Water-tank experiment and static numerical analysis of the mooring system of a controllable depth cage

Recent variations in marine environments have increased the risk of aquaculture accidents at sea. This risk can be reduced by installing fish cages at the desired depth, based on environmental conditions such as wave height, the vertical profiles of water temperature, algal concentration, and dissolved oxygen concentration. Most submergible fish cages can be located at only two depths: the sea surface and a submerged depth. In the present study, a fish cage installed at various depths, i.e., a controllable depth cage (CDC), is proposed for avoiding undesirable environments. A water tank experiment is conducted to measure the drag of the cage and the static deformation of the mooring system using a scale model of the actual cage. Then, a simple numerical model based on the balance of forces on each component is developed to analyze the position and the attitude of the cage and the mooring tension of the system. The numerical model is verified by comparing the experimental and numerical results. The outcome showed that the cage and floats moved downstream at an increasing velocity. The results of the numerical simulation supported those of the water tank experiment. However, the simulated vertical positions of a cage and floats were higher compared with experimental results. Additionally, the inclination of angle increased alongside increasing velocity in the numerical simulation, whereas a complex variation was observed in the experiment. This happened because of underestimating the drag on the mooring rope in lower water current velocities; additionally, cage lift was not considered in the numerical model. Despite these discrepancies, the tension of each mooring rope was well predicted because of the dominant tension of the horizontal component. In future studies, the balance of forces on the rope should be predicted more precisely, and variations in cage drag and inclination angle should be included in the numerical model. Additionally, the effect of waves should be considered alongside water currents to ensure the safety of the CDC.     

An automatic fishing machine based on acoustic conditioning

An automatic fishing machine based on acoustic conditioning was developed and tested in a reservoir. The floating fishing machine is remotely and automatically controlled to operate an underwater speaker, a feeder and underwater cameras that monitor fish behavior in real time. An open net pen installed under the system can be automatically closed to capture gathering fish. The dynamics of fish populations in the open net pen was monitored and discussed. Fish were conditioned to associate an acoustic signal with food by classical conditioning. Periodic retraining was used to reinforce the conditioned behavior of fish freely swimming in the reservoir. Fish were captured by calling them to the fishing machine using the acoustic signal and automatically closing the net pen around them: 2.5 times more fish were captured in such capture events than in control events, in which the net pen was automatically closed without calling the fish.     

水流作用网衣过程的数值模拟

网衣是深水网箱的主要组成部分,也是整个网箱系统中受力最为复杂的部件之一。文章介绍了水动力作用下一种基于集中质量法的网衣数学模型,并引用前人的试验结果对数学模型进行了验证。在此基础上,利用此数学模型在不同配重（CW1=400kg和CW2=800kg）和流速（U=0．3—0．6m·s^-1）条件下分别对网衣所受的水流力、网衣形状和网衣运动位移三者随时间的变化过程进行了数值模拟,给出了网衣达到稳定状态前、后网衣受力变形的计算结果,揭示了网衣在水流作用过程中的动态变化规律,并进一步分析了配重及流速大小对网衣受力、变形及运动特性的影响。     

Examination of net volume reduction of gravity-type open-net fish cages under sea currents

Owing to heavy criticisms of nearshore fish farming for causing environmental pollution and encroaching on sea space used for shipping, boating, recreational sea activities and marine eco-tourism, offshore fish farming has now being seriously considered. Moreover an offshore site provides more pristine water and greater space for increased fish production. However, offshore fish farming poses challenges such as a more energetic sea environment. A higher sea current can lead to large deformation of fish net and hence a net volume reduction which compromises fish welfare. With the view to identifying the effects of various important parameters on net volume reduction of a gravity-type open-net fish cage, this paper adopts a mass-spring model for the dynamic analysis of current-induced net deformations of cylindrical fish nets with discrete weights hanging at the bottom edge of the nets. In this model, the net mesh comprises knot nodes and bar nodes connected by tension-only massless springs. The spring stiffness is determined from the net bar diameters and material properties. The current-induced loads are applied to each node and calculated based on Morrison’s equation. The governing equation system for nodal motions can be established according to Newton’s second law, and solved by using the Runge-Kutta method for the real-time net deformations. The effects of net string reinforcements, weight distributions and net shapes on the net volume reduction are studied with the view to shed insights into how one may improve fish cage designs to effectively mitigate net deformation under high sea current speeds in offshore fish farming sites.     

水流作用下重力式网箱浮架结构的动力响应研究

近年来深水网箱养殖设施得到了广泛的应用,浮架作为网箱结构的重要组成部分,其安全性对网箱的设计至关重要。基于有限单元法采用SHELL单元建立了浮架结构的荷载-变形数值模型,对水流作用下浮架的应力和变形进行了数值模拟,并开展物理模型试验对该数值模型进行验证。结果表明,该数值模型可以准确模拟浮架的变形,采用该数值模型,分析了不同流速条件下浮架的变形和应力,数值模拟结果显示:随着流速的增加,浮架的变形和应力逐渐增加。相同流速条件下,注水下潜后,浮架的应力和变形能够显著减小;因此,在强流条件下,可以采用浮架注水的方式,使浮架处于下潜状态,以改善浮架结构的变形和应力分布。     

波流作用下深水网箱受力及运动变形的数值模拟

基于已建立的浮架和网衣数学模型,对不同波况和流速共同作用条件下HDPE深水网箱所受的锚绳力、波流力、容积损失率以及浮架倾角进行数值计算,设计的波流要素值为:波高H=4～6 m,周期T=6.0～8.6 s,流速U=0.3～0.9 m/s.结果表明,网箱锚绳受力、波流力和容积损失率均与波高和流速成正比,与周期的关系不明显,且网箱系统所受的波流力约为网箱迎浪侧两根锚绳受力的合力.在波高H=4～6 m、流速U=0.75 m/s时,网箱容积损失率达到47%～56%,网箱变形较为严重,为此建议网箱养殖区域应选择流速小于0.75 m/s的海区较为适宜.周期对网箱容积损失率的影响很小,对浮架倾角的影响较为明显,波高和流速不变时,随着周期的增大,浮架倾角会有所减小.本研究旨在探讨波浪流对深水网箱受力及运动变形的影响,为高海况网箱养殖的风险评估提供参考依据.     

The Influence of Stocking Density on the Swimming Behavior of Adult Atlantic Cod,Gadus morhua,in a Near Shore Net Pen

Cage design and stocking density are important aspects of aquaculture farm design, therefore understanding how fish behave at different stocking densities is critical information for farm managers. In this study, high resolution acoustic telemetry was used to investigate the swimming behavior of adult Atlantic cod, Gadus morhua, that were stocked at four densities (5, 10, 25, and 45 kg/m³). Acoustic tags were placed into the abdominal cavity of five fish per density treatment so their swimming behavior could be continuously monitored throughout the study. An array of hydrophones made it possible to calculate the position of each fish in three dimensions, at ～2–5 sec intervals, for 4–30 d. Three underwater cameras were used to obtain additional data about the distribution of fish in the cage during the daytime. At the lowest density, the cod spent the majority (64.3 ± 0.08%) of their time in the bottom third of the net pen. As density increased, the fish moved higher in the water column, and this behavior was most evident at night, at all densities. At no time throughout the entire study were there any obvious occurrences of schooling behavior, even at the highest density (45 kg/m³).     

HDPE圆形重力式网箱受力变形特性的数值模拟

该研究旨在综合探讨不同网箱周长、浮管管径、网衣高度及网目大小对整体网箱受力变形的影响,为网箱的科学合理选型提供数据参考。设定的网箱周长40—80m,浮管管径250～630mm,网衣高度6～20m,网目大小45～115mm。通过数值模拟方法对4种规格高密度聚乙烯圆形网箱在不同组合条件下网箱锚绳受力、波流力以及容积损失率进行了数值计算。结果表明,大规格网箱的锚绳受力、波流力更大,容积损失率更小,锚绳数量的增加可以大大降低锚绳受力。相比浮管管径,网衣高度和网目大小对网箱受力变形的影响更显著。整体网箱的受力变形随着网衣高度的增加而增大,随网目的增大而减小。     

Numerical simulation of the effects of structure size ratio and mesh type on three-dimensional deformation of the fishing-net gravity cage in current

In our previous research, the hydrodynamic behavior of a two-dimensional (2D) plane fishing net in current was simulated. Based on the research, a model of a three-dimensional (3D) net is established by using the lumped mass method. To verify the validity of the numerical model, model test results by other authors are cited and compared with the numerical results. The simulated results are in good agreement with experimental ones. In this paper, the 3D net model is applied to investigate the effects of structure size ratio (RDH) and mesh type on the 3D net deformation of the gravity cage in current. The numerical results indicate that the decrease of RDH is practically feasible in improving the cage net deformation. With a sinker system the net deformation with diamond mesh is greater than that with square mesh. When the bottom-collar sinker system is applied, with an increase in current velocity, the net deformation with diamond mesh is less than with square mesh. The results of this study provide a better understanding of the hydrodynamic behavior of the gravity cage.     

Typical hydrodynamic models for aquaculture nets: A comparative study under pure current conditions

The net is regarded as the most critical component in marine aquaculture facilities as it is the only barrier which protects the environment from fish escapes. Accurate predictions of the net cage deformation and drag force on the nets are needed, both for ensuring fish welfare and for dimensioning of the mooring system. Thus, an appropriate hydrodynamic model is essential. In practice, two types of hydrodynamic force models, i.e., the Morison type and the Screen type, are commonly used to calculate the hydrodynamic forces on nets. Application of the models depends on the underlying structural model and the availability of data. A systematic review of hydrodynamic models is therefore undertaken to compare the models and various parameterisations, in aid of model selection during the design. In this study, eleven commonly used hydrodynamic models, i.e., five Morison models and six Screen models, are reviewed comprehensively, and implemented into a general finite element (FE) solver for dynamic simulations. Sensitivity studies on different current velocities, inflow angles and solidities of the nets are carried out. Moreover, different wake effects are also considered in numerical simulations. The numerical results from different models are compared against existing experimental data under pure current conditions. Suggestions for selection of suitable hydrodynamic models are provided, based on the model comparison.     

Effects of water flow velocity and fish culture on net biofouling in fish cages

The effects of water flow, fish feed and cage position on net biofouling was examined in a floating cage fish farm. Fouling of 16 mm mesh net panels suspended inside and outside net cages and exposed to different treatments were monitored weekly until net apertures were completely occluded by the fouling organisms (8 weeks). Results indicate a dramatic reduction in water flow velocity throughout the fish farm due to the cage units themselves and net biofouling. The reduced water flow (<10 cm s^?1) inside net cages promoted rapid net biofouling, while rapid water flow outside the net cages (>25 cm s^?1) kept the net fouling organisms at bay. Although fish rearing in net cages with inputs of commercial pellet feed increased sessile biofouling (222% higher than outside the net cages) and non‐sessile biofouling (570% higher), the type of fish feed used did not significantly affect biofouling development. The study recommends that the geometry of serially arranged net cages, as commonly deployed in tropical tidal estuaries, be reconfigured to improve flow through in order to minimize the impact of fouling.     

Numerical simulation of the flow field inside and around gravity cages

A three-dimensional (3D) numerical model is established to simulate the flow field inside and around the gravity cages in a current. The realizable k–ɛ turbulence model was chosen to describe the flow, and the governing equations are solved by using the finite volume method. In the numerical model the cylindrical cage is divided into 16 plane nets around the circumference and a bottom net, and the net is modeled using the porous media model. The unknown porous coefficients are determined from the hydrodynamic force on the net under different flow velocities and attack angles using the least squares method. In order to validate the numerical model, the numerical results of the plane nets were compared with the data obtained from two physical model tests. The comparisons show that the numerical results are in good agreement with the experimental data. Using the present model, this paper presents the flow field inside and around the gravity cages with different spacing distances and cage numbers. This study provides information about the flow field inside and around the fishing net cages.     

Automatic submerging and surfacing performances of model submersible fish cage system operated by air control

An automatic, submersible fish cage system using air control was developed and a set of model experiments were conducted to examine the automatic submerging characteristics of the cage. The components of the fish cage consist of a rigid frame assembly with 6 variable ballast tanks and 6 fixed ballast tanks. The variable ballast tanks were used to change the buoyancy characteristics of the system by air control so that the fish cage can either be placed at the surface or submerged. The cage is free to move vertically within a water column by adjusting the weight and the buoyancy with an air control system. The model of this system, with dimensions of 2.20 m in diameter and length and 1.04 m in net cage depth, was constructed to be 1/10 the size of the full-scale system. In the model experiments, the submerging and surfacing characteristics of the cage were regulated with measurements from a water-pressure gauge and a gyroscope incorporated into the automatic control system. Model tests were performed in a still water tank and a large wave tank to develop the algorithm required to control the cage system and to verify the ability of the automatic submersion mechanism to function. The control system was designed so that when the variable ballast tanks were flooded with water, the model descended. To raise the system, compressed air is injected into the tanks by opening the main evacuation valve on the manifold. After the required amount of compressed air is supplied, the main evacuation valves can be shut and as a result, the fish cage becomes buoyant. Measured performance results in a still water tank are then compared with calculations from a previously developed numerical technique. The submerging and surfacing characteristics of the fish cage were relatively similar to the measurements obtained with the physical model experiments using air control. The cage was submerged to a target depth when incidence wave heights were higher than the critical wave height and raised when little wave actions were detected in a wave tank. On the other hand, the cage was placed at the surface when incidence wave heights were the same as the critical wave height or lower.     

Installation,operation and evaluation of a submerged cage at 45M depth in crete for the rearing of red porgy Pagrus pagrus

Socio‐economic factors related to management of costal zones and biological parameters related to species‐specific requirements set limitations for the rearing location of net‐pen sea cages. A cage was designed and installed at 45 m depth based on the REFA Tension Length Cage technology at the cage facility of the Hellenic Centre for Marine Research in Crete. The cage‐net module consisted of a net‐pen contained within a hexagonal rigid frame to maintain the shape of the net. Vertical beams of the frame glider on six tension legs were placed in a circle. The operation of the cage was evaluated using a group of red porgy (Pagrus pagrus) for a rearing period of 4 months whereas a second group reared in a surface net‐pen cage served as control. Growth performance was similar between the experimental groups. The rearing method significantly affected fish skin pigmentation. Individuals reared in the submerged cage had a brighter skin colour and lower skin melatonin content compared with the ones reared in the surface cage. The presented submerged cage may provide a feasible alternative for rearing fish in exposed areas and particularly species requiring specific biological conditions, such as narrow temperature ranges and low light intensity.     

Growth of pen reared pink salmon fry,Oncorhynchus gorbuscha,feeding on available marine zooplankton

Pink salmon fry, Oncorhynchus gorbuscha, were held in a small floating net pen for nearly 7 weeks. The pen was moored in a narrow channel where tidal current velocities reach values near 0.4 m/s. The fry were maintained solely by marine zooplankton carried into the enclosure by surface currents.Samples of approximately 100 fry were removed weekly from the pen to monitor growth and diet. For the 1049 fry which began the experiment, fork length ($\text{FL}$) averaged 32.7 mm and wet weight ($\text{W}$) 0.23 g. Forty-eight days later 330 fry remained in the pen; $\text{FL}$ had increased to 44.1 mm and $\text{W}$ to 0.76 g. Natural mortality in the pen was less than 2% during this period.     

多波束深水网箱声学监测仪的研究

研究了一种声学监测设备,使网箱养殖者能在海水能见度较低时实现对鱼群的实时监测,进而掌握深水网箱中鱼群的生存状况。该声学监测仪由7个独立通道构成,能同时输出探测图像并通过监测仪的评估软件对数据进行分析。试验结果表明该设备能够对深水网箱中的生物量进行估计,可用于鱼类生长情况研究并监测鱼群的逃逸情况。     

圆形网衣在水流作用下的运动变形特性

研究网衣在水流作用下的运动变形是深入了解网箱耐流特性的关键。本研究基于集中质量点法,建立了圆形网衣在水流作用下的数值计算模型,并利用前人的试验结果对数值模型进行了验证。验证结果表明,无论是网衣的变形还是受力情况,模拟结果都与试验结果吻合良好。在此基础上,针对实际应用中的圆形网衣,在不同配重(GW1=400kg、GW2=800kg)和流速(U=0.3～0.9m/s)条件下,对网衣变形、网衣底端前后点位移、网衣底端倾角进行了数值模拟,探讨了流速、配重大小对网衣运动变形特性的影响。结果表明,水流速度的增大会加剧网衣的变形,在配重GW1、流速U=0.75m/s时,网衣容积损失率达到50%以上。网衣底端前点的水平位移大于后点的水平位移,前点的垂直位移则小于后点的垂直位移,随着流速的增大,网衣底端前后点的位移差异将更为明显。当配重增加时,网衣底端位移、倾角以及网衣容积损失率可相应减小,网衣变形得到一定改善。数值模拟证明,本研究所建立的数值计算模式具有较好的稳定性和解的收敛性,可为下一步波浪、流联合作用下的整体网箱模拟奠定良好的理论基础。     

Control of dissolved oxygen levels of water in net pens for fish farming by a microscopic bubble generating system

ABSTRACT:   A microscopic bubble generating system (MBGS) has been developed to control dissolved oxygen (DO) levels suitable for fish farming. The MBGS has been tested to confirm its capability in net pens. Water conditions in a fish farm were monitored every two hours from June to October 2004 by setting an online vertical profiling system (OVPS) close to the net pen. DO in the net pen water decreased to physiologically stressful levels for the fish during the night (4.84–5.51 mg/L), while the DO was kept in saturated conditions during the day, due to oxygen supply from phytoplankton. The MBGS was operated from the evening to the morning of the next day for 16 h, to successfully create DO-saturated conditions in the net pen water at night. By using microscopic bubbles during the warm seasons, DO levels in the net pen water could be improved to a level suitable for fish farming. However, the low DO levels (<5.0 mg/L) of the bottom water occasionally extended to the net pen layers, despite the supply of microscopic bubbles to the water. To maintain the DO of the net pen water at levels suitable for fish farming, DO supply to the net pen water and the bottom water needs to be increased, and the organically enriched sediment just below the net pens needs to be treated.     

波浪作用下一种鲆鲽类方形网箱水动力特性数值模拟研究

针对鲆鲽类方形重力式网箱在纯波浪条件下的水动力特性,采用数值模拟的方法对鲆鲽类方形深水重力式网箱的主要部件——浮架系统、配重系统、锚碇系统进行了模拟。将模拟结果与实验结果进行分析比较表明,数模与物模各量值吻合良好,平均相对误差均不超过9%,表明数值模拟的方法能较好地模拟鲆鲽类方形深水重力式网箱的水动力特性。在此基础上,用数值模拟的方法,进一步研究了底框质量和网衣高度的改变对网箱各参数的影响,模拟结果显示,底框质量改变主要影响锚绳受力和底框倾角,底框质量从80 g升至100 g再升至140 g的过程中,锚绳受力平均增幅分别为13%和19%,同时增大底框质量有助于减小底框运动倾角,其平均降幅分别为8.8%和9.3%;网衣高度由20 cm增至30 cm后,锚绳受力平均增加15%,同时网衣的增高导致对浮架和底框的牵制作用加大,使得浮架和底框的运动幅度有所减小,降幅不超过10%。在以上研究结果的基础上,为鲆鲽类网箱的设计与优化提供了参考建议。     

深水网箱养殖中的声学监测问题探讨

针对深水网箱养殖过程出现的一些诸如网衣安全、鱼类逃逸等问题,介绍了几种类型的监测技术应用:(1)声学警戒带方式构成的被动式网衣安全监测技术,监视声纳可以是单波束,也可以是机械扫描的多波束或电子扫描的多波束,同时采用水下机器人进行巡视;(2)基于网箱中鱼的目标强度,实现对网箱中养殖品种大小、数量的统计监测,鱼的目标强度主要取决于鱼的体态特征,同时也与发射声波的波长有关;(3)数量识别技术以及饵料投饲过程中的声学监测技术,通过将声纳输出信号反馈到投饵机,实现饵料投放自动化。