Improvement of the survival in the seven-band grouper Epinephelus septemfasciatus larvae by optimizing aeration and water inlet in the mass-scale rearing tank

The water flow in larval rearing tanks has been indicated to cause mass mortality of the seven-band grouper Epinephelus septemfasciatus larvae. Therefore, a new aerating method was tested in an actual scale intensive rearing tank (8.0 m in diameter, 1.87 m of water depth, 100 m³ of volume), in which an aerator was positioned at the center of the rearing tank surrounding cylindrical drain (1.2 m in diameter) to generate the flow field, and seven larval rearing trials were performed. The survival rate with the former aeration methods were compared, in which several aerators were located in the rearing tank. The survival rate at 10 days after hatching with the new aeration method (61.5±5.1%, n=7) was approximately three times higher than the former methods (21.2±13.7%, n=6). The flow environment of rearing tanks was also examined by quantifying the flow field, and the relationship between the flow field in the rearing tank, behavior of larvae and survival discussed. It was confirmed that the vertical circulating flow was observed in rearing tanks, and determined effectively the survival and the behavior of grouper larvae in patchiness.     

Effect of tank proportions on survival of seven-band grouper Epinephelus septemfasciatus (Thunberg) and devil stinger Inimicus japonicus (Cuvier) larvae

We examined the effects of rearing‐tank proportions on early survival, surface death and growth of the seven‐band grouper Epinephelus septemfasciatus (Thunberg) and the devil stinger Inimicus japonicus (Cuvier). Fertilized eggs were introduced into three differently shaped 100 L rearing tanks. The three tanks had different water surface areas, and included a shallow tank (S; 71 × 26 cm in diameter and depth, respectively), an intermediate tank (I; 57 × 39 cm) and a deep tank (D; 44 × 70 cm). Both species showed their highest survival rate and the lowest numbers of surface death in the D tank (P<0.05). There were no significant differences between fish reared in the three tank shapes in notochord length, total length, growth rate and dry weight. Rearing‐tank shape affected larval movement in the water column, with the duration of larval movement under the water surface being the shortest in the D tank. These results suggest that using a rearing tank of a suitable shape could significantly reduce the surface death of marine fish larvae.     

Screening of enzyme activity for assessing the condition of larvae in the seven-band grouper <Emphasis Type="Italic">Epinephelus septemfasciatus</Emphasis> and devil stinger <Emphasis Type="Italic">Inimicus japonicus</Emphasis>

We conducted screening tests to determine whether enzyme activity is a suitable biomarker for assessing the physiological condition of marine fish larvae. The rearing experiments consisted of three trials, of which two were conducted using the seven-band grouper Epinephelus septemfasciatus for a period of 5 days after hatching (DAH), and one was conducted using the devil stinger Inimicus japonicus for 10 DAH. The trials were conducted under three different rearing-tank environments (shallow tank, intermediate tank, deep tank) in a water volume of 100 l and an aeration rate of 50 ml/min. We determined survival, surface death, growth, and enzyme activities (trypsin, esterase, and alkaline phosphatase). The highest survival rates and lowest surface deaths in both species occurred among the larvae grown in the deep tank. There was a significant and negative correlation between survival at 5 DAH and alkaline phosphatase activity at 0 DAH in the seven-band grouper. The same correlation was found between survival at 10 DAH and trypsin and alkaline phosphatase activity at 1 DAH in the devil stinger. Based on these results, we conclude that the activity of a specific enzyme is a candidate for assessing the physiological condition of marine fish larvae.     

Observations of flow patterns in a model of a marine fish larvae rearing tank

There are few studies on flow fields generated by aerators in fish larvae rearing tanks. The flow varies with aeration rates and tank proportions resulting in different larvae survival rates. The effects of aeration rate and aspect ratio AR (the ratio of liquid depth to tank radius) on overall flow patterns were investigated experimentally using flow visualization techniques. Two distinct types of flow patterns, a single-pair vortex system and a two-pair vortex system, were observed as the value of tank aspect ratio varied from about 1.0 to 2.0. In addition, corner vortex structures were observed in both the region between the free surface and the upper sidewall, and between the bottom wall and the lower sidewall of the tank. On the sidewall, reattachment and separation points were found, which were closely related with these vortex structures.     

Swim bladder inflation and survival of Mugil cephalus to 50 days

Excessive inflation of the swim bladder causes immobility and mortality of 12-day old larvae of Mugil cephalus and other marine fish. Increased aeration or mechanical agitation of the water in rearing tanks maintained the larvae below the water surface, and the swim bladder formed and functioned normally. Survival was dramatically increased. Two experimental production tanks, stocked initially with 15 000 and 20 000 larvae, at a density of 5.25 and 7.0/l, respectively, yielded 33.5% and 16.9% juveniles on day 50. Only an outbreak of ciliated parasites prevented both tanks from having survival figures of over 30%.     

七带石斑鱼繁殖生物学及人工繁育技术研究进展

石斑鱼是我国南方海水养殖的主要种类之一,而大部分石斑鱼种类均为暖水性鱼类,养殖水温要求在15℃以上,限制了石斑鱼在北方的发展。七带石斑鱼是唯一一种可以在较低温下生活,被人们称为“冷水石斑”,适合在北方养殖,如今已成为中、日、韩三国海水鱼类繁育研究的热点。由于七带石斑鱼人工繁殖和苗种培育过程中有许多制约条件,其人工繁育仍未达到稳定大批量生产的规模。主要问题有以下几个：①雄性亲鱼难获得,数量较少,限制了苗种的大量繁育;②初孵仔鱼的个体弱小、口径小,对开口饵料要求严格,适口饵料极少;③稚鱼期鱼苗互相残杀极其严重,出现大鱼吃小鱼的现象。此外,还有卵质不良,仔稚幼鱼疾病等难题。本文简要概述了国内外七带石斑鱼繁殖生物学与人工繁育研究进展情况,并对其养殖过程中出现的问题进行探讨和总结,为今后开展七带石斑鱼规模化人工养殖研究提供参考。     

Effect of water surface condition on survival,growth and swim bladder inflation of yellowfin tuna,Thunnus albacares (Temminck and Schlegel), larvae

Early‐stage mortality due to surface water tension‐related death and due to sinking to the tank bottom was investigated for yellowfin tuna, Thunnus albacares (YFT), larvae. Different aeration rates and rearing water surface conditions were examined to evaluate the effect on larval survival, swim bladder inflation and growth. The percentage survival of yolk sac larvae was significantly higher when the rearing water surface was covered with fish oil at aeration rates of 0 and 50 mL min^?1. The highest mortality occurred at the highest aeration rate of 250 mL min^?1 regardless of surface water condition. A second experiment was conducted twice under different water surface conditions: the water surface was covered by fish oil (FO), skimmed of fish oil (SS), and was not treated (NC). The percentage survival was not significantly different between treatments after 7 days of feeding. In contrast to the survival, the proportion of larvae with inflated swim bladders was significantly higher for the NC and SS groups than that of the FO group. Results of these experiments indicate that the addition of oil to the rearing water surface without its removal interferes with the initial swim bladder inflation in YFT larvae. These results also indicate that YFT larvae need to obtain (gulp) air at the water surface for initial swim bladder inflation, and success of initial swim bladder inflation may be crucial for their survival.     

Effects of hydrodynamic factors on Pecten maximus larval development

Hatchery production of great scallop, Pecten maximus, remains unpredictable, notably due to poor larval survival. Large‐scale flow‐through systems up to 3500 L have been developed to avoid the use of antibiotics in static systems. Alternatively, small‐scale flow‐through systems have been successfully applied for oysters but they proved to be unsuitable to rear scallop larvae. By focusing on physical factors presumed to limit P. maximus larval development, this study aimed to optimize great scallop larvae rearing parameters under controlled conditions. First, the influence of aeration on larval performances, energetic metabolism and antioxidant defences were studied both in static and flow‐through systems. Aeration depressed larval food intake, regardless of the intensities of flow tested (100 ml/min, 155 ml/min and 270 ml/min). On the other hand, antioxidant enzyme activities remained constant or decreased, suggesting that antioxidant defences were inactivated. The increase in citrate synthase activity suggested an increase in metabolic rate possibly due to a turbulent stressful environment. All larvae exposed to such turbulence died before reaching metamorphosis, whereas those reared without aeration survived well (≈ 95%). The effects of water renewal were thereafter studied in 50‐L flow‐through flat‐bottomed tanks. No differences in survival (20.4 ± 0.5%), growth (3.8 ± 0.2 μm/d), competence (5.6 ± 0.2%), energetic metabolism level and antioxidant enzyme activities were observed when comparing 12.5 and 25 L/hr water renewal. Whereas air bubbling leads to detrimental effects, flow‐through in small flat‐bottomed tanks appears to be a suitable technique for scallop larvae rearing.     

Ceramic clay reduces the load of organic matter and bacteria in marine fish larval culture tanks

Ceramic clay has been increasingly used to improve contrast and prey detection in tanks for rearing of fish larvae. In contrast to live microalgae or algae pastes, clay increases turbidity without contributing to the organic matter load. In addition, clay may aggregate and sediment organic matter and bacteria, facilitating its removal. Marine larvae are sensitive to infections by opportunistic bacteria. Fish, algae, and live feed increase the microbial carrying capacity of the rearing water which allow exponential growth of bacteria and favor fast-growing opportunists. Reducing substrate levels by replacing microalgae with clay may reduce bacteria proliferation and benefit larvae. We compared the effects of three rearing regimes including live Isochrysis galbana, Nannochloropsis oculata paste, and ceramic clay on the bacterial community, concentration of organic matter, and growth and survival of Atlantic cod larvae (Gadus morhua L.). The application of clay resulted in reduced substrate levels for bacteria in the rearing water compared to the addition of live algae or algae paste. To some extent, clay aggregated and transported organic matter to the bottom of the larval fish tanks, where it could be effectively removed. Fish tanks receiving clay showed a lower abundance of bacteria in the water than tanks added algae paste or live algae. Fish tanks with algae paste showed a higher abundance of bacteria and a higher share of cultivable bacteria and TCBS counts than the other two treatments. Tanks with live algae showed low relative abundances of opportunistic bacteria and TCBS counts in both water and rotifers. Cod larvae in tanks with clay or live algae initiated exponential growth earlier than larvae in tanks with algae paste. Larvae in tanks receiving clay had significantly higher dry weight than larvae in tanks receiving algae paste at day 5 and 20 post hatching. The survival of larvae in the tanks added clay was variable. Two of the three tanks with clay had significantly higher larval survival than the tanks with live algae or algae paste. However, one tank with clay underwent 100% mortality. It is not possible to conclude whether this was related to the use of clay or an incidental development of a harmful microbial community in this tank. The effects of clay addition on larval performance should be studied further. Clay addition appears to be an easy way to reduce bacterial load during early first feeding of marine larvae without compromising the beneficial effects of turbidity.     

Aeration rate adjustment at night to prevent sinking syndrome‐related death in the tiger grouper Epinephelus fuscoguttatus (Perciformes:Serranidae) larvae

The effects of different aeration rates at night to prevent sinking syndrome‐related death (SSRD) of the tiger grouper, Epinephelus fuscoguttatus were examined. The aeration rates were fixed at 300 mL min^?1 at daytime (07:00–19:00 hours) and regulated to 0, 300 and 900 mL min^?1 at night (19:00–07:00 hours). Larval survival, growth, feeding intake, sinking velocity, distribution and behaviour, stress level, surface tension‐related death (STRD) and flow velocity distribution were assessed. The occurrence of SSRD in the tiger grouper was observed through the accelerated sinking velocity (Vl) (from 0.15 ± 0.09 cm s^?1 at 4 days AH to 0.41 ± 0.09 cm s^?1 at 12 days AH) coupled with larval passive swimming behaviour at night‐time. On the final day of experiment (15 days AH), larvae reared in 900 mL min^?1 at night had attained significantly higher (P < 0.05) survival (34.4 ± 5.5%), growth (5.8 ± 0.5 mm) and feeding intake (60.46 ± 6.98 ind. larva^?1). A favourable flow field for the tiger grouper was produced in 900 mL min^?1 at night‐time, in which larvae were transported 15–25 cm above the tank bottom and 1.0 cm beneath the water surface. Under these night‐time rearing conditions, larval stress level and number of STRD reared in 900 mL min^?1 compared with those observed in 300 mL min^?1 remained insignificant, indicating that strong turbulence of flow velocity was not detrimental for larvae. Our findings recommend aeration at 900 mL min^?1 at night as this could improve larval survival by reducing SSRD.     

Effects of salinity, aeration and light intensity on oil globule absorption, feeding incidence, growth and survival of early-stage grouper Epinephelus coioides larvae

ABSTRACT: A series of experiments were conducted to examine the effects of salinity, aeration and light intensity on oil globule absorption, feeding incidence, and growth and survival of early-stage Epinephelus coioides larvae. Newly hatched larvae were transferred to 40-L aquaria at a density of 1500 individuals/aquarium. Larvae were exposed to different levels of aeration (0 mL/min per L, 0.62 mL/min per L, 1.25 mL/min per L, 2.50 mL/min per L, or 3.75 mL/min per L); salinity (8 ppt, 16 ppt, 24 ppt, 32 ppt, or 40 ppt); and light intensity (0 lx, 120 lx, 230 lx, 500 lx, or 700 lx) for 4–6 days. Twenty larvae were sampled daily at 11:00 hours to measure for total length (TL), oil globule volume, and feeding incidence. Survival rates were determined by counting the total number of larvae remaining in each aquarium at the end of the experiment. Significantly higher survival rates ( P   <  0.05) were observed at aeration levels of 0.62 mL/min per L and 1.25 mL/min per L, at salinity levels of 16 ppt and 24 ppt, and at light intensities of 500 lx and 700 lx. The influence of aeration level, salinity and light intensity on oil globule absorption, feeding incidence, and growth and survival of early-stage grouper larvae are discussed.     

Food ingestion,consumption and selectivity of pompano,Trachinotus ovatus (Linnaeus 1758) under different rotifer densities

The growth, survival, food selection and consumption of pompano larvae under different rotifer densities as well as their colour preference during the rotifer feeding stage were examined in this study. Growth and survival of fish larvae were not significantly affected when rotifer density was between 10 and 20 mL^?1. Fish larvae grew slower at 1 and 40 rotifers mL^?1 than at 10 and 20 rotifers mL^?1, and higher fish survival was achieved when fish larvae were exposed to 10 and 20 rotifers mL^?1. The rotifer density of 1 mL^?1 not only reduced food ingestion during the early stage, but also delayed diet switch from rotifer to copepod nauplii. On 5 days post hatching (DPH), larval pompano ingested more rotifers in dark‐coloured tanks and ingested more rotifers when prey colour was green. Based on the results obtained in the present study, the culture of larval pompano larvae is recommended using dark wall tanks with a feeding density of 10–20 rotifers mL^?1 during the initial feeding stage. This study proposes a management protocol to use appropriate type and quantity of live food to feed pompano larvae in a hatchery rearing condition, which could be applicable to the culture of fish larvae in other marine fish species.     

Intensive rearing system for fish larvae research: I. Marine fish larval rearing system

Larvae nutrition and in general larvae culture is considered to be the ‘bottle neck’ for marine finfish culture. Fish larvae rearing experiments investigating nutritional factors or rearing protocols are carried out in various systems, from small beakers to very large commercial tanks, making it difficult to compare data across systems.

A continuous supply of live or dry feeds and a controlled environment, i.e. temperature, filtration, photoperiod, oxygen and pH, are essential for any experimental or commercial system. These environmental factors are best controlled automatically in order to minimize variations between tanks. However, only a few automatic systems have been developed for marine finfish hatcheries.

An experimental larval rearing system was developed to reduce variability amongst tanks (due to manual feeding and other parameters) and enhance control of environmental parameters while reducing the workload. The system includes 24 conical tanks with the option of either an up-welling or bottom draining flow through water delivery system. The inlet water passes through a gas exchange column that saturates the water with dissolved oxygen and stabilizes the pH. The system was originally designed for nutritional experiments using formulated feeds. The use of an up-welling water inlet method extends the suspension time of inert particles in the water column and also helps to suspend very small or passive swimming larvae. However, when the system is used to grow-on larvae or juvenile fish it can easily be switched to bottom draining to provide self-cleaning water dynamics for high organic loads.

A unique outlet filter was developed that eases the daily routine of replacing screens when enriched live food is used. This filter can be exchanged with a screened standpipe and outlet surface skimmer when the bottom draining flow characteristics are engaged.

The system is fully controlled by a single programmable logic controller (PLC). The PLC controls the light intensity, photoperiod, dimming time, live food and algae pumping intervals, substantially reducing labor requirements.     

Use of avoidance response by rainbow trout to carbon dioxide for fish self-transfer between tanks

Convenient, economical, and reduced labor fish harvest and transfer systems are required to realize operating cost savings that can be achieved with the use of much larger and deeper circular culture tanks. To achieve these goals, we developed a new technology for transferring fish based on their avoidance behavior to elevated concentrations of dissolved carbon dioxide (CO₂). We observed this behavioral response during controlled, replicated experiments that showed dissolved CO₂ concentrations of 60–120 mg/L induced rainbow trout (Oncorhynchus mykiss) to swim out of their 11 m³ “growout” tank, through a transfer pipe carrying a flow with ≤23 mg/L dissolved CO₂, into a second 11 m³ “harvest” tank. The research was conducted using separate groups of rainbow trout held at commercially relevant densities (40–60 kg/m³). The average weight of fish ranged from 0.15 to 1.3 kg during the various trials. In all trials that used a constant flow of low CO₂ water (≤23 mg/L) entering the growout tank from the harvest tank, approximately 80–90% of the fish swam from the growout tank, through the transfer pipe, and into the harvest tank after the CO₂ concentration in the growout tank had exceeded 60 mg/L. The fish that remained in the growout tank stayed within the area of relatively low CO₂ water at the entrance of the transfer pipe. However, the rate of fish transfer from the growout tank to the harvest tank was more than doubled when the diameter of the transfer pipe was increased from 203 to 406 mm. To consistently achieve fish transfer efficiencies of 99%, water flow rate through the fish transfer pipe had to be reduced to 10–20% of the original flow just before the conclusion of each trial. Reducing the flow of relatively low CO₂ water near the end of each fish transfer event, restricted the zone of relatively low CO₂ water about the entrance of the fish transfer pipe, and provided the stimulus for all but a few remaining fish to swim out of the growout tank. Results indicate that the CO₂ avoidance technique can provide a convenient, efficient, more economical, and reduced labor approach for fish transfer, especially in applications using large and well mixed circular culture tanks.     

Using a macroalgae Ulva pertusa biofilter in a recirculating system for production of juvenile sea cucumber Apostichopus japonicus

A simple indoor recirculating system for production of juvenile sea cucumber (Apostichopus japonicus) was operated on a commercial scale for 90 days during winter. The system consists of three 70 m³ sea cucumber rearing tanks and one biofilter tank where macroalgae (Ulva pertusa) was used as a biofilter in order to reduce water requirements. Effluent from the sea cucumber tanks drained into the macroalgae biofilter tank and were then returned to the sea cucumber tanks by a discontinuous-flow recirculation system. Survival and growth rates in the sea cucumber culture tanks were similar to those in the control tank (with one water exchange per day). The survival rate averaged about 87%. The average body weight increased from 3.5 ± 0.3 g to 8.1 ± 0.8 g and total sea cucumber biomass production over the experimental period was 745 g m⁻² after initial stocking densities of 375 g m⁻². The growth rate of U. pertusa was 3.3% day⁻¹. U. pertusa was efficient in removing toxic ammonia and in maintaining the water quality within acceptable levels for sea cucumber culture; there were only small daily variations of temperature, pH and DO. The U. pertusa tank removed 68% of the TAN (total ammonia-nitrogen) and 26% of the orthophosphate from the sea cucumber culture effluent; the macroalgae biofilter removed ammonia at an average rate of 0.459 g N m⁻² day⁻¹. It would be efficient to use the U. pertusa biofilter in a recirculating system for production of A. japonicus juveniles in winter.     

Design and performance of a recirculating aquaculture system for oyster larval culture

The major objective of this study was to introduce a newly designed recirculating aquaculture system (RAS) for oyster (Crassostrea angulata) larval culture. The system includes a culture tank, a suspended circular inlet‐pipe, an upwelling aeration pipe, combined “banjo” sieves and a bioreactor chamber containing microalgae life keeping installation. The system was designed to resolve three problems: (i) stranding of larvae caused by water level changes and aeration, (ii) physical clogging of the screens and also (iii) deterioration of diet microalgae. The culture tank, “banjo” sieve size, water flow rate and light intensity for maintaining microalgae activity were all designed according to the pattern of larval movement and feeding behaviour. Results of this study showed the best average SGR for larval length was 6.36%/d (9.5 μm/d) and survival rate was 80%, with initial rearing density of 50 larvae/ml, indicating the problems above were fully resolved. Consequently, the system is fit for larval culture in mass production of oysters.     

Turbulence effect on survival and feeding of pacific bluefin tuna <Emphasis Type="Italic">Thunnus orientalis</Emphasis> larvae,on the basis of a rearing experiment

ABSTRACT:   Physical conditions such as oceanic turbulence related to food availability are considered to be important factors affecting fish larval survival. Rearing experiments were conducted to elucidate the effects of turbulence on the survival and feeding rates during the initial feeding period of Pacific bluefin tuna Thunnus orientalis . Six levels of turbulence intensity were provided by changing flow rates from pipes set on the bottom of rearing tanks. The result showed a dome-shaped relationship between turbulence level and survival rate, in which the feeding rate appeared higher at a logged turbulence energy dissipation rate of −6.32, and decreased at both higher and lower turbulence levels. Compared with the turbulence intensity in the ocean, the optimal turbulence level for Pacific bluefin tuna larvae corresponded to the turbulence caused by sea surface winds with speeds of 4–12.5 m/s. The estimated optimal turbulence intensity for Pacific bluefin tuna larvae is comparable to that for yellowfin tuna Thunnus albacares .     

Influence of California halibut (Paralichthys californicus) on the vertical distribution of dissolved oxygen in a raceway and a circular tank at two depths

The design and operation of aquaculture tanks should minimize stagnant areas especially in the immediate vicinity of the fish. In tanks with pelagic fish, mixing caused by the water flow and by fish swimming is sufficient to maintain dissolved oxygen and metabolite concentrations in the immediate vicinity of the fish that are similar to those in the main water body. Given the behavior of sedentary benthic species, such as the California halibut (Paralichthys californicus), and their tendency to remain motionless on the bottom of aquaculture tanks, often in layers that are several fish deep, water quality may stratify with the worse conditions occurring in the area where they fish are lying. The purpose of this study was to evaluate the influence that California halibut (450 g average weight) may have on the vertical profile of oxygen concentration in a raceway (239 cm long, 28 cm wide) and a circular tank (92 cm diameter) operated at two water depths (10 and 20 cm). Oxygen was measured at each centimeter of the vertical profile both in an area with fish and without fish to assess their influence.

Results showed a lower oxygen concentration in the near-bottom region of the raceway and circular tanks. The phenomenon was most pronounced in the raceway operated at a 20 cm depth, but was also observed in the circular tank operated at 20 cm and in the raceway at 10 cm.

Measurements were also taken in samples collected just in front of or directly from a fish's mouth. A zone of depressed oxygen concentration in the immediate vicinity of the fish was documented, with oxygen concentrations as low as 50% of the measured tank effluent concentration. The magnitude of the depression was greater in raceways than in circular tanks and in 20 cm water depth than in 10 cm depth. The fish remained sedentary in these zones of depressed oxygen concentration for extended periods of time and frequently exhibited hyperventilation. The oxygen concentrations in the vicinity of the fish were consistently lower than the concentrations measured in the tank effluent. Therefore, effluent measurements did not provide an accurate representation of conditions to which the fish were exposed.     

Efficacy of different types of live and non-conventional diets in endangered clown knife fish Chitala chitala (Hamilton-Buchanan) during its early life stages

In the present study, the growth and survival of early life stages (ELS) of Chitala chitala were studied in nylon hapa for 28 days, followed by rearing in fibreglass reinforcement plastic (FRP) tanks for a period of 30 days. Ten‐day‐old ELS of C. chitala reared in hapa were fed with three different diets namely Indian Major Carp (IMC) spawn (<8 mm), live tubifex and fresh fish eggs. In the second phase, 28‐day‐old ELS were stocked in 200‐lit FRP tank and supplied four different live diets namely live tubifex worm, chironomous larvae, zooplanktons and mosquito larvae. Fish accepted all types of diets in the experimental rearing period in both the systems. The experiments conducted in hapa showed a higher specific growth rate (SGR), weight gain per cent and survival rate in larvae fed with live tubifex (SGR=1.76±0.02) than fish eggs (0.77±0.31) and IMC spawn (0.46±0.12). The study carried out in FRP tanks revealed that SGR was higher in ELS fed on chironomous larvae (4.44±0.61), followed by mosquito larvae (3.29±0.40) and live tubifex (3.28±0.36), whereas minimum SGR was recorded with zooplanktons (2.84±0.66). A significant difference (P<0.05) in SGR, final mean weight and weight gain (%) was also recorded. The highest mean survival rate (100%) of ELS in an FRP tank was observed in chironomous larvae and zooplanktons, whereas with live tubifex and mosquito larvae the same survival rate (80%) was recorded. The rate of survival of the ELS reared in hapa varied from 65% to 85%. The experiments showed that ELS of C. chitala could be reared successfully in hapas and fibreglass reinforcement tanks for attaining better survivability and growth.     

Flow field control via aeration adjustment for the enhancement of larval survival of the kelp grouper Epinephelus bruneus (Perciformes:Serranidae)

Flow field control via aeration adjustment for the enhancement of larval survival of the kelp grouper Epinephelus bruneus was examined. Aeration rate of 300 mL min^?1 was introduced during daylight (07:00–19:00 hours) and adjusted to 0, 300 and 900 mL min^?1 at night (19:00–07:00 hours). Larval sinking velocity±SD increased from 0.08 ± 0.05 to 0.26 ± 0.24 cm sec^?1 from 4 to 12 days after hatching (DAH), indicating their susceptibility to sink. Larvae reared in 300 mLmin^?1 attained the highest survival rate at 24.9 ± 3.4%, but remained significantly smaller in growth: 4.54 ± 0.56 mm compared with 4.82 ± 0.53 mm in 900 mL min^?1. The flow field in 300 and 900 mL min^?1 was at 10–20 and 15–25 cm above the bottom of the tank and 8.0 and 1.0 cm beneath the water surface. A favourable rearing condition was observed in 300 mL min^?1 as larvae were away from the bottom and surface areas, thus preventing them from dying due to sinking and surface tension‐related death (STRD). Although sinking death was decreased with an increasing aeration rate, the stronger flow had increased larval susceptibility to STRD. Our findings suggest that aeration at 300 mL min^?1 could enhance larval survival by reducing both sinking death and STRD.