Influence of larval co-feeding with live and inert diets on weaning the tongue sole Cynoglossus semilaevis

The tongue sole Cynoglossus semilaevis, an inshore fish in China, has showed great potential in aquaculture recently. However, poor survival was recorded during the period of weaning from live Artemia to artificial diets. In this paper, the influence of co‐feeding larvae with live and inert diet on weaning performance was described. The C. semilaevis larvae were reared at 21 ± 1 °C and fed four different feeding regimes from 6 days post‐hatching (dph): A, Artemia (10 individuals mL^?1); B, Artemia (5 individuals mL^?1); C, mixed diet (10 Artemia individuals mL^?1 and 12 mg L^?1 inert diet); and D, mixed diet (5 Artemia individuals mL^?1 and 12 mg L^?1 inert diet). Rotifers were also supplied in all cases during the first days of feeding. Mixed diets of commercial formulated feed and live prey (rotifers and Artemia) allowed larvae to complete metamorphosis, achieving similar specific growth rate (SGR) (18.5 ± 1.4% and 18.7 ± 1.6%) and survival (40 ± 7.6% and 48.5 ± 6.8%) compared with larvae fed on live feed alone (SGR of 18.3 ± 1.2%, 19.3 ± 1.9% and survival of 41.2 ± 11.3%, 38 ± 4.9%). However, in metamorphosed fish, when live feed was withdrawn on 31 dph, there was significant difference (P < 0.05) in survival and growth among treatments. Metamorphosed fish, previously fed mixture diets during larval stages, had similar survival (62.1 ± 7.6% and 62.8 ± 3.9% for regimes C and D, respectively) but higher than that obtained for fish that previously fed on live feed (49.3 ± 2% and 42.1 ± 3.9% for regimes A and B, respectively) after weaning (day 60). The SGR of weaned fish previously fed live feed was similar (3.1 ± 0.6% and 2.92 ± 0.6% for regimes A and B, respectively) but lower than that recorded for fish that was fed from day 6 to day 30 on the mixed diet (4.5 ± 1.1% and 4.9 ± 0.3% for regimes C and D, respectively). It is suggested that weaning of C. semilaevis from early development would appear to be feasible and larval co‐feeding improves growth and survival.     

The effects of feeding frequency on growth of juvenile Atlantic halibut, Hippoglossus hippoglossus L.

Four experiments were conducted to investigate the effects of feeding frequency on growth of juvenile Atlantic halibut, Hippoglossus hippoglossus L. Fish (22–75 g) fed three (3 ×) or five times per day (5 × day^?1) under constant light and temperature (13±1°C) consumed significantly more feed than fish fed 1 × day^?1 but by the end of the experiment only fish fed 5 × day^?1 were heavier and had greater specific growth rates (SGR). Under simulated winter conditions (9L:15D, 5±1°C), halibut (～300 g) fed every other day consumed more feed, had a greater SGR and final weight compared with fish fed every third day. Feed conversion ratios were not different among treatment groups in any of the experiments. These results suggest that growth rates may be improved by feeding juvenile halibut more than 1 × day^?1.     

Interactions between temperature and size on the growth,size heterogeneity,mortality and cannibalism in cultured larvae and juveniles of the Asian catfish,Pangasianodon hypophthalmus (Sauvage)

Temperature effects vary throughout the ontogeny, but are proportionally more variable during the early life stages than in older fish. The larvae of a few species have been studied but contrasting trajectories were observed, thereby making it difficult to predict how temperature impacts on the growth, survival and size heterogeneity in a particular species. This study examined these interactions in young [0.9 mg–1 g wet mass (WM)] Pangasianodon hypophthalmus, one of the most extensively cultured tropical fish. Fish were raised at five temperatures from 23 to 33 °C in a recirculated water system, fed in excess with a high‐energy feed (Artemia then a formulated feed, >50% protein), and examined at 1‐day (first feeding days) or 4‐day intervals (older fish). The temperature that produced the fastest growth (T°_opt) was 31 °C at the start of exogenous feeding (0.9 mg); it increased to 32.7 °C at 8 mg then decreased by 0.7 °C for each 10‐fold increase of WM. Size heterogeneity was lower and survival was higher (70–85% from 0.9 mg to 1 g) when the temperature was close to T°_opt. Comparisons with other species suggest that the slope of the relationship between T°_opt and fish size has a strong latitudinal component, and may also reflect the fish sensitivity to oxygen.     

A modeling approach to evaluate growth and movement for recruitment success of Japanese sardine (Sardinops melanostictus) in the western Pacific

A two‐dimensional individual‐based fish movement model coupled with fish bioenergetics was developed to simulate the observed migration and growth of Japanese sardine (Sardinops melanostictus) in the western North Pacific. In the model, derived from the observed ocean–environmental data as the driving force, fish movement was adapted as a kinesis behavior. The model successfully simulated the observed transport patterns during the egg and larval stages and the northward migrations during the juvenile stage in 2005, 2006 and 2007. The model results showed that both temperature during the larval stage in the Kuroshio Extension and the prey availability during the early juvenile stage in the Kuroshio–Oyashio transitional area are important factors for growth of Japanese sardine. In autumn, the observed juvenile sardine were mainly distributed in the subarctic water region off the Kuril Islands, which is an area (158–165°E, 43–47°N) with a high chlorophyll‐a (Chl‐a) concentration. The model reproduced the fish distribution, which has a high density in this region. The high Chl‐a concentration area in autumn may contribute to increasing the survival rate of Japanese sardine by cascading up the food chain, from the high primary production, and is an important habitat for recruitment success of Japanese sardine.     

Growth compensation in juvenile tongue sole,Cynoglossus semilaevis (Güther, 1873): responses to thermal stress and feed restriction

Responses of compensatory growth to high temperature and feed restriction in juvenile tongue sole, Cynoglossus semilaevis (Güther, 1873), were investigated during a 56‐days experiment. Fish were divided into seven groups including three high‐temperature treatments which were reared at 28°C for 1, 2 and 3 weeks, respectively, then returned to 22°C (recorded as T1, T2 and T3), three feed‐restricted treatments which were fed 25% satiation for 1, 2 and 3 weeks, respectively, then fed ad libitum (recorded as R1, R2 and R3) and a control (continuously reared at 22°C). The results showed that juvenile tongue sole exposed to thermal stress for 1 week exhibited overcompensatory growth. However, complete compensation occurred in T2, T3 and R1 fish, while partial compensation occurred in R2 and R3 fish. The body composition and energy content of fish were not significantly different among all treatments (P > 0.05). The specific growth rate and feed efficiency of fish in T1 were significantly higher than other treatments during the whole experiment period (P < 0.05). Higher feed efficiency and apparent digestion rate during recovery might account for the compensatory growth in tongue sole in the present study. Hyperphagia played an important role in compensatory growth for feed‐restricted fish, but was not involved in the compensatory response in thermal stressed fish in the present study. The results from the present study suggested that a suitable thermal stress could be more feasible to elicit compensatory growth than feed restriction in juvenile tongue sole.     

The effect of different numbers of feeding days on feed consumption and growth of rainbow trout [Oncorhynchus mykiss (Walbaum)]

A trial was conducted to evaluate the effects of the number of feeding days on feed consumption and growth of rainbow trout, Oncorhynchus mykiss (Walbaum) (initial mean weight 53.6 ± 11.3 g). Fish were held under natural winter–spring (40°57′N) photoperiod (L:8 D:16 – L:14 D: 10) and ambient water temperatures (4–12 °C) for 128 days, and fed on day 7 (F₇), day 5 (F_5.1: no feeding on weekends and F_5.2: no feeding on Sundays and Wednesdays) or every other day (F_EOD). Feeding treatments were replicated with 50 fish held in 200‐L tanks. Number of feeding days affected final live weight, with fish in group F₇ achieving a mean weight of 285.6 g, followed by F_5.1 (256.6 g), F_5.2 (247.6 g) and F_EOD (223.2 g). Final condition factors (CF) differed between the F_EOD group and the others. Feed conversion ratios (FCR) exhibited significant differences among some of the groups. The results suggest that depriving rainbow trout of feed for more than 1 day a week has a negative effect on growth, but little influence on feed conversion.     

Feeding Larval Red Drum on Microparticulate Diets in a Closed Recirculating Water System1

A feeding protocol was developed for red drum larvae based on combining a commercial microparticulate diet (Kyowa Fry Feed) with live prey (rotifers) in a closed, water reuse system. In five trials, growth and survival were measured on larvae reared on a combination of live and microdiet for 1–5 d and then microdiet alone. Results in each trial were compared to control larvae reared on live rotifers Brachionus plicatilis and brine shrimp nauplii Artemia salinas. The most satisfactory combination was feeding live food and microdiet together for the first five days and then completely discontinuing live prey, eliminating the need to feed brine shrimp to the larvae. Growth rates of larvae fed progressively larger sizes of the microdiet were as good as larvae reared on live prey. Both groups metamorphosed to the juvenile stage at less than one month. Survival rates on the five day live food and microdiet combination were a remarkable 60% from egg to the juvenile stage. The successful weaning of red drum to microdiets paves the way to produce a semipurified diet to test nutrient requirements of larval fish.     

MyoD,myogenin and proliferating cell nuclear antigen expression in growing Nile tilapia (Oreochromis niloticus L.)

In the present study, immunohistochemical and morphometric analysis was used to characterize variations in muscle growth performance during muscle fibre recruitment and hypertrophy. As in fisheries, fish were classified into four age stages: alevin 35 days (0.65±0.08 g); juvenile 60 days (13.67±1.35 g); adult 90 days (73.18±4.70 g) and adult 190 days (349.76±34.62 g). The number of nuclei expressing MyoD and myogenin was similar in alevin, juvenile and adult 90 days; however, in adult 190 days, the number of nuclei expressing myogenin was higher than the number expressing MyoD. The number of proliferating cell nuclear antigen‐stained nuclei in each stage was higher than MyoD and myogenin staining with peaks in alevin and adult 90 days. These data suggest that growth per se stimulated cellular proliferation and nuclei accretion of Nile tilapia muscle fibres in alevin, juvenile and adult 90 days. Muscle fibre differentiation was more pronounced in adult 190 days.     

Larval development in European hake (Merluccius merluccius L.) reared in a semi-intensive culture system

Eggs of European hake (Merluccius merluccius L.) were stripped from fish caught at sea. Larvae were kept under semi‐intensive conditions at around 12°C. In addition, eggs were incubated in single wells at 9.2, 12.7 and 14.5°C, where hatching, development and survival were closely examined. During the larval stage, a total of 299 larvae were sampled to follow development and growth. In addition a small number of juveniles were sampled. Larvae hatched approximately 4 days after fertilization, and were 2.9 mm in total length (TL). At 6‐day post hatching (dph), the larvae were 4.1 mm (TL), the jaw apparatus was developed, and the larvae had started to feed. Most of the growth during the early larval period is restricted to the head, and there is almost no increase in length for the first 3–4 weeks post hatching. Teeth and pelvic fins appear at 25 dph. Development of unpaired fins at approximately 30 dph marks the start of the larval–juvenile transition. Weaning to formulated feed was accomplished 50 dph, when external morphology was similar to that of adult hake.     

Stepwise temperature regulation and its effect on growth,feeding and muscle growth patterns of juvenile Atlantic halibut (<Emphasis Type="Italic">Hippoglossus hippoglossus</Emphasis> L.)

To investigate the possible direct effect of a stepwise reduction in temperature with increasing size on growth, feeding parameters and muscle growth patterns of juvenile Atlantic halibut (Hippoglossus hippoglossus L.), 804 juvenile halibut (mean initial weight individuals: 14.2 g ± 0.2 SEM) were reared at constant 9, 12 and 15°C or shifted (T-step, i.e. 15–12°C after 36 days) for 99 days. Despite indications of lower optimal temperature for growth with increasing size, equal end weights were obtained between the constant 12°C, constant 15°C and T-step groups. Best overall growth was observed for the group kept at constant 12°C. The limited effect of the T-step group may relate to the size at movement (too big), the temperatures investigated (close to optimum) and the time and size interval investigated (too narrow). Differences in growth were reflected more by alterations in feed intake (C _T and F%) than by differences in feed conversion efficiencies (FCE). Differences were found with respect to the density of muscle cells, whereas no differences were found between the average muscle cell diameters. The mean diameter of muscle cells tended to increase only slightly with increasing fish weight, while the mean density of muscle cells tended to decrease. Using an optimum temperature of 12°C, an indication of a possible increased rate of hyperplasia in relation to higher growth was seen.     

Effect of diets on growth and reproductive development of juvenile pikeperch, Stizostedion lucioperca (L.), reared under intensive culture conditions

Juvenile pikeperch 3.66 ± 0.23 cm total length (TL) and 0.32 ± 0.05 g were reared in water at 22 ± 0.5^°C and fed with live zooplankton or alternatively a commercial trout feed. Fish fed with dry diet grew faster and had higher condition factor than fish fed live zooplankton. Sex differentiation in pikeperch was visible at different stages of ontogenesis, but was related to the fish growth rate. The gonads offish that were 3.43-5.69 cm TL revealed undifferentiated characteristics. Clear indication of oogenesis in females was observed in pikeperch at 7.9 cm TL and 3.47 g, whereas evident spermatogenesis was not observed till the end of the experiment.     

Effects of salinity on growth characteristics and osmoregulation of juvenile cobia,Rachycentron canadum (Linnaeus 1766), reared in potassium‐amended inland saline groundwater

The suitability of inland saline groundwater as a medium to culture juvenile cobia, Rachycentron canadum, was assessed. In the first experiment, juvenile cobia stocked in raw (unamended) saline groundwater at salinities of 5, 10, and 15 g/L exhibited complete mortality after 108, 176, and 195 hr, respectively. The second experiment evaluated the rearing of juvenile cobia (mean weight ~9.23 ± 0.12 g) in potassium (K⁺)‐amended saline groundwater (100% K⁺ fortified) and reconstituted seawater at salinities of 5, 10, and 15 g/L to assess growth and osmoregulation in distinct culture media. Following 60 days of culture, all fish survived the experimental period. Final mean bodyweight of cobia reared in K⁺‐amended saline groundwater (103.2–115.8 g) and seawater (111.2–113.8 g) of different salinities did not vary significantly (p > .05). No differences (p > .05) were observed in specific growth rate, weight gain (%), and feed conversion ratio between treatment groups. Serum osmolality increased with salinity and was significantly higher (p < .05) for fish in K⁺‐amended saline groundwater (353–361 mOsmol/Kg) than in reconstituted seawater (319–332 mOsmol/Kg), although differences were not observed between salinities by water type. Cobia stocked in saline groundwater of different salinities were osmoregulating normally, and the higher values observed may be because of variations in ionic composition and other interfering ions in saline groundwater. Trial results suggest that juvenile cobia can achieve optimal growth in K⁺‐amended saline groundwater of low and intermediate salinities.     

Effect of salinity and temperature on marine leech,Zeylanicobdella arugamensis (De Silva) under laboratory conditions

The high prevalence (80–100%) of the marine leech Zeylanicobdella arugamensis (De Silva) on cage‐cultured Asian sea bass Lates calcarifer (Bloch) led us to investigate the percentage of juvenile leeches hatched from deposited cocoons, survival of juvenile and adult marine leeches at different salinity and temperature. The results showed that the hatching percentage of juvenile leeches was highest at salinity of 30 ppt (32.5 ± 2.8%) followed by 20 ppt (18.0 ± 4.3%) and 10 ppt (12.1 ± 1.4%), respectively. It was found that the adult and juvenile leeches could live up to an average range of 4–7 days at salinity ranging from 10 to 40 ppt. The juvenile leeches were able to hatch at temperature ranging from 25 to 35 °C but unable to hatch at 40 °C. The survival period of adult and juvenile leeches ranged from 11 to 16 days at 25 °C, which was comparatively longer than 5–13 days and 10 h – 5 days at 27–30 °C and 35–40 °C, respectively. The study provided the information on the physical parameters of salinity and temperature which are most optimal for the marine leech Z. arugamensis to propagate.     

Effect of rearing temperatures on the growth and maturation of Arctic charr (Salvelinus alpinus) during juvenile and on‐growing periods

The effect of rearing temperature on the growth and maturation of Arctic charr (Salvelinus alpinus) was investigated. Arctic charr juveniles were reared for 6 months (phase I, October–April, size range 20–500 g) at constant temperatures of 9, 12 and 15 °C and according to two temperature‐step groups (T_step) i.e. fish transferred from 15 to 12 °C or from 12 to 9 °C. All the previous treatments were then reared either at 7 °C or at 12 °C for an additional 4 months (phase II, size range 300–1000 g) and then slaughtered in August 2008. The overall growth rate was the highest at a constant temperature of 15 °C for the first 6 months of the trial, with the fish in this group being 44% and 78% heavier than the fish reared at a constant temperature of 12 or 9 °C respectively. Arctic charr showed a negative response in terms of the growth rate when transferred from higher to lower temperatures, especially for groups previously reared at 15 °C. There was a trend for higher gonadosomatic index values at the end of the experiment for groups of fish that were exposed to higher rearing temperatures during the juvenile phase i.e. 4.18% (±0.79) and 7.29% (±0.89), for the temperature groups of 12 and 15 °C, respectively, compared with 2.49% (±0.74) for the 9 °C group. Our results suggest that for the production of fish >1000 g, moderate or low temperatures (here 9 °C) should be applied during the juvenile phase in order to reduce the negative effects arising from maturation. Farmers with access to heat sources should accordingly choose more moderate rearing temperatures during the juvenile stage, especially if the fish is to be moved down in the temperature regime during the on‐growing period.     

Fine tuning of feeding practices for hatchery produced Persian sturgeon,Acipenser persicus and Beluga sturgeon,Huso huso

In this work, we investigated the effects of various feeding treatments on the survival and growth of Huso huso and Acipenser persicus larvae during a 20‐day culture period. Three replicate groups (250 fish/replicate) of first‐feeding larvae were fed according to four main feeding regimes: (1) live food (live nauplii of brine shrimp Artemia urmiana); (2) indirect transition (5–7 days live food followed by gradual transition to formulated diet); (3) direct transition (using different combinations of live and formulated diet from the start feeding onwards); and (4) formulated feed (FD) from the start of feeding. In H. huso larvae, combining live food and manufactured diets (co‐feeding) from the first feeding stage onwards (direct transition) resulted in significantly higher weight gain than the other regimes. Survival was significantly higher in H. huso larvae fed solely live food or the direct transition regimes compared with indirect transition and FD. In A. persicus larvae, growth and survival were higher in the indirect transition feeding regime than in the other regimes. On the basis of the results of this study, we recommend co‐feeding of H. huso immediately from the commencement of exogenous feeding, but co‐feeding of A. persicus should start 7 days after prior feeding with live food.     

Effects of probiotic doses on the survival and growth of hatchlings,fry, and advanced fry of Rohu (Labeo rohita Hamilton)

ABSTRACT

Rohu (Labeo rohita) is the most popular fish in South Asia. Its farming is often constrained by shortages of seed due to low survival (20%–30%) during nursing. Hypothesizing that probiotics might help, varying doses of two multistrain commercial probiotics were tested at hatchling (days 8–38), fry (days 38–68), and advanced fry (days 68–98) stages. The first probiotics (P1) consisted of Bacillus subtilis, Pediococcus acidilactici, and yeast (Saccharomyces cerevisiae) and the second probiotics (P2) of Bifidobacterium and Lactobacilli plus yeast (Saccharomyces cerevisiae), microalgae (Spirulina sp.), and phytase. Low, medium, and high doses of P1 (1.0, 2.0, and 3.0 g/kg) and P2 (0.5, 1.0, and 2.0 g/kg) were tested, supplementing the control diet, which consisted of mustard oil cake (40%), rice bran (30%), wheat flour (20%), and fish powder (10%). At the hatchling stage results showed that medium and high doses of P1 increased survival by 14.4% and 16.2% respectively over the control. Similarly, medium and high doses of P2 increased survival by 22.1% and 22.3% respectively compared to the control. Survival of hatchlings and fry increased linearly (P < 0.01) with the increase in the doses of P1. The dose of P2 had a quadratic relationship with the survival based on which dose of 1.5 g and 1.7 g/kg feed was found to maximize the survival of hatchlings (72%) and fry (99%) respectively. Regression analysis also showed that maximum specific growth rate (SGR%/day) of fry could be obtained with the estimated dose of 3.3 g/kg diet for P1 and for 1.5 g/kg diet for P2. Therefore, tested probiotics are beneficial during the nursing of Rohu hatchlings and fry or before the age of 68 days but not for nursing afterwards.     

Culture of the mysid, Mysidopsis almyra (Bowman), (Crustacea: Mysidacea) in a static water system: effects of density and temperature on production, survival and growth

The effects of animal density and water temperature on the culture of the mysid, Mysidopsis almyra (Bowman), in a static water system were evaluated. An initial set of experiments tested the effects of mysid density on production. Densities of 25, 37.5, 50, 100 and 200 mysids L^–1 were placed in trays with 20 L of sea water. Temperatures were maintained at 26 ± 2 °C. A second set of experiments was conducted in the same system at three different temperatures (18 ± 1, 22 ± 1 and 26 ± 2 °C) using a mysid density of 50 mysids L^–1 (1000 mysids tray^–1). All experiments had a duration of 30 days. The mysids in all trials were cultured at 20 ± 2‰ salinity and fed Artemia nauplii enriched with marine fatty acids. There was a positive correlation between production and mysid densities up to populations of 100 mysids L^–1; maximum production was 273 ± 99 hatchlings day^–1. At a population density of 200 mysids L^–1, high mortality and low production were recorded 4 days after the start of the experiment. The experiments testing different temperatures showed that mysid production was higher at 22 ± 1 °C, although this result was not significant (P > 0.05). Growth rates and hatchling survival after 7 days were significantly higher (P < 0.05) at 26 ± 2 °C compared to survival and growth at 18 or 22 °C.     

Dietary nucleotides improve the growth performance,antioxidative capacity and intestinal morphology of turbot (Scophthalmus maximus)

A growth trial was conducted to evaluate the effects and safety of nucleotides in low fish meal diets on the growth performance, antioxidative capacity and intestinal morphology of turbot (Scophthalmus maximus). High fish meal control diet was formulated with 500 g kg^?1 fish meal. Seven levels (0.075, 0.15, 0.225, 0.300, 1.5 and 3.0 g kg^?1, respectively) of nucleotides were added to a low fish meal basal diet, which was formulated with 400 g kg^?1 fish meal. The eight experimental diets were fed to groups of juvenile turbot (initial weight: 6.0 ± 0.03 g) for 60 days. Results showed that compared with high fish meal control diet, low fish meal basal diet treatment had lower total antioxidative capacity (T‐AOC), glutathione peroxidase activity, fold height of proximal and distal intestine, enterocyte height of all evaluated enteric section and microvillus height of mid‐intestine and distal intestine (P < 0.05). However, supplemented nucleotides in diets could significantly improve growth (specific growth rate, SGR), feed utilization, antioxidative capacity and intestinal morphology of turbot (P < 0.05). Broken‐line regression analysis of SGR and T‐AOC showed that the optimal supplemental levels of dietary nucleotide for juvenile turbot were 0.366 and 0.188 g kg^?1, respectively. In summary, 0.300 g kg^?1 of dietary nucleotides was helpful in improving growth, feed utilization, antioxidative capacity and intestinal morphology of turbot fed with low fish meal diet. Excessive dietary nucleotides (3.0 g kg^?1) might cause oxidative stress and morphological damage in intestine and then reduce the growth of turbot.     

Dietary threonine requirement of fingerling Indian major carp,Labeo rohita (Hamilton)

An 8‐week feeding experiment was conducted in a water flow‐through system (26–28 °C) to determine the dietary threonine requirement of fingerling Labeo rohita (3.90±0.03 cm; 0.58±0.02 g). Growth, feed utilization and body composition of fish fed test diets (40% crude protein; 17.9 kJ g^?1 gross energy) with graded levels of l ‐threonine (0.75%, 1.0%, 1.25%, 1.50%, 1.75% and 2.0% dry diet) to apparent satiation were response variables used to assess threonine adequacy. Diets were made isonitrogenous and isoenergetic by adjusting the levels of glycine and dextrin. The amino acid profiles of the test diets were formulated to that of 40% whole chicken egg protein except for threonine. The performance of fish fed experimental diets was evaluated using calculated values for weight gain (g fish^?1), feed conversion ratio (FCR), protein efficiency ratio (PER) and protein productive value (PPV) data. Maximum weight gain (g fish^?1) (1.79), lowest FCR (1.39), highest PER (1.76) and PPV (0.33) were recorded at 1.50 g per 100 g dietary threonine. Statistical analysis of weight gain, FCR, PER and PPV data reflected significant differences (P<0.05) among treatments. Except for reduced growth performance in fish fed threonine‐deficient diets, no deficiency signs were noted. Weight gain, FCR, PER and PPV data were also analysed using second‐degree polynomial regression analysis to obtain a more accurate threonine requirement estimate, which was found, using each response variable, to be at 1.70, 1.63, 1.65 and 1.51 g per 100 g of dry diet, corresponding to 4.2, 4.07, 4.12 and 3.77 g per 100 g of dietary protein respectively. Based on the second‐degree polynomial regression analysis of the live weight gain, FCR, PER and PPV data, the optimum dietary level of threonine for fingerling L. rohita was found to be in the range of 1.51–1.70 g per 100 g of the dry diet, corresponding to 3.77–4.2 g per 100 g of dietary protein.