An Analytical Model for Predicting the Carrying Capacity of Submerged Biofilters Used in Aquaculture

Laboratory data and information from the literature were used to develop an equation that models ammonia removal by submerged biofilters. The equation is based on the half-order/zero-order kinetics model and fixed biofilm nitrification, which indicates that the nitrification rate is a linear function of the ammonia concentration at ammonia concentrations <2.0 mg/l. Input data for the equation include easily gathered information of flow rate, biofilter size, maximum permissible ammonia level, biofilter influent and biofilter effluent ammonia concentrations, temperature, and ammonia production rate. The equation can be used to determine the carrying capacity of the submerged biofilters, to estimate biofilter sizes needed to support various fish loads, and to investigate the effects of changes in biofilter system parameters on carrying capacity. Comparisons of the predicted and actual carrying capacities of several experimental biofilters indicated that the equation accurately predicted carrying capacity under most conditions. Studies of the sensitivity of the equation to changes in system parameters demonstrated that flow rate limits biofilter performance and suggested an approach for determining cost-effective flow rates for biofilter operation.     

Metagenomic analysis shows diverse,distinct bacterial communities in biofilters among different marine recirculating aquaculture systems

While biofilters are widely used to metabolize ammonia and other metabolic waste products in recirculating aquaculture systems, their microbial communities are not thoroughly characterized. While inroads have been made characterizing microbial communities within single biofilters, replicated comparisons across biofilters and facilities have been lacking. We hypothesized that microbial communities might differ among filter types and facilities. We characterized and compared the bacterial communities of nine nitrification biofilters in five commercial recirculating marine aquaculture operations by amplifying and sequencing the 16S rRNA gene using the Illumina-MiSeq DNA sequencing platform. Our results demonstrated the usefulness of the approach for elucidating bacterial community structure in aquaculture biofilters; among almost 249,000 usable DNA sequence reads—a mean of 27,663 for each biofilter—we detected a mean of 682 operational taxonomic units. Higher species diversity was observed in the submerged biofilters at farms 3 and 4 (HF_SB1, HF_SB2, HF_SB3, MB_SB1, MB_SB2, and MB_SB3), and a bead filter at farm 2 (XYF-MBBR) than in a bead filter at farm 1 (DF_MBBR) and a fluidized sand filter at farm 5 (TY_FSF). At the phylum level, Proteobacteria were the most frequently observed taxa (representing 36–50 % of reads in the overall data set for a given filter); other frequently observed phyla were Bacteroidetes (13–34 %), Chloroflexi (2–23 %), Nitrospirae (1–7 %), Planctomycetes (1–4 %), and Actinobacteria (2–5 %). However, in fluidized sand filters, after Proteobacteria, the subdominant phyla were Bacteroidetes (19 %), Nitrospirae (17 %), and Planctomycetes (11 %). At the genus level, the nitrite-oxidizing genus Nitrospira was frequently observed in sand filter TY_FSF (16.4 %), bead filter DF_MBBR (7.6 %), submerged biofilter MB_SB1 (7 %), and bead filter XHF_MBBR (7.36), and less frequently in submerged biofilters HF_SB3 (1.94), HF_SB2 (1.77 %), and HF_SB1 (1.63 %), and bead filters MB_SB2 (0.8 %) and MB_SB1 (0.2 %). Observations of the ammonia-oxidizing genus Nitrosomonas varied widely within and among filter types, ranging from 0.06 % in submerged bed filter HF_SB3 to 2.82 % in bead filter DF_MBBR. Principal components and cluster analyses classified the bacterial communities in the nine biofilters into groups corresponding to the respective recirculating marine aquaculture operations and the associated filter types.     

Use of fluidized bed biofilter and immobilized Rhodopseudomonas palustris for ammonia removal and fish health maintenance in a recirculation aquaculture system

Intensive recirculating aquaculture relies on biofilters to sustain satisfactory water quality in the system. Fluidized bed and immobilized cell technologies were used to remove ammonia from the water and maintain fish health. A high‐rate nitrifying fluidized bed biofilter combined with valveless filter was designed for use in a recirculation aquaculture system (RAS). The suspended solids produced during fish culture could automatically be removed using a valveless filter. Natural porosity with fitting proportion, steady fluidization and expanding rate was chosen as the fluidized carrier. The technology of bacterial separation and cultivation was used. The immobilized Rhodopseudomonas palustris (R. palustris) produced through a biotechnologically embedding medium is suitable for fish and could help prevent diseases. Nitrification was promoted through the selective rearing of nitrobacteria in a fluidized bed biofilter. Water quality was improved using fluidized bed biofilter and immobilized R. palustris in the RAS. In addition, the proposed system was able to reduce costs. Maximum fish load was 45 ± 3 kg m^?3 in the closed recirculating water fish culture system, and water use was reduced by 80–90%. The total ammonia nitrogen removal rate of the technology was 80–95%, and nitrite N removal rate was above 80%.     

A case study of biofilter activation and microbial nitrification in a marine recirculation aquaculture system for rearing Atlantic salmon (Salmo salar L.)

Marine recirculation aquaculture system (RAS) is a prominent technology within fish farming. However, the nitrifying bacteria in the biofilter have low growth rates, which can make the biofilter activation a long and delicate process with periods of low nitrification rates and variations in water quality. More knowledge on the microbial development in biofilters is therefore needed in order to understand the rearing conditions that favour optimal activation of the biofilters. In this case study, we investigated the activation of two biofilters in a marine RAS for Atlantic salmon post‐smolt associated with either high or low stocking densities of fish by monitoring the microbial communities and chemical composition. The results showed that the microbial communities in both biofilters were similar during the first rearing cycle, despite variations in the water quality. Nitrifying bacteria were established in both biofilters; however, the biofilter associated with low stocking density had the highest relative abundance of ammonia‐oxidizing Nitrosococcus (1.0%) and nitrite‐oxidizing Nitrospira (2.1%) at the end of the first rearing cycle, while the relative abundance of ammonia‐oxidizing Nitrosomonas (2.3%–2.9%) was similar in both biofilters. Our study showed that low fish stocking density during the first rearing cycle provided low and steady concentrations of ammonium, nitrite and organic load, which can stimulate rapid development of a nitrifying population in new marine RAS biofilters.     

Performance characteristics of fluidised bed biofilters in a novel laboratory-scale recirculation system for rainbow trout: nitrification rates, oxygen consumption and sludge collection

A laboratory-scale recirculating aquaculture system for fluidised bed biofilter evaluation was engineered. The design included all components found in typical full-scale commercial production systems. The system included two identical units each with oxygenation, UV treatment, cooling, biofiltration and a particulates separation device. Water from the two systems was mixed in a degassing unit. A 1 month test period after biofilter maturation revealed stable concentrations of total ammonia nitrogen (TAN), nitrite and nitrate within the system. Mean nitrification rate was 0.27 and 0.21 g TAN m⁻² day⁻¹. Oxygen consumption in the biofilters ranged between 56 and 64% due to nitrifying activity. Mass balances on nitrogen indicated that 48%, added via the feed, was converted to nitrate within the system, with 6% of the added nitrogen being found in the sludge. The remaining 43% was either used during fish growth, left the system, as organic nitrogenous compounds (or unidentified nitrogenous compounds), via the outlet, or was lost to the atmosphere. At least 61% of the nitrate produced was generated by the biofilters. The system proved to be an exceptional set-up for evaluation of the performance of fluidised bed biofilters, allowing both pre- and post-filter measurements of various water quality criteria.     

Effects of formaldehyde on nitrification in biofilters of small‐scale recirculating systems

Florfenicol (Aquaflor®) is the only U.S. Food and Drug Administration (FDA) approved drug for treating diseased fish reared in recirculating aquaculture systems (RAS). Treating diseased fish in RAS is challenging because of the potential to damage nitrifying bacteria in the biofilters. Impaired nitrification can lead to concentrations of ammonia and nitrite that compromise fish welfare. The objective of this study was to determine the effects of a FDA‐approved parasiticide and fungicide, Parasite‐S^® (formalin), on biofilter nitrification. Stable biofilters were exposed once to 0, 9.25, 18.5, 37, or 55.5 mg/L formaldehyde. Total ammonia nitrogen (TAN) and nitrite nitrogen were monitored daily before and throughout the study to quantify biofilter function. Formaldehyde concentrations ≥37 mg/L increased TAN and nitrite nitrogen concentrations, and nitrification did not recover to pre‐exposure concentrations up to 8 day postexposure. On the basis of those results, a second trial was conducted. Stable biofilters were exposed once or on four consecutive days to 9.25 or 18.5 mg/L formaldehyde. Biofilters repeatedly exposed to formaldehyde showed signs of impairment and had variable recovery relative to single exposures. Results of this study may help identify formaldehyde concentrations that can be safely applied to RAS when treating diseased fish.     

Microbial dynamics in RAS water: Effects of adding acetate as a biodegradable carbon-source

This study evaluated the effect of an abrupt increase in easily biodegradable carbon (acetate) on bacterial activity and abundance in the water of recirculating aquaculture systems (RAS). The study included a batch experiment with RAS water only, and an experiment at system level where twelve pilot scale RAS were used. The batch experiment was made to test how acetate concentration would influence the microbial state in RAS water. Further, we wanted to observe if the selected microbial analysis tools would be able to detect these changes. The second experiment was carried out in twelve identical and independent RAS that had been operated under constant loading conditions (1.6 kg/m³ make-up water) for five months prior to the trial. The twelve RAS were divided into four treatment groups in triplicates: i) control with submerged biofilter (Ctrl + bf); ii) control without submerged biofilter (Ctrl-bf); iii) acetate addition in RAS with submerged biofilter (Ac + bf); and iv) acetate addition in RAS without submerged biofilter (Ac-bf). The biofilter media from the groups without submerged biofilter (Ac-bf and Ctrl-bf) was removed just 5 h prior to the start of the trial. The two acetate treatment groups (Ac + bf and Ac-bf) were spiked with 40 mg/L of acetate three consecutive times (0, 24 and 48 h). Consumption of acetate, bacterial abundance and bacterial activity were followed for 72 h after the first acetate spike for both experiments. Bacterial activity was quantified by BactiQuant^® and hydrogen peroxide (HP) degradation assay. Bacterial abundance was assessed by quantifying micro-particles and free-living bacteria. In the batch experiment we observed a significant increase in bacterial activity proportional to the amount of acetate added, and a corresponding significant increase in microparticles (1–3 μm). In the pilot scale RAS experiment, the acetate addition in RAS with submerged biofilter did not cause an increase in bacterial activity, or in the number of microparticles in the water phase but a significant increase in bacterial activity and number of microparticles were observed in the RAS without submerged biofilter (Ac-bf). These changes were particularly pronounced shortly after each acetate spike.In RAS with submerged biofilters, the acetate was presumably consumed primarily by the bacterial community within the biofilm, and consequently, only minor changes were observed in densities of free-living bacteria in the water phase. The results of the study suggest that heterotrophic bacteria in the submerged biofilter have a high capacity to handle fluctuation of organic matter loading in RAS, thereby stabilizing the abundance and activity of bacteria in the water column.     

Experiments in rearing rabbitfish (Siganus rivulatus) in sea water

The initial experiments in rearing rabbitfish (Siganus rivulatus) in sea water indicate that this fish may be suitable for commercial cultivation. Juveniles are available in large numbers during summer and fall along the Mediterranean and Red Sea coast of Israel. Juveniles collected in Eilat (Red Sea) were kept in tanks and submerged cages and fed fish pellets. After 29 days in tanks, the first group of fish which averaged 0.91 g upon capture increased in weight to an average of 3.7 g. A subsequent school of juveniles kept in sea cages for 84 days increased in weight from an average of 7.3 g to 28.5 g.     

Effect of Partial and Total Replacement of Fish Meal on Growth and Body Composition of Sunshine Bass Morone chrysops×M. saxatilis Fed Practical Diets

With the increasing emphasis to replace fish meal (FM) with less expensive protein sources in aquaculture diets without reducing weight gains, an 8-wk feeding trial was conducted with juvenile (15 g) sunshine bass Morone chrysops×M. saxatilis) to evaluate growth and body composition when fed diets with different levels of FM (0, 7.5, 15, and 30%). Six practical floating diets were formulated to contain 40% protein and similar energy levels, with various percentages of FM, meat-and-bone meal (MBM), soybean meal (SBM), poultry by-product meal (PBM), and/or distillers grains with solubles (DGS). Ten fish were stocked into each of 24 110-L aquaria and were fed twice daily ad libitum (0730 and 1600 h). At the conclusion of the feeding trial, final weights of fish fed diet 2 (0% FM, 29% SBM, 29% MBM, and 10% DGS), diet 3 (0% FM, 32% SBM, and 28% PBM), diet 5 (15% FM and 44% SBM), and diet 6 (30% FM and 26% SBM) were not significantly different (P > 0.05) and averaged 72 g. However, final weights of sunshine bass fed diet 1 (0% FM, 30% SBM, and 31% MBM) and diet 4 (7.5% FM and 54% SBM) were significantly lower and averaged 55 g. Specific growth rate (SGR) of sunshine bass fed diet 4 was significantly lower (2.14) than fish fed diet 2 (2.70), diet 3 (2.80), diet 5 (2.68), and diet 6 (2.84), while feed conversion ratio (FCR) of fish fed diet 4 was significantly higher than sunshine bass fed diets 2, 3, 5, and 6. Carcass (fish were decapitated) composition of sunshine bass fed diet 4 had a significantly higher percentage of moisture (70%) and protein (54% on a dry-matter basis) than fish fed all other diets. Percentage lipid was similar among fish fed all diets and averaged 41% (dry-matter basis). Results from the present study indicate that diets in which all of the FM is replaced with a combination of animal- and plant-source proteins can be fed to sunshine bass without adverse effects on weight gain, growth rate, and body composition. Further feeding trials are needed to refine diet formulations used in the present study and should be conducted in aquaria and ponds.     

Limited water exchange production systems for freshwater ornamental fish

Two biofilter designs and a control were tested in triplicate to determine if inexpensive bioremediation devices could increase production and decrease water use on ornamental fish farms in Hawaii. Koi (Cyprinus carpio L.) were used as the model species and the experiment was conducted outdoors in greenwater. When fish density was 9.7 kg per 2.08 m³ and they were eating 125 g day^?1, the 20 L trickle filters were able to maintain acceptable water quality. Tanks with the same size submerged filters suffered significantly lower dissolved oxygen levels compared with tanks with trickle filters and control tanks with no biofilters exhibited significantly higher nitrite‐nitrogen (about 20 mg L^–1) and nitrate levels (about 400 mg L^–1). As typical ornamental fish weigh 3 g, the trickle biofilter system described here can produce 1.55 fish L^?1 (compared with the industry standard 0.25 fish L^?1) and use very little water other than the water originally in the tanks.     

Effects of the Prebiotics GroBiotic®‐A and Inulin on the Intestinal Microbiota of Red Drum,Sciaenops ocellatus

Two separate feeding trials examined the effects of dietary supplementation of the prebiotics GroBiotic^®‐A and inulin on growth performance and gastrointestinal tract microbiota of the red drum, Sciaenops ocellatus. In the first feeding trial, fish meal‐based diets without prebiotics or supplemented with either GroBiotic^®‐A or inulin at 1% of dry weight were fed to triplicate groups of juvenile red drum (initial weight of 2.6 g) in 110‐L aquaria operated as a brackish water (7 ppt) recirculating system for 8 wk. In the second feeding trial, soybean meal/fish meal‐based diets supplemented with either GroBiotic^®‐A or inulin at 1% of dry weight were fed to triplicate groups of red drum (initial weight of 15.8 g) in 110‐L aquaria operated as either a common recirculating water system or closed system with individual biofilters (independent aquaria) for 6 wk. Supplementation of the prebiotics in either feeding trial did not alter weight gain, feed efficiency ratio, or protein efficiency ratio of red drum fed the various diets. In the second feeding trial, the culture system significantly affected weight gain, feed efficiency ratio, and protein efficiency ratio although there were no effects of dietary treatments on fish performance or whole‐body protein, lipid, moisture, or ash. Denaturing gradient gel electrophoresis (DGGE) analysis of the gastrointestinal tract microbial community showed no effect of the dietary prebiotics as the microbial community appeared to be dominated by a single organism with very low diversity when compared with other livestock and fish species. DGGE of the microbial community in the biofilters of the independent aquariums showed a diverse microbial community that was not affected by the dietary prebiotics.     

Design and operations of fine media fluidized bed biofilters for meeting oligotrophic water requirements

Fine media fluidized bed biofilters (FBB) have some unique characteristics, which become very important when extremely high water quality is required. They provide greater surface area per unit volume than other fixed film biofilters and are capable of operating as a plug flow on the liquid phase and mixed flow on the biological phase type reactor. As the concentration of pollutants decreases in an aquaculture system, the removal rate per unit surface area in a biofilter decreases, hence being able to obtain very high surface areas per unit cost becomes critical. As the concentration further decreases, conventional bioreactors that are either, mixed flow biological phase and mixed flow liquid phase (i.e. moving bed type reactor), or plug flow liquid and fixed biological phase (trickling filter or submerged filter) reach the minimum substrate concentration (S_Min), below which the bacteria cannot grow under steady state conditions. However, in a fine media FBB the discharge concentration can be below S_Min. This allows filters to be designed and operated in commercial aquaculture settings with over 90% removal of NH₃, and related biochemical oxygen demand (BOD) per pass. Fine media FBBs can be designed and operated for biological removal of 99.95% of slow biodegrading refractory organic pollutants like methyl tertiary butyl ether (MTBE) in a single pass with discharge concentrations <1 ppb (inlet 2000 ppb, 20 min contact time, S_Min = 20 ppb). The details of how and why these high performances at low concentrations are possible and why this oligotrophic water quality is desirable for maturation and larva rearing will be discussed.     

Design and management of conventional fluidized-sand biofilters

Fluidized-sand beds are an efficient, relatively compact, and cost-competitive technology for removing dissolved wastes from recirculating aquaculture systems, especially in relatively cool or coldwater applications that require maintaining consistently low levels of ammonia and nitrite. This paper describes several types of flow injection mechanisms used in commercial fluidized-sand biofilters and provides criteria for design of flow distribution mechanisms at the bottom of the fluidized bed. This paper also summarizes the most critical aspects of sand selection, as well as methods for calculating or experimentally measuring fluidization velocities and pressure drop for a given filter sand size distribution. Estimates of nitrification rate, ammonia removal efficiency, carbon dioxide production, and oxygen consumption across fluidized-sand biofilters are also provided for various conditions. Fluidized-sand biofilter operational and management practices are also described.     

Effect of feeding delay after self‐feeder activation on growth performance and feeding behaviour of red porgy (Pagrus pagrus L. 1758): application to submergible cages with surface hopper

Red porgies (Pagrus pagrus) of 20.6±1.5 g mean weight were reared in tanks under four delayed self‐feeding conditions (0=control, 7, 30 and 90 s) after rod activation. The time intervals were chosen to simulate the time taken for a commercial pelleted feed to transit the tube linking a surface‐mounted feed hopper and the depth at which a cage might be submerged. Daily feeding rate, growth, food conversion index and condition factor were quite similar but fish behaviour differed among conditions. Fish remained close to the feeding point in the first two treatments (0 and 7 s delay) but foraged more widely in the remaining treatments (30 and 90 s delay), only reconsolidating around the feeding point a few seconds before feed was released. Results are discussed in relation to learning capacity and adaptation of fish to feeding behaviour flexibility, in order to develop reliable self‐feeding systems suitable for submerged cages.     

Performance of sequencing microbead biofilters in a recirculating aquaculture system

Biological filtration, or biofiltration, is the key technology in recirculating aquaculture systems. Sequencing microbead biofilters, in which the media maintains a continuous up-and-down movement, are based on traditional microbead filters but offer superior filtration properties. The performance characteristics of a sequencing microbead biofilter installed in a recirculating aquaculture system for rearing Barcoo perch at 29 ± 1 °C were examined. The total ammonia-nitrogen (TAN) concentrations and the nitrite-nitrogen concentrations during a 52-day culture period were maintained blow 1.6 mg/L and 0.9 mg/L. In order to ensure efficient biofiltration, the optimal actual application of hydraulic retention time was determined to be approximately 3–5 min. The water flow produced by the reciprocating motion of the media served to wash away suspended solids, ensuring the occurrence of optimal nitrification processes. Additionally, the reciprocating motion of the media enhanced ammonia treatment efficiency significantly by improving the transport of nutrients and nitrification activity. Compared to a static situation the ammonia removal rate increased by 27% based on the application of up-and-down reciprocating movement. The biofilm on the microbead forms as a compact, complex, and homogeneous structure, consisting of numerous microscopic thin sheets. Additionally, a multitude of pores, interstitial voids, and vertical channels were widely observed to convey obviously advantageous properties in support of fluid passage, thus enhancing mass transfer and ultimately contributing to biofiltration effectiveness. The optimum biofilm thickness for providing efficient biofiltration was determined to be approximately 70 μm for this filter.     

Degradation and effect of hydrogen peroxide in small‐scale recirculation aquaculture system biofilters

From an environmental point of view, hydrogen peroxide (HP) has beneficial attributes compared with other disinfectants in terms of its ready degradation and neutral by‐products. The rapid degradation of HP can, however, cause difficulties with regard to safe and efficient water treatment when applied in different systems. In this study, we investigated the degradation kinetics of HP in biofilters from water recirculating aquaculture systems (RAS). The potential effect of HP on the nitrification process in the biofilters was also examined. Biofilter elements from two different pilot‐scale RAS were exposed to various HP treatments in batch experiments, and the HP concentration was found to follow an exponential decay. The biofilter ammonia and nitrite oxidation processes showed quick recuperation after exposure to a single dose of HP up to 30 mg L^?1. An average HP concentration of 10–13 mg L^?1 maintained over 3 h had a moderate inhibitory effect on the biofilter elements from one of the RAS with relatively high organic loading, while the nitrification was severely inhibited in the pilot‐scale biofilters from the other RAS with a relatively low organic loading. A pilot‐scale RAS, equipped with two biofilter units, both a moving‐bed (Biomedia) and a fixed‐bed (BIO‐BLOK^®) biofilter, was subjected to an average HP concentration of ～12 mg L^?1 for 3 h. The ammonium‐ and nitrite‐degrading efficiencies of both the Biomedia and the BIO‐BLOK^® filters were drastically reduced. The filters had not reverted to pre‐HP exposure efficiency after 24 h, suggesting a possible long‐term impact on the biofilters.     

Circadian periodicity of biological oxidation under three different operational conditions

Daily cycles of biological oxidation efficiency were studied in three different biofilters: high-load trickling filter A, 64 kg fish m⁻³, BIO-NET^® material (Norddeutsche Seekabelwerke), 260 m² active surface area per m³ volume; low-load trickling filter B, 1·2 kg fish m⁻³, lower section Hydropak^®-foil (Friedrich Uhde GmbH), 200 m² m⁻³, upper section BIO-NET^® material, 260 m² m⁻³; and low-load submerged rotating contactor (SRC), BIO-NET^® material, 380 m² m⁻³.The dissolved BOD₅ removal efficiency of trickling filter A was dependent on the pH and on the space-load of organic matter. The total ammonia-nitrogen oxidation efficiency decreased directly after feeding from 60% to just over 20% and returned 4 h later to the earlier oxidationrate of 60% fluctuating between 60% and 30% (initial total ammonia-nitrogen concentrations ranged between 0·78 and 2·89 mg N litre⁻¹ 8 h after feeding). This decreasing efficiency was caused by an increasing initial carbonaceous (BOD) level from 4 to 20 mg O₂ litre⁻¹. In the low-load trickling filter B the total ammonia oxidation efficiency ranged between 35% early in the morning to 60%, 10 h after the first feeding. The removal efficiency in the SRC increased constantly from nearly 2% to more than 40% 7 h after the first feeding and decreased during night time to values of about 4%.The degradation efficiency of total nitrogen in both trickling filters fell drastically after feeding (from 75 to 23% and from 88 to 42%). The SRC showed a relatively constant increase from 28% directly after feeding to 58% 7 h later.     

浅海围网养殖鱼类捕捞网箱的设计与试验

为解决浅海围网设施养殖鱼类的捕捞和分级,以浮绳式围网养殖为研究对象,根据养殖围网设施的结构和大小、敷设技术和主要性能,结合围网养鱼方式和养殖生产的特点,设计并制作了一种分级捕捞网箱。网箱由框架钢筋、网衣、绳索、分级栅等构件制作而成,网箱网身呈圆柱形,顶部呈倒圆台形,整体高1 515 mm,体积约为1 m3,分为入鱼口、分级栅、取鱼口等3部分。在实验室用鲫鱼对分级栅进行了试验,结果表明,分级栅间距为15 mm和20 mm时,小规格鱼残留率平均为7.78%和4.75%,大规格鱼逃逸率平均为2.38%和3.00%。海上试验结果显示,大规格鱼捕获率90%以上。研究表明,通过日常投饵养鱼驯化将养殖鱼引诱至网箱内,通过提升网箱、驱使箱内小规格鱼群从分级栅游离,剩下的大规格鱼提出水面后迅速放入活鱼舱,从而达到捕大留小的目的。     

Nitrification kinetics of biofilm as affected by water quality factors

Various types of fixed film biofilters have been used in recirculating aquaculture systems under different water quality and operating conditions. The effectiveness of the nitrification process can be evaluated by nitrification kinetics. Nitrification in the bacterial film of the biofilter involves physical, chemical and biological processes that are governed by a variety of parameters such as substrate and dissolved oxygen concentrations, organic matters, temperature, pH, alkalinity, salinity and turbulence level. The impacts of these parameters upon nitrification kinetics make predicting the performance of a biofilter for a given application an engineering challenge. Knowing the performance of a biofilter is critical for both designers and managers. This paper summarizes the current knowledge on nitrification kinetics as affected by the aforementioned factors based on literature and the results from the authors’ laboratories. These factors were ranked according to their significance of impact on biofilter nitrification performance. The information presented can be used as a reference for the design and operation of biofilters in recirculating aquaculture systems.     

Experimental deepwater culture of the Sydney rock oyster (Crassostrea commercialis): IV. Pilot production of faft oysters

Rafts carrying five permanently submerged oyster trays were moored in nine New South Wales estuaries during 1976–1977 to determine how rapidly culled spat oysters (25–29 g whole weight) or seconds (29–40 g) could be grown into first grade (plate) oysters (40–67 g).In the first series of trials results were obtained from seven localities after 3 months summer growth. The average weight increase was 51%; mortality averaged 17%. A further series of trials were conducted for 3 months in autumn at four localities. Weight increases averaged 40% and mortality 8%.The growth rates calculated from weight increases were two to three times greater than those expected from normal intertidal culture traditional of Australia. The estimated time to convert spat to plate oysters was 4 months for summer and autumn crops.