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.     

Evaluation of partial water reuse systems used for Atlantic salmon smolt production at the White River National Fish Hatchery

Eight of the existing 9.1 m (30 ft) diameter circular culture tanks at the White River National Fish Hatchery in Bethel, Vermont, were retrofitted and plumbed into two 8000 L/min partial water reuse systems to help meet the region's need for Atlantic salmon (Salmo salar) smolt production. The partial reuse systems were designed to increase fish production on a limited but biosecure water resource, maintain excellent water quality, and provide more optimum swimming speeds for salmonids than those provided in traditional single-pass or serial-reuse raceways. The two systems were stocked with a total of 147,840 Atlantic salmon parr in May of 2005 (mean size 89 mm and 8.5 g/fish) and operated with 87–89% water reuse on a flow basis. By the time that the smolt were removed from the systems between March 28 to April 12, 2006, the salmon smolt had reached a mean size of 24 cm and 137 g and hatchery staff considered the quality of the salmon to be exceptional. Overall feed conversion was <1:1. The Cornell-type dual-drain circular culture tanks were found to be self-cleaning and provided mean water rotational velocities that ranged from a low of 0.034 m/s (0.2 body length per second) near the center of the tank to a high of 39 cm/s (2.2 body length per second) near the perimeter of the tank. The fish swam at approximately the same speed as the water rotated. System water quality data were collected in mid-September when the systems were operated at near full loading, i.e., 24 kg/m³ maximum density and 52.1 and 44.1 kg/day of feed in system A and system B, respectively. During this evaluation, afternoon water temperatures, as well as dissolved oxygen (O₂), carbon dioxide (CO₂), total ammonia nitrogen (TAN), and total suspended solids (TSS) concentrations that exited the culture tank's sidewall drains averaged 14.8 and 15.9 °C, of 7.9 and 8.2 mg/L (O₂), 4.0 and 3.2 mg/L (CO₂), 0.72 and 0.67 mg/L (TAN), and 0.52 and 0.13 mg/L (TSS), respectively, in system A and system B. Dissolved O₂ was fairly uniform across each culture tank. In addition, water temperature varied diurnally and seasonally in a distinct pattern that corresponded to water temperature fluctuations in the nearby river water, as planned. This work demonstrates that partial reuse systems are an effective alternative to traditional single-pass systems and serial-reuse raceway systems for culture of fish intended for endangered species restoration programs and supplementation programs such as salmon smolt.     

Effect of increased water recirculation rate on algal supply and post-larval performance of scallop (Pecten maximus) reared in a partial open and continuous feeding system

In a commercial scallop hatchery spat production depends on a culture system which ensures high survival and good growth. Reuse of water with algae may increase the food exploitation and hence reduce the costs. Post-larvae of great scallop (Pecten maximus) were studied in a commercial hatchery using a partial open and continuous feeding tank system. Three different water recirculation rates (67, 83 and 92%) were tried out in two experiments with post-larvae originating from three spawning groups of ages between 43 and 57 days post-spawn, 316–886 μm shell-height and 1.1–9.6 μg ash-free dry weight. The post-larvae were held in sieves in tanks of 2500 l where a downwelling flow was maintained by airlifts. New water with a mix of monocultured algae was continuously added to the tanks at algal concentrations of 10 and 15 cells μl⁻¹ in experiment 1 (groups 1 and 2) and 2 (group 3), respectively. The algal supply to each sieve was reduced along with increased recirculation rate, but was kept between 6 and 13 cells μl⁻¹. Generally no significant differences in survival, growth or chemical content were found between the three recirculation rates, while few differences were found between and within groups. Large variation in survival was found between and within groups (1–81%). Highest survival was found in experiment 1, and where post-larvae from two settlements were used, the first settlement survived better than the second. The daily growth ranged from 15 to 62 μm shell-height and from 0.3 to 2.6 μg ash-free dry weight. The scallop post-larvae could well be reared at all three recirculation rates studied as an increase from 67 to 92% did not seem to affect the post-larval performance seriously. The algal supply, however, had to be compensated by an increasing number of cells (>10 cells μl⁻¹) when increasing the recirculation rate.     

Constructed wetlands as a treatment method for effluents from intensive trout farms

This study examined the effects of different hydraulic loading rates on the treatment efficiency of subsurface flow (SSF) constructed wetlands treating effluents from trout farming over a period of 6 months. Six identical wetland cells with a pre-sedimentation zone of 9.6 m² and a root zone of 23.6 m² were loaded with effluents from intensive trout farming (> 2.1 kg feeding stuff per L/s and day). The total runoff of 13.2 L/s was treated in the wetland cells, where two duplicate cells received equal hydraulic loads of 3.9, 1.8 and 0.9 L/s. All examined wetland cells had significant treatment effects on the nutrient fractions containing particulate matter [total nitrogen (TN), total phosphorous (TP), biological oxygen demand in 5 days (BOD₅), chemical oxygen demand (COD), and total suspended solids (TSS)].

Efficiency was between 5.5% for TN and 90.1% for TSS. The SSF wetland also had a high treatment effect on total ammonia nitrogen (TAN), with efficiencies of 61.2 to 87.8%. Nitrate nitrogen (NO₃–N) and phosphate phosphorous (PO₄–P) showed a significant increase in the wetland effluent by 8.4 to 209%. Nitrite nitrogen (NO₂–N), had no significant, or significant effluent increase depending on the inflow rate. Treatment efficiency for particulate nutrients and TAN increased with decreasing hydraulic load, while the differences between 1.8 and 0.9 L/s were not significant. The treatment efficiency for TP was constant for all cells, at around 40%. The wetland receiving 3.9 L/s was over-flooded after 10 to 12 weeks due to colmatation. Nevertheless, the wetland still showed high treatment efficiencies. For commercial trout farms, SSF wetlands are a highly effective method of effluent treatment. A hydraulic load of 1 L/s on 13.3 m² wetland area (1.8 L/s on the examined wetland) seems most suitable. Higher loads lead to accelerated wetland colmatation, while lower loads waste space.     

Water quality and waste production in a recirculating trout-culture system with feeding of a higher-energy or a lower-energy diet

Higher-energy fish feeds can reduce waste discharges and might also improve water quality in recirculating fish-culture systems. A higher-energy diet, Zeigler Salmon High Energy feed (HE; 45% protein, 20% fat, 17.4 MJ digestible energy kg^-1) and a lower-energy diet, Zeigler Hi-Fat Trout Grower (LE; 38% protein, 12% fat, 14.6 MJ digestible energy kg ^-1) were fed ad libitum at different times to rainbow trout, Oncorhynchus mykiss (Walbaum), in a semi-closed recirculating culture system by means of demand feeders. The system contained two 10-m³ fish-culture tanks, each with a downstream microscreen (80 μn) filter. Feeding rates per day and per unit biomass were not significantly different between diets. In general, use of HE was associated with higher levels of total ammonia nitrogen (TAN) and NO₂-N, lower BOD₅ and total suspended solids (TSS), and lower effluent releases of suspended solids per unit feed, NO₃-N per unit feed, and dissolved phosphorus per unit feed. Although total effluent P per unit feed or P fed did not differ significantly between diets, HE had significantly more of the total effluent P in the settleable solids, 85% vs. 76%. Differences in water quality in the system were probably not of great importance with respect to fish health.     

Preliminary tests of an aerated tank system for tilapia culture

A simple tank system, using artificial aeration to supply oxygen and incoming water to dilute metabolic by-products and other wastes, was tested. Blue tilapia, Oreochromis aureus, were grown under a wide range of loads, 7–31 kg liter⁻¹ min⁻¹, and densities, 19–77 kg fish m⁻³ culture unit. Average yield ranged from 6–10 kg m⁻³ culture unit per month. Approximately 14–20 m³ of water was used for each kilogram of fish produced.     

Water quality and sludge characterization at raceway-system trout farms

Studies were conducted to characterize raceway water quality, effluent water quality, and waste solids within three, raceway-type trout farms. No significant differences were found in effluent water quality between the three farms during 7 months of monitoring. Average effluent quality over the course of the day was not found to be impaired. However, effluent quality was found to change significantly during times of feeding and harvesting. In a concrete/earthen-lined farm, normalized total suspended solids (TSS) concentrations were as high as 115 and 63 mg/l during harvesting and feeding events, respectively. Total Kjeldahl nitrogen (TKN) and ortho-phosphate (OP) also increased with higher TSS loads. The majority of particles (by weight) measured in effluent samples at all three farms were evenly divided between the smallest range (1.5–30 μm) and the largest (>210 μm). For settled sludge samples, the majority of the particles were in the size range of 1.5–30 μm. Particle size in the raceway was positively correlated with fish size during feeding events, but this correlation dissipated during the 4-h period after feeding. The accumulation and characteristics of sludge in a sediment trap were also monitored over a 22-day period.     

Exemple de système de réchauffement de l''eau de mer utilisable dans les dispositifs aquacoles

This paper deals with the possibility of heating cold seawater entering a seaside aquacultural nursery during winter. The proposed model contains a ‘warm’ water source and two heat exchangers in series (heating and pre-heating). The model evolves for the system a set of values for various pairs of exchanger surface areas. The values of the working temperature are calculated for different flowrate ratios and for several surface areas given a set value of 15°C for the ‘warm’ water source. The paper further develops the optimising of the ratio S₂/S₁ for minimum capital investment and takes the further step of investigating the possibility of failure to achieve a set T_i for a specific pair of heat exchangers during the cold season at a specific seawater site.     

Phosphorus utilization in rainbow trout (Oncorhynchus mykiss) fed practical diets and its consequences on effluent phosphorus levels

Excessive dietary phosphorous (P) concentrations in effluents from aquaculture present a major environmental problem. We therefore studied the effect of dietary P and vitamin D₃ on P utilization by rainbow trout-fed practical diets and on P concentrations in the soluble, particulate and settleable components of the effluent from fish tanks. Rainbow trout (average weight: 78 g, initial biomass: 13 kg in 0.7 m³ tanks) were fed for 11 weeks, practical diets that varied in total P, available P, and vitamin D₃ concentrations. Soluble, particulate (10–200 μm) and settleable (>200 μm) P in the effluent were sampled every 0.5–6 h for 1–3 days in the third and eleventh weeks of the experiment. Trout in all diets more than doubled their weight after 11 weeks. Increasing the concentrations of available dietary P from 0.24% to 0.88% modestly enhanced growth rate. Feed conversion ratio (FCR) and biomass gain per gram P consumed decreased as dietary P concentrations increased. Carcass P, daily P gain, and plasma P concentrations were lower in fish fed with low P diets. Soluble P concentrations in the effluent peaked immediately after and again 4–6 h after feeding, and is a linear function of available dietary P. No soluble P would be produced during consumption of diets containing less than 0.22±0.02% available P. Above this dietary concentration, soluble P would be excreted at 6.9±0.4 mg/day/kg for each 0.1% increase in available dietary P. Particulate P concentrations in the effluent were independent of dietary P concentrations. Settleable, presumably fecal, P concentrations tended to increase with dietary P concentrations. In trout fed with low P (0.24% available P, 0.6% total P) diets, 60% of total dietary P were retained by the fish and the remaining 40% were excreted in the effluent as settleable P (20–30%) and particulate or soluble P (10–20%). In trout fed with high P (0.59–0.88% available P; 0.9–1.2% total P) diets, 30–55% of total dietary P was retained by fish, and the remaining 15–25% appeared in the effluent as settleable P, 20–55% as soluble P, and 5–10% as particulate P. Vitamin D₃ did not affect fish growth nor effluent P levels. Physicochemical management of aquaculture effluents should consider the effect of diets on partitioning of effluent P, the peaks of soluble P concentration following feeding, and the contributions of particulate P to total P in the effluent. Increasing our understanding of how dietary P is utilized and is subsequently partitioned in the effluent can contribute significantly towards alleviating this important environmental and industry problem.     

Food Production and Water Conservation in a Recirculating Aquaponic System in Saudi Arabia at Different Ratios of Fish Feed to Plants

An indoor aquaponic system (i.e., the integration of fish culture with hydroponic plant production in a recirculating setup) was operated for maximizing water reuse and year-round intensive food production (Nile tilapia, Oreochromis niloticus , and leaf lettuce) at different fish feed to plants ratios. The system consisted of a fish culture component, solid removal component, and hydroponic component comprising six long channels with floating styrofoam rafts for holding plants. Fish culture effluents flowed by gravity from the fish culture component to the solid removal component and then to the hydroponic component. Effluents were collected in a sump from which a 1-horsepower in-line pump recirculated the water back to the fish culture tanks at a rate of about 250 L/min. The hydroponic component performed as biofilter and effectively managed the water quality. Fish production was staggered to harvest one of the four fish tanks at regular intervals when fish attained a minimum weight of 250 g. Out of the total eight harvests in 13 mo, net fish production per harvest averaged 33.5 kg/m³ of water with an overall water consumption of 320 L/kg of fish produced along with the production of leaf lettuce at 42 heads/m² of hydroponic surface area. Only 1.4% of the total system water was added daily to compensate the evaporation and transpiration losses. A ratio of 56 g fish feed/m² of hydroponic surface effectively controlled nutrient buildup in the effluents. However, plant density could be decreased from 42 to 25–30 plants/m² to produce a better quality lettuce.     

Pharmacokinetics and the active metabolite of enrofloxacin in Chinese mitten-handed crab (Eriocheir sinensis)

The pharmacokinetics and active metabolite of enrofloxacin were estimated after single intramuscular administration (10.0 or 20.0 mg/kg body weight) to the Chinese mitten-handed crab (Eriocheir sinensis) in fresh water at 25.0 ± 1.0 °C. Levels of enrofloxacin and its metabolite ciprofloxacin in the main tissues (hemolymph, hepatopancreas, muscle, ovary and spermary) were simultaneously detected by HPLC. Enrofloxacin concentration–time profiles for the hemolymph in both tests were described by a two-compartment open model with first-order absorption. Distribution half-time (T_1/2), elimination half-time (T_1/2β), body clearance (CL/F), mean residence time (MRT_0–∞), area under the concentration–time curve from 0 to ∞ h (AUC_0–∞) and apparent volume of distribution (Vd/F), which derived from the pharmacokinetic model, were 0.427 h, 21.3 h, 0.133 l/h/kg, 60.0 h, 96.9 μg/ml/h and 4.08 l/kg, respectively, at a dose of 10.0 mg/kg body weight, and 0.216 h, 12.3 h, 0.189 l/h/kg, 85.8 h, 187 μg/ml/h and 3.35 l/kg, respectively, at a dose of 20.0 mg/kg body weight. Similarities were found between the hemolymph concentration–time curves of the two tests; for example, instant absorption process followed by the distribution phrase, and a second absorption peak at 6 h post-treatment. After intramuscular administration of 10.0 mg/kg body weight, absorption of enrofloxacin was observed in the main edible tissues (hepatopancreas, muscle, ovary and spermary), and the drug residue was the highest in the hepatopancreas, where the ‘drug sink’ phenomenon occurred. Comparative pharmacokinetics showed fast absorption, broad distribution and fast elimination of enrofloxacin in E. sinensis after intramuscular dosing. Regarding ciprofloxacin, the main active metabolite of enrofloxacin, though relatively low levels were detected in all the main tissues of the crab, its kinetics in the hemolymph in the two tests were not described by a one- or two-compartment open model.     

Measurements of oxygen consumption and ammonia excretion of Atlantic salmon (Salmo salar L.) in commercial-scale, single-pass freshwater and seawater landbased culture systems

The oxygen consumption of Atlantic salmon was measured in large culture tanks for a period of 20 months from the parr to the adult stage. In addition, diurnal sampling was conducted for estimation of both oxygen consumption and ammonia excretion. The oxygen consumption was affected especially by temperature, season and smoltification. For parr the oxygen consumption rate was 1–6 mg O₂/kg min and the ammonia excretion rate was 0·037–0·13 mg N/kg min from autumn to spring. The corresponding rates for adult salmon during the period October to July were 1·5–4·5 mg O₂/kg min and 0·075–0·13 mg N/kg min.     

Performance and operation of a rotating biological contactor in a tilapia recirculating aquaculture system

This paper describes the performance characteristics of an industrial-scale air-driven rotating biological contactor (RBC) installed in a recirculating aquaculture system (RAS) rearing tilapia at 28 °C. This three-staged RBC system was configured with stages 1 and 2 possessing approximately the same total surface area and stage 3 having approximately 25% smaller. The total surface area provided by the RBC equaled 13,380 m². Ammonia removal efficiency averaged 31.5% per pass for all systems examined, which equated to an average (± standard deviation) total ammonia nitrogen (TAN) areal removal rate of 0.43 ± 0.16 g/m²/day. First-order ammonia removal rate (K₁) constants for stages 1–3 were 2.4, 1.5, and 3.0 h⁻¹, respectively. The nitrite first-order rate constants (K₂) were higher, averaging 16.2 h⁻¹ for stage 1, 7.7 h⁻¹ for stage 2, and 9.0 h⁻¹ stage 3. Dissolved organic carbon (DOC) levels decreased an averaged 6.6% per pass across the RBC. Concurrently, increasing influent DOC concentrations decreased ammonia removal efficiency. With respect to dissolved gas conditioning, the RBC system reduced carbon dioxide concentrations approximately 39% as the water flowed through the vessel. The cumulative feed burden – describes the mass of food delivered to the system per unit volume of freshwater added to the system daily – ranged between 5.5 and 7.3 kg feed/m³ of freshwater; however, there was no detectable relationship between the feed loading rate and ammonia oxidation performance.     

Modeling industry-wide sediment oxygen demand and estimation of the contribution of sediment to total respiration in commercial channel catfish ponds

Data collected from 45 commercial channel catfish, Ictalurus punctatus, ponds were used to develop empirical models predicting sediment oxygen demand (SOD). Seven acceptable models were combined with a Monte-Carlo sampling distribution to predict industry-wide sediment oxygen demand (SOD_i). The SOD_i values obtained from the best equation were used in simulations to assess the effect of diurnally varying water column dissolved oxygen (DO) concentrations on SOD and the effect of pond water depth on the contribution of SOD to overall pond respiration. Estimated SOD_i ranged from 62 to 962 mg m⁻² h⁻¹, with a mean of 478 mg m⁻² h⁻¹. There was a 95% probability of mean SOD_i being ≥700 mg m⁻² h⁻¹. The effects of diurnal variation in DO concentration in the water column on expression of SOD was modeled by combining maximum SOD_i, an empirical relationship between DO and SOD, and simulated pond DO concentrations. At DO concentrations >15 mg l⁻¹, diel SOD in catfish ponds exceeded 20 g O₂ m⁻² day⁻¹. But when average diel DO was <4 mg l⁻¹ and the range of DO concentration was 6–8 mg l⁻¹, SOD decreased to 13 g O₂ m⁻² day⁻¹ because DO availability limited the full expression of potential SOD. Respiration totals for sediment (average SOD_i), plankton, and fish respiration were calculated for pond water depths ranging from 0.25 to 4 m. Although whole-pond respiration increases as pond depth increases, the proportion of total respiration represented by sediment decreased from 48 to 10% by increasing water depth over this range. The results of these studies show that SOD is a major component of total pond respiration and that certain management practices can affect the impact of SOD on pond oxygen budgets. Mixing ponds during daylight hours, either mechanically or by orienting ponds for maximum wind fetch, will increase oxygen supply to sediments, thereby allowing maximum expression of SOD and maximum mineralization of sediment organic matter. Given a mixed condition caused by wind or other artificial means, the construction of deeper ponds increases the total mass of DO available for all respiration, causing nighttime DO concentrations to decline at a slower rate, reducing the need for supplemental aeration. Because a pond’s water volume decreases over time from sediment accumulation, annual aeration costs will increase with pond age. Constructing ponds with greater initial depth will therefore reduce long-term cost of aeration, allow more flexible management of pond water budget, and reduce the long-term expense associated with pond reconstruction.     

Alum residuals as a low technology for phosphorus removal from aquaculture processing water

Aquaculture process waters are often scrutinized for loading phosphorus discharges into surface water. With the growing regulatory control of discharge from aquaculture process industries, it has become very important to address low cost and effective technological solution for aquaculture facilities. This study aims to investigate the effectiveness of alum residuals, which were generated during drinking water treatment for adsorption of phosphorus from aquaculture process water. Alum residuals were dried using an oven at 105 °C for 24 h. Particle size (d₆₀) was similar to conventional adsorbent, granular activated carbon. Bench scale experiments (batch and fixed bed column tests) were conducted using oven dried alum residuals. Fixed bed column tests also looked at the effect of influent pH on the effectiveness of oven dried alum residuals. Experimental results observed phosphorus removal of 94–99% using an alum residuals concentration of 4–16 g/L. Freundlich adsorption isotherm was effective in explaining partitioning among solid and liquid phases. Oven dried alum residuals were a better adsorbent for orthophosphate phosphorus than total phosphorus. Effluent pH levels for both batch and fixed bed column tests were within range of 6–9 for most of the samples tested and therefore, suitable for surface water disposal. There were no effects of pH observed on the breakthrough pore volume processed during fixed bed column test. There was aluminum leaching from oven dried alum residuals, however, not high enough to cause toxicity for aquatic species if disposed in surface water. Oven dried alum residuals were also able to adsorb organic matter from aquaculture process water. The effluent BOD₅ was below 30 mg/L for most of the samples with an exception of a few samples where BOD₅ was beyond the limit for surface water disposal guidelines. The results indicated that oven dried alum residuals have potential to provide a technological solution for small aquaculture facilities.     

Use of treated mine water for rainbow trout (Oncorhynchus mykiss) culture: a production scale assessment

In previous proof-of-concept work, it was shown that the use of treated coal mine water for rainbow trout (Oncorhynchus mykiss) culture in a cage was technically feasible, though only a 50-fish bioassay was grown and no work on production-related issues was conducted. To further advance the use of treated mine water, an under-utilized water resource throughout Mid Appalachia, work was conducted to assess the effects of using treated coal mine water for the intensive production of rainbow trout in a flow-through system. During this study, comprehensive water quality data were collected to supplement fish weight and length data taken during routine monthly sampling events. The 8000 fish grew well in the raceway system over the 9 months of production, where a feed conversion ratio of 1.4 and a condition factor of 5.1 × 10⁻⁴ were measured with stocking and harvest densities of 26.4 and 50.2 kg/m³, respectively. Further, total net production was 3275 kg (7220 lb) with 98.6% survival. Throughout the study, dissolved ion concentrations (Fe, Al, Mg, Ca, and SO₄) often exceeded recommended tolerance limits. Further, elevated ammonia nitrogen concentrations generated from a component of the mine water-treatment process were identified as a potential limiting factor for aquaculture development. However, when the non-ideal effects of high ionic strength and the speciation of dissolved metal–ligand complexes were taken into account, the concentrations of free metal ions were within recommended tolerance limits.     

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.     

Air bubbling results in carbon loss during microalgal cultivation in bicarbonate-enriched media: experimental data and process modeling

Turbulence is a major variable controlling growth of microalgae. When turbulence is provided by means of air bubbling, mass transfer phenomena take place between the culture medium and the air bubbles. A mathematical model has been developed to evaluate the effects of air bubbling in bicarbonate-enriched media for microalgae cultivation, and in cultures of the halotolerant alga Dunaliella viridis. Simulations have been made using a four order Runger–Kutta method combined with Marquardt's algorithm to establish the parameters of the system. The model permits: (i) simulation of the experimental behavior of organic (cellular) and inorganic (CO₂ + H₂CO₃ + HCO₃⁻ + CO₃²⁻) carbon in the cultures; (ii) performance of a sensitive analysis of the effects of varied operative conditions and (iii) development of strategies for optimizing the use of culture media and subsidiary energy. Simulation of the model demonstrated that in air-bubbled HCO₃⁻ enriched media there is a loss of CO₂ from the medium to the air bubbles, which escapes from the culture vessel, and no transference of CO₂ from the air bubbles to the water occurred. Validation of the model has been carried out by comparison of experimental and predicted values under different conditions.     

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.     

New techniques for the assessment and optimal management of growth and standing crop variation in the cultured freshwater prawn, Macrobrachium rosenbergii

The current ‘traditional’ prawn pond management system really does not ‘manage’, but rather accommodates prawn production characteristics and was instituted with a minimum knowledge of prawn biology and husbandry. As such, the traditional system is not optimal because it does not rely on a strong empirical knowledge base. Prawn culture has succeeded up until now because Macrobrachium rosenbergii is easily cultured and will give 500–1000 kg ha⁻¹ year⁻¹ without much effort. This is sufficient in many areas of the world but not in others where economic conditions have break-even production requirements of nearly 1 tonne greater than this. The traditional system relying on selective harvesting with large seine nets is seriously inefficient which not only leads to lost revenue but under-manages pond growth since unculled large animals suppress the growth of unculled smaller ones. The degree to which this occurs was not known to the designers of the traditional system who had no way of knowing (as we do now from our research results) the extremely large compensatory growth capability of small prawns in the absence of large ones. The traditional system also does not manage sexual dimorphic growth because no technology exists which can be used to create monosex broods or manipulate the sex ratio in ponds. Accurate production models are not available because traditional ponds are rarely sampled and/or rarely drained. This, along with inefficient harvests, results in a co-mingling of cohort stocking classes. It is impossible to get accurate survival and growth data because of this and because it is so time consuming to sample commercial ponds and process the data on a regular basis. This paper describes four new engineering and ‘bioengineering’ techniques which are under development and can overcome all the drawbacks of the traditional system: (1) surgical sex reversal to create monosex broods; (2) genetic tagging of stocking and resident cohorts to assess survival and growth; (3) size grading and 100% efficient harvesting using pond draining and a machine grader-harvester; and (4) a semi-automated computer assisted prawn sample data management system which uses sonic digitization of prawn sample data.