modification, carbohydrate levels in this yeast were raised to 60% of the cell dry weight. Through additional genetic selection these levels are likely to be increased even further.     

Growth, body composition and hepatic tyrosine aminotransferase activity in cortisol-fed channel catfish, Ictalurus punctatus Rafinesque

Metabolic consequences of chronic elevation of cortisol in the diet of yearling channel catfish, Ictalurus punctatus , were studied. Cortisol was incorporated into the diet in concentrations of 1, 10, 50 and 100 μg/g of food. This diet was offered at 3% of the body weight per day for 10 weeks. Fish were individually weighed and measured at 2-week intervals and feeding rates were adjusted. Body weight, liposomatic index and condition factor were significantly lower and food conversion was significantly higher in fish fed 50 or 100 μng cortisol/g of food when compared with controls. The hepatosomatic index of fish fed cortisol at the rate of 100 μg/g of food decreased significantly. Specific activity of hepatic tyrosine aminotransferase was significantly higher at the two highest cortisol doses. Long-term cortisol administration can reduce growth and condition factor by activating gluconeogenic mechanisms in which lipids and amino acids, rather than carbohydrates, are used for energy production. The metabolic effects of exogenous cortisol in this study offer an explanation for the decreased growth of fish under conditions that activate the secretion of endogenous cortisol.     

Growth rate of tank-reared Mediterranean amberjack, Seriola dumerili (Risso 1810) fed on three different diets

Mediterranean amberjack Seriola dumerili (Risso 1810) has aquaculture potential. The growth rate and food conversion ratio of S. dumerili on three different types of food (A: 100% frozen sardines; B: 50% frozen sardines and 50% pellet; C: 100% pellet) were measured, and analysed with respect to temperature, condition index and chemical composition of the fish fillet. Wild S. dumerili, average body weight 248 g and average total length 26.9 cm were caught in August and September 1994 in the South Adriatic Sea near Dubrovnik, Croatia and kept in three tanks (n=15 in each tank; duration of experiment, 226 days). The fish that were fed on diet A [initial weight, 252 ± 71 g; total length (TL), 24.3 ± 2.6 cm] started feeding immediately; however, fish assigned to diet C began to feed entirely on the pelleted diet 1 month after the start of the experiment. The mortality of fish fed on diet A was negligible, the registered growth rate was 313 ± 74 g (124.2%), specific growth rate was 0.32% day^?1 and the food conversion rate was 6.7. The fish fed on diet B (initial weight, 246 ± 74 g; TL, 28.2 ± 2.5 cm) started to feed on day 3 and achieved a total growth rate of about 98% (final weight gain, 241 ± 69 g) and specific growth rate of 0.24% day^?1 (feed conversion rate of 9.00 and mortality 13%). The fish fed on diet C (initial weight, 246 ± 74 g; TL, 28.2 ± 2.5 cm) started to feed on the pellets after 1 month and had a growth rate of 87% (weight gain 214 ± 85 g), a specific growth rate of 0.24% day^?1 and a food conversion rate of 10.6 with considerable mortality (27%). In all three diet groups the fish grew with considerable variation in food consumption and growth rate, depending on seasonal temperature variations of the ambient sea water supplied to the rearing tanks. Chemical analysis showed that the protein level (amount) in the fish meat exceeded 20% in all three fish fillet samples.     

Gastric emptying rate in Pleuronectes platessa L.

X-ray studies of gastric emptying of Pleuronectes platessa force-fed a moist, artificial diet give the relationship:
where GET is gastric emptying time (h), S is meal size (g), W is body weight (g) and T is temperature (°C).
The corresponding gastric emptying curve is predicted to be:
where stomach contents (g net weight) are S ₀ at start and S_t at time th. Direct observations of residual stomach contents using serial slaughter are in close agreement with the predicted curve, save for small plaice (below 50 g body weight) which empty at only 50–60% of the rate found in larger fish.
The small effects of body weight on gastric emptying rate may reflect the relatively small role of the stomach [Volume (ml) = 0.024 body weight (g)] in digestion when compared with other flatfish.
The emptying curve established here can be used in field studies to estimate feeding rate on natural food items, provided fish greater than 50 g body weight are used and the energy density of the food is greater than 0.5 kcal/g wet weight. Smaller fish will have emptying rates which are only 30–60% of those found in larger fish; the change appears to be abrupt and corresponds to the size at which young plaice move offshore from their nursery grounds.     

Dietary habits of the fanray Platyrhina tangi (Batoidea: Platyrhinidae) in Ariake Bay, Japan

The dietary habits of the fanray Platyrhina tangi were investigated by analyzing the stomach contents of specimens collected in Ariake Bay, Japan. Of 334 stomach specimens, 324 contained food and 10 (3.0%) were empty. The mean percentage weight of stomach contents per unit of body weight was 0.59%. Thirty-seven taxonomic levels of prey were identified. The most common prey was shrimp, followed by fish and mysids. There were no differences in the composition of the diet between sexes, but an ontogenetic dietary shift was observed. Shrimps were the most common prey in all size classes. In addition, smaller individuals frequently ate mysids, and larger individuals often consumed fish. Dietary breadth values increased with size. Trophic level analysis revealed that trophic level increased with size; however, this species is consistently a secondary consumer. Dental sexual dimorphism was also observed. Specifically, mature males had much longer and sharper cusps than females and immature males. Since males and females had similar diets, dental sexual dimorphism may be related to their reproductive behaviour.