Pseudomonas chlororaphis strain JF3835 reduces mortality of juvenile perch, Perca fluviatilis L., caused by Aeromonas sobria

Motile aeromonad septicaemia caused by Aeromonas sobria is a cause of disease in farmed perch, Perca fluviatilis L., in Switzerland. We have evaluated the potential of a Pseudomonas chlororaphis isolate, obtained from perch intestine, to control A. sobria infection. Inoculation of juvenile perch with P. chlororaphis strain JF3835 prior to infection with A. sobria caused a reduction in A. sobria associated mortalities. Infection of perch with xylE -labelled P. chlororaphis indicated the bacterium is able to transiently colonize juvenile fish and fingerlings.     

New production strategy for silver perch (Bidyanus bidyanus); over‐wintering fingerlings in a tank‐based recirculating aquaculture system

Slow growth and losses to bird predation and infectious diseases in winter can compromise the profitability of silver perch farming. To evaluate over‐wintering silver perch (Bidyanus bidyanus) in a recirculating aquaculture system (RAS), fingerlings (38 g) were stocked in either cages in a pond at ambient temperatures (10–21 °C) or tanks in the RAS at elevated temperatures (19–25 °C) and cultured for 125 days. Mean survival (96%), final weight (146 g), specific growth rate (1.07% day^?1) and production rate (28.1 kg m^?3) of fish in the RAS were significantly higher than for fish over‐wintered in cages (77%, 73 g, 0.53% day^?1, 11.1 kg m^?3). Fish from both treatments were then reared in cages for a further 129 days. Final mean weight of fish originally over‐wintered in the RAS was 426 g, while fish over‐wintered in cages were only 273 g. To determine optimal stocking densities, fingerlings (11.8 g) were stocked at 500, 1000 or 1500 fish m^?3 in tanks in the RAS and cultured for 124 days. Survival was not affected, but growth was significantly slower and feed conversion ratio higher at 1500 fish m^?3 compared with 500 or 1000 fish m^?3. Results demonstrate that over‐wintering silver perch in an RAS can produce large fingerlings for grow‐out in early spring. This strategy could eliminate bird predation, reduce losses to diseases and shorten the overall culture period.     

Efficacy of Sarafloxacin Hydrochloride in Treating Naturally-Induced Edwardsiella ictaluri Infections in Channel Catfish,Ictalurus punctatus

Sarafin (sarafloxacin hydrochloride), a new antibacterial, was evaluated in the field on a naturally induced infection of Edwardsiella ictaluri in channel catfish, -Ictalurus punctatus. Healthy channel catfish (mean weight = 50 g) were stocked into nine cages at 200 fish per cage in a pond with an undergoing E. ictaluri infection. Seven days after stocking, dead fish were observed in the cages with clinical signs of enteric septicemia of channel catfish (ESC). After E. ictaluri was confirmed through isolative biochemical tests, medicated feed was applied for five consecutive days. During this period, fish in three control cages received a commercial 32% protein floating feed, three other cages of fish served as positive controls and were fed Romet, and three cages received the test feed with Sarafin. Both medicated feeds reduced the increase in cumulative percent mortality. In the control cages, cumulative percent mortalities continued to increase throughout the study period. Average daily mortality rates were significantly lower following both treatments of medicated feed, and treatments receiving Sarafin showed the greatest reduction in average daily mortality rates. Average daily mortality rates in the control did not change after the medicated feeding period. Toward the end of the study, temperatures reached 30°C, above the active range of ESC infections, and all mortalities ceased.     

Development of a feeding strategy for silver perch, Bidyanus bidyanus (Mitchell), based on restricted rations

To develop a feeding strategy for the Australian freshwater fish silver perch (Bidyanus bidyanus Mitchell), a series of eight experiments was done in 1 m³ cages in an aerated, earthen pond to determine the effects of feeding rate (% body weight) and feeding frequency (no. of feeds day^?1) on the growth and feed conversion ratio (FCR) of fingerlings and larger fish under ambient water temperatures over the range 13.8–30.6°C. Fish were fed extruded pellets of a silver perch diet containing 34% digestible protein and 14 MJ kg^?1 digestible energy. Commercial silver perch farmers were consulted about feeding practices for large fish (>500 g) and at water temperatures below 12°C, and winter feeding practices for other warmwater species were used to complete the strategy. In the feeding experiments, growth and FCR increased with increasing feeding rates to a level above which only FCR increased. Optimal feeding rates and frequencies were those which resulted in maximal growth, while minimizing effort (feeding frequency) and FCR. The highest feeding frequency required for maximal growth, including that of small fingerlings was twice (2 ×) daily, and the optimal feeding rates varied with water temperature and size of fish. The optimal daily regimes were: small fingerlings (initial mean weight, 2.0 g) 7.5% 2 × at a mean temperature of 23.3°C; fingerlings (14.9–27.7 g) 7.5% 2 × at 27.1°C, 5.0% 2 × at 23.7°C and 2.0% 1 × at 16.8°C; and large silver perch (162.5–510.6 g) 0.5% 1 × daily or 1.0% on alternate days at 15.6°C, 1.0% 1 × at 17.3°C, 3.0% 2 × at 24.1°C and 2.0% 2 × at 27.9°C. It is suggested that regimes of 0.5% 1 × daily for fingerlings (<50 g) and 0.5% 1 × on alternate days for larger fish are used at temperatures of 9–12°C, and 0.5% 3 days week^?1 and 0.5% 1 day week^?1 for fingerlings and larger fish, respectively, at 6–9°C. Feed inputs should not exceed 150 kg ha^?1 day^?1 in ponds less than 0.3 ha and 100 kg ha^?1 day^?1 in larger ponds. Our research has established a feeding strategy for silver perch based on restricted rations.     

Increasing Yields of Channel Catfish Using a Combination of Cage and Open Pond Production Systems1

Channel catfish (lctalurus punctatus) fingerlings stocked at a rate of 450 fish/0.04 ha pond were simultaneously cultured with fingerlings stocked in 1.25 m³ cages (0, 250, 350, or 450 fishlcage; one cage/pond). The fish in the cages were cultured and harvested for a 90–330 g (whole fish) market. The fish in the open ponds were cultured and harvested for a 490–1,140 g market. Harvest weights of open pond fish in all treatments were similar indicating that the presence of the caged fish and the associated higher daily pond feeding rates did not affect open pond production. Ninety-five to 99% of the caged fish and 96 to 98% of the open pond fish were of marketable size at harvest. Survival and food conversion ratios were similar among treatments. Results of this study indicate that total pond production can be increased (in this case up to 19%) by using a combination of open pond and cage techniques and by simultaneously producing fish for two markets.     

Survival, growth and feeding of Atlantic salmon, Salmo salar L., smolts after transfer to sea water in relation to the failed smolt syndrome

Abstract. An increase in mortalities, due to the failure of some smolts to feed after transfer to sea cages, has been observed in many fish farms during the last 2 years. In the present study mortalities were as high as 9–5% and the majority of these appeared 7–9 weeks post-transfer. Comparisons of length-weight relationships between the initial group of smolts released in the cage and at 3-, 6- and 8-wcek intervals after transfer showed that a proportion of the fish had lost weight. The condition factor of individually measured fish also confirmed that with increasing time after transfer some fish were in a much poorer condition.
The feeding of smolts after transfer was not related to their size. The fork length distribution of feeding and non-feeding fish showed that feeding fish were recruited from all size classes of the initial population.
The lack of appetite in smolts after transfer was demonstrated by the reduced number of fish feeding and by the amount of food consumed. In the first week after transfer only 10% of the fish were feeding in the sea cage, increasing to 65% after 5 weeks. In the tanks (where there were no failed smolts) 95% of the smolts were feeding within 5 weeks after transfer, whereas in the sea cage it took 8 weeks before this figure was reached.     

Effect of Density at Harvest on the Growth Performance and Profitability of Hatchery‐reared Spotted Rose Snapper,Lutjanus guttatus,Cultured in Floating Net Cages

This study evaluated the effect of the density at harvest on the performance and profitability of hatchery‐reared spotted rose snapper cultured in cages. The fish were stocked at harvest densities of 15, 20, and 22 kg/m³ in cages of 222 and 286 m³. More than 39,000 snapper fingerlings with an initial weight of 14 g were stocked. The fish were fed an extruded diet and cultured over a 360 d period. The thermal growth coefficient ranged from 0.04 to 0.05 and survival was 95% for all treatments, with the highest final weight (436.8 g) observed for fish reared at a density of 20 kg/m³. The allometric value b indicated that hatchery‐raised, cage‐cultured snapper were heavier than their wild counterparts. The major costs were feed (ranging from 44.7–45.9%), labor (22.4–32.6%), and seed costs (20.2–26.1%). The total production cost ranged from US$ 6.5 to US$ 7.5/kg. The baseline scenario was not economically feasible. However, a 10% increase in the sales price resulted in increases in the internal rate of return (183%) and net present value (US$ 97,628.9). These results suggest that L. guttatus has the potential for commercial production in cages.     

Temporary Sequestration of Fingerling Channel Catfish in Cages as a Means to Improve Production of Multiple-Crop Ponds

Abstract

Two studies were conducted to determine if temporary sequestration of fingerling channel catfish, Ictalurus punctatus, in cages improves production of multiple-crop ponds. In the first study, 0.04-ha ponds were stocked with 295 large (mean weight = 566 g) and 780 fingerling (mean weight = 21 g) catfish. Fingerlings were stocked into cages or open ponds. At 120 days after stocking, fish in cages were released. After an additional 40 days, ponds were clean-harvested to remove large (>500 g) and small (<500 g) fish. In the second study, ponds (0.08-ha) were stocked with 750 large (mean weight = 46 g) and 750 small (mean weight = 20 g) fingerling catfish. Small fingerlings were stocked into cages or open ponds. At 60 days after stocking, fish in cages were released. Market-ready fish were selectively harvested at five and nine months after stocking. Results from the first study indicated that individual weight, weight gain, and yield of both size classes of fish raised in sequestered ponds were significantly greater (P< 0.05) than that of fish raised in open ponds. In addition, total weight gain and yield of fish in sequestered ponds was significantly greater (P< 0.05) than those in non-sequestered ponds. Results from the second study indicated that a significantly greater (P< 0.05) number of market-ready fish were harvested from sequestered ponds than from non-sequestered ponds. Mean yield of sequestered ponds was 31% greater than that of non-sequestered ponds; however, differences were not significant (P> 0.05). Amount of feed fed to fish raised in sequestered ponds was significantly greater (P< 0.05) than amount of feed fed to fish raised in non-sequestered ponds in both studies. It is unclear which factor or factors were responsible for the enhanced production of sequestered ponds; however, temporary sequestration may reduce agonistic behavior and competition for feed between fish size groups.     

High incidence of death due to collision of hatchery-reared Pacific bluefin tuna Thunnus orientalis juveniles in sea cages, as revealed by head and spinal injuries

High mortality of hatchery-reared Pacific bluefin tuna (PBT) juveniles is frequently observed in sea cages after transfer from indoor tanks, and is likely due to collisions of the fish with the net wall of the sea cages. Because collisions frequently cause bone injuries due to physical impact, such injuries can be considered an index of obvious collisions. In the present study, bone injuries of dead juveniles were investigated in order to examine the prevalence of net collision as a cause of death among hatchery-reared PBT juveniles in sea cages. Juvenile PBT were reared in sea cages for 90 days after transfer, and dead fish were collected and examined by the clearing and staining method or by dissection to detect injury to the vertebral column and parasphenoid. The prevalence of injuries to the vertebral column and parasphenoid until day 30 after transfer was low (0.0–12.0 %) when the fish were 5.5–15.2 cm in total length (TL). However, it drastically increased to 17.8–78.0 % for fish 21.0–39.2 cm TL from day 31 onward after transfer. These results suggest that the mortalities of juvenile PBT larger than 20 cm TL in cages were induced by net collisions.     

Growth of Tilapia aurea in Seawater Cages

Fingerling Tilapia aurea were reared for 90 days in three 1.0 m³ floating cages in seawater (36 ppt) at Lee Stocking Island, Bahamas. Fish stocking density (100, 200 and 400 fish/m³) apparently did not affect growth rate but it appears salinity inhibited growth. Daily weight gain and specific growth rate (G) averaged 0.34 g/day and l.08%/day, respectively, for Tilapia aurea fingerlings. Infection of the Tilapia aurea by Bacillus sp. was associated with a significant number of mortalities. The relatively low growth rate and the high incidence of disease and mortality of Tilapita aures in seawater indicate that it may not be a good candidate for cage culture in full-strength seawater.     

The Toxicity of Hydrogen Peroxide to Rainbow Trout Oncorhynchus mykiss and Cutthroat Trout Oncorhynchus clarki Fry and Fingerlings

Fungal and parasitic infections of fish can significantly impact the survival of cultured fish. Formalin is currently used to control such infections; however, concern has arisen over its safety to users and to the environment. Hydrogen peroxide has been designated as a low priority fungicidal drug by the United States Food and Drug Administration (FDA), yet, little information is available on treatment concentrations or its toxicity to trout. Rainbow trout Oncorhynchus mykiss and cutthroat trout Oncorhynchus clarki fry and Angerlings were exposed to hydrogen peroxide concentrations of 0, 70, 170, 280, 420 and 540 ppm for 30, 60 or 120 rnin at 15 C to determine the chemical's toxicity. Rainbow trout fry and fingerlings experienced elevated mortalities (>20%) during treatments using 420 and 540 ppm for 30 min; 280, 420 and 540 ppm for 60 min; and >170 ppm for 120 min. Cutthroat trout fry experienced elevated mortalities (>23%) during treatments using 540 ppm for 30 min; 420 and 540 ppm for 60 min; and >170 ppm for 120 min. Cutthroat trout fingerlings experienced elevated mortalities (>60%) during treatments using 540 ppm for 60 min and >280 ppm for 120 min. No control mortalities were encountered for both life stages of either species. The lethal concentrations (LC₅₀) of both age classes and species for each of the three durations ranged from 514–636 ppm for 30 min treatments, 322–506 ppm for 60 min treatments, and 189–280 for 120 min treatments. Mortalities for all four toxicity tests which occurred during a 96-h post-treatment period were centered around the following treatments: 30 min, 540 ppm; 60 min, 280–540 ppm; 120 min, 170 ppm. Tissue damage to gills was found only among fish that did not survive the initial chemical exposure. Test concentrations proved to be relatively stable during a 24-h period, retaining better than 85% of their original strength for all five dilutions. At a water temperature of 15 C concentrations should not exceed 280 ppm for a 30-min treatment.     

Influence of pond fertilization and feeding rate on growth performance, economic returns and water quality in a small-scale cage-cum-pond integrated system for production of Nile tilapia (Oreochromis niloticus L.)

The effects of pond fertilization and feeding rate on growth, economic returns and water quality were investigated to develop a low‐cost cage‐cum‐pond integrated system for production of Oreochromis niloticus (L.). Hand‐sexed male fingerlings averaging 19±0.39 and 32±0.69 g were stocked in cages and open ponds at 150 fish cage^?1 and 2 fish m^?2 respectively. Fish were cultured for 114 days in five triplicate treatments. Cages were installed into ponds and caged fish were fed a 24% protein diet at 3% (T1) and 6% (T2) body weight day^?1 (BWD) without pond fertilization, and 6% BWD with pond fertilization (T3). The open water in the fourth treatment (T4) was not stocked but contained caged fish, which were fed 6% BWD for the first 57 days followed by 3% BWD for the remaining period. Ponds in the control (T5) had no cages and were neither fertilized nor open‐pond fish fed. Feeding rate and pond fertilization significantly (P<0.05) affected fish growth, profitability and water quality among treatments. Fish growth, feed utilization, fish yield, water quality and profits were significantly (P<0.05) better in T3 than the other treatments. It was concluded that fish production and economic returns were optimized at 6% BWD in fertilized ponds.     

Molecular evidence for the lack of transmission of the monogenean Sparicotyle chrysophrii (Monogenea, Polyopisthocotylea) and isopod Ceratothoa oestroides (Crustacea, Cymothoidae) between wild bogue (Boops boops) and cage-reared sea bream (Sparus aurata) and sea bass (Dicentrarchus labrax)

In the Mediterranean area, wild fish have often been suggested as either the reservoirs of the causative agents or at least the carriers of the pathogens responsible for disease outbreaks in cultured fish. However, no epidemiological investigations on actual pathogen/disease interactions between farmed and wild fish have been conducted even for the most important fish pathogens. Only sporadic isolations and identifications of various pathogens in wild fish have been done and real associations with the pathological conditions that exist within the farm environment and vice versa have not been established. Monogenean ectoparasite Sparicotyle chrysophrii and isopod Ceratothoa oestroides are commonly found in the Mediterranean cage-reared sea bream and sea bass and in the surrounding wild fish population. Both species were recognized as pathogens that seasonally inflict serious losses in fingerlings and juveniles of sea bream and sea bass, being potentially propagated and exchanged during wild and caged fish interaction. In order to evaluate the degree of pathogens transfer between wild and farmed fish, we investigated genetic population structures of these two important parasitic pathogens inferred by mtDNA cytochrome oxidase I locus. Parasites isolated from wild and farmed fish on the two most productive Adriatic fish farms showed genetic heterogeneity, contradicting widely accepted hypothesis of cross-contamination, at least in case of S. chrysophrii and C. oestroides. As far as we know, this is the first report that in a study of pathogen transfer molecular evidence was employed to asses the genetic population structure of shared parasites.     

Effects of three dietary protein levels on the growth of pacu, Colossoma mitrei Berg, in cages

Abstract. Artificially reproduced pacu, Colossoma mitrei Berg, fingerlings (initial weight 41 g) were stocked (100/m³) in plastic cages (1·0m³) within a freshwater reservoir. Three diets containing 30, 35 and 40% protein were tested in quadruplicate over a 76-day experimental feeding period. Fish were fed 7 days a week, twice daily at a feeding regime of 4% body weight/day from stock to 100g and thereafter at 3% until harvest. Mean water temperature over the trial was 26·7°C. Significant differences were observed in total and daily gain and in protein efficiency ratio, while no differences were obtained in the feed conversion ratio between treatments. Based on this study a 35% protein ration is recommended for pacu within floating cages.     

Monitoring of oxygen fluctuations in seabass cages (Dicentrarchus labrax L.) in a commercial fish farm in Greece

Dissolved oxygen and temperature were monitored in cages stocked seabass at a commercial fish farm in Greece during summer and autumn. During the first part of the study, October–November 2012, current velocity monitoring outside the cages supported the sampling program. The latter sampling took place during May–July 2014. Including both periods, temperature fluctuated within the range 18–26°C while the current velocity, dominated by the tidal flow, varied from 0 to 28 cm/s in autumn. During sampling, seabass of 400–550 g individual weight was stocked at a density of 10–15 kg/m³. Within the cages, dissolved oxygen (DO) saturation decreased by 10% to 50% compared with outside levels. In‐cage oxygen concentrations seemed to be affected by current speed, temperature and diurnal fluctuations due to the fish stock's activity and the day‐night rhythm of algae. Hypoxia within the range 40%–70% of oxygen saturation was shown in several cases, despite the fact that the feeding intensity was quite low in the farm. The diurnal pattern of DO saturation showed that minimum levels coincided with feeding of the fish, while photosynthesis played a minor role in this case. This level of DO saturation although does not result in any mortalities may have other severe implications, especially at high temperature, such as lower feed utilization, increased metabolic expenses due to osmo‐respiratory compromise, increased stress level, lower disease resistance and diminished fish welfare.     

Capture depth related mortality of discarded snapper (Pagrus auratus) and implications for management

Variables affecting the short-term survival of snapper (Pagrus auratus) captured using commercial fish traps and subsequently released were investigated by holding the fish in cages. A logistic regression model showed that capture depth had the greatest affect on short-term survival of snapper, with no mortalities observed from depths of less than 21 m and 2% from depths of less than 30 m. Mortality of snapper increased rapidly after 30 m and was 39% between capture depths of 30 and 44 m and 55% between capture depths of 45 and 59 m. Survival was also effected by fish length, with smaller fish being more likely to die. The rate of ascent of captured snapper and the density of fish in cages were kept reasonably constant and did not appear to affect survival. The number of snapper swimming upside-down prior to being returned to the sea floor in cages was not a good predictor of mortality. Future studies that use cages to assess discard mortality rates would benefit from underwater video observations of fish behaviour. The results demonstrate that the discard mortality of snapper should be considered when managing the fishery in New South Wales, Australia.     

Challenge studies of European stocks of redfin perch, Perca fluviatilis L., and rainbow trout, Oncorhynchus mykiss (Walbaum), with epizootic haematopoietic necrosis virus

A challenge model for comparison of the virulence of epizootic haematopoietic necrosis virus (EHNV) to European stocks of redfin perch, Perca fluviatilis L., and rainbow trout, Oncorhynchus mykiss (Walbaum), was tested. The model investigated intraperitoneal (IP), bath and cohabitation routes at 10, 15 and 20 °C for 5–6 g fish and 15 °C for 20 g perch. In the IP challenges of perch, significant mortality occurred at 15 °C and 20 °C. In challenge trials for rainbow trout, significant mortalities were observed in IP and bath challenges at 20 °C. The mortality observed in IP challenged 20 g perch was not significantly different from that recorded for 6 g fish challenged IP. No significant mortality was observed in any other treatment groups. Re-isolation of ranavirus was confirmed by IFAT and was consistently associated with dead or moribund fish in the trial groups challenged with EHNV. The findings indicate that EHNV does not pose a high risk for wild perch and trout populations in Europe by natural exposure. Mortality appears to be primarily a function of environmental factors, with temperature playing an important role, and not just the presence of the virus in the fish.     

Development of Aquaculture Methods for Southern Flounder,Paralichthys lethostigma

ABSTRACT

A detailed spawning and larval rearing protocol for the production of southern flounder, Paralichthys lethostigma, is presented. The protocol is based on the results of spawning, larval culture and fingerling production trials with the southern flounder, which were completed during 1998 and 1999. Seventy-six brood fish (0.1-3.8 kg) were collected from the wild, acclimated to captivity, and stocked in matura tion tanks. Sex ratio (male:female) was 1.1:1. Males (0.1-1.0 kg; mean = 0.5 kg±0.30 SD) were significantly smaller (P > 000.1) than females (0.5-3.8 kg; mean =1.8 kg±0.75 SD). Spermatogenesis was achieved using temperature manipulation during a 3-month period. Vitellogenesis was nearly completed when most females were caught, and final ovulation was accomplished using gonadotropin releasing hormone-analogue (GnRH-a) implants and human chorionic gonadotropin (HCG). Eggs were obtained from both tank and strip spawnings. At 24-26°C, fertilized eggs hatched within 24-28 hours. Weaning to artificial diets was completed after 5 weeks, when metamorphosis was completed. Epizootics of Vibriosis and Amyloodinium ocellatum caused massive mortalities. Average survival rate from eggs to fully metamorphosed fingerlings was 30%, ranging from 5-40%. A total of 14,562 juveniles measuring 5 cm were produced in these experimental trials. Approximately 25% of the fingerlings exhibited pigment abnormalities known as hypomelanosis or pseudo-albinism on the dark side and hypermelanosis or ambicoloration on the blind side. Reversed asymmetry was observed in 5% of the fingerlings and a few individuals (0.02%) exhibited no ocular migration at all. A large size variation was observed during early larval development stages, with larger individuals growing faster through post-larval, juvenile, and adult stages. Results suggest that hatchery production of southern flounder fingerlings should rely on early culling of runts.     

Short-term mortality of Australian bass, Macquaria novemaculeata, after catch-and-release angling

Abstract  Australian bass, Macquaria novemaculeata Steindachner, is angled in large numbers from coastal impoundments and rivers throughout south-eastern Australia. Many are released in the belief that most survive with few negative short-term impacts. This assumption was investigated in three experiments involving a range of conventional angling scenarios. A total of 313 Australian bass were angled and then released along with appropriate controls into either land-based tanks or floating cages, where they were monitored for up to 5 days. No controls died. Short-term mortalities of angled fish were low (0–6%) and attributed to the effects of bait type, hook location and fish size ( P  <   0.05). Specifically, fish that ingested hooks or were caught with natural baits were more likely to die than those that were mouth-hooked or caught on lures. Mortality negatively correlated with fish size. The results supported previous assertions that Australian bass is a resilient, hardy species that should tolerate short-term impacts associated with catch-and-release angling. However, other sublethal physiological impacts that could contribute towards delayed mortalities or reduced reproductive output warrant further investigation, particularly in relation to native populations in coastal rivers that form gauntlet fisheries during their annual spawning season.     

Effects of dietary protein levels on the growth,body composition,serum biochemistry and digestive enzyme activity in Chinese rice field eel (Monopterus albus) fingerlings

A 12‐week feeding trial was conducted to determine the influence of dietary protein levels on growth performance, body composition, haematological parameters and digestive enzyme activity of Chinese rice field eel (Monopterus albus) fingerlings. Six isolipidic experimental diets (27.89%, 32.23%, 35.95%, 40.06%, 43.31% and 46.53%) were formulated to randomly assign to triplicate groups of 50 rice field eel fingerlings (5.0 ± 0.5 g) cultured in plastic cages. Results indicated that the best growth performance of fish with the highest WG (158.68 ± 31.59%) and SGR (2.28 ± 0.21%) was obtained at 40.06% group. Significant alterations were demonstrated on whole‐body crude protein, crude lipid and amino acid profile by different dietary protein levels. Besides, haematological parameters including triacylglycerol, aspartate aminotransferase and alkaline phosphatase decreased with various levels by the increasing of dietary protein levels. The lowest pepsin activity was observed in 27.89% protein group, which suggested lower availability of protein as substrate for protease activity. Meanwhile, the highest amylase activity in fish of 27.89% group indicated higher carbohydrate level in this group. Based on the polynomial regression analysis of WG and SGR, the dietary protein requirement for fingerlings of Monopterus albus was estimated to be 386.4–390.8 g/kg, respectively, with the lipid level of 10.08%.