Net Panels Research Articles

Flow Turbulence Characteristics and Mass Transport in the Near-Wake Region of an Aquaculture Cage Net Panel

Cage-based aquaculture has been growing rapidly in recent years. In some locations, cage-based aquaculture has resulted in the clustering of large quantities of cages in fish farms located in inland lakes or reservoirs and coastal embayments or fjords, significantly affecting flow and mass transport in the surrounding waters. Existing studies have focused primarily on the macro-scale flow blockage effects of fish cages, and the complex wake flow and associated near-field mass transport in the presence of the cages remain largely unclear. As a first step toward resolving this knowledge gap, this study employed the combined Particle Image Velocimetry and Planar Laser Induced Fluorescence (PIV-PLIF) flow imaging technique to measure turbulence characteristics and associated mass transport in the near wake of a steady current through an aquaculture cage net panel in parametric flume experiments. In the near-wake region, defined as ~3M (mesh size) downstream of the net, the flow turbulence was observed to be highly inhomogeneous and anisotropic in nature. Further downstream, the turbulent intensity followed a power-law decay after the turbulence production region, albeit with a decay exponent much smaller than reported values for analogous grid-generated turbulence. Overall, the presence of the net panel slightly enhanced the lateral spreading of the scalar plume, but the lateral distribution of the scalar concentration, concentration fluctuation and transverse turbulent scalar flux exhibited self-similarity from the near-wake region where the flow was still strongly inhomogeneous. The apparent turbulent diffusivity estimated from the gross plume parameters was found to be in reasonable agreement with the Taylor diffusivity calculated as the product of the transverse velocity fluctuation and integral length scale, even when the plume development was still transitioning from a turbulent-convective to turbulent-diffusive regime. The findings of this study provide references to the near-field scalar transport of fish cages, which has important implications in the assessment of the environmental impacts and environmental carrying capacity of cage-based aquaculture.

Improving species and size selectivity in the Baltic cod trawl fishery with two simple codend designs

The eastern Baltic cod (Gadus morhua) stock is at a critically low level and recruitment of the western Baltic cod stock is at the lowest observed level in over 30 years. The EU (European Union) landing obligation was introduced in 2015 for cod and in 2017 for plaice (Pleuronectes platessa). Size selective codend designs have been used in the Baltic cod trawl fishery for 25 years; however, there are still relatively high discard rates for cod, plaice and European flounder (Platichthys flesus; henceforth flounder) in the trawl fishery and landings of individuals below the minimum conservation reference size (MCRS). These problems call for additional improvements of species and size selectivity in the cod trawl fishery. In this study, two trawl codend designs were developed and tested to improve species and size selectivity in the Baltic cod trawl fishery. The codends were tested onboard a commercial trawler and the test codends were compared to the conventional Bacoma codend, which was fished simultaneously in a twin trawl rig. New codend designs were based on the Bacoma design, where the first test codend had a bottom net panel made of larger diamond meshes (nominal 130 mm mesh size) to improve selectivity in a cod fishery with high catch rates of flatfish. This codend showed a reduction of plaice and flounder, and cod below 45 cm, in the catches. The other test codend was a full square mesh codend (nominal 120 mm mesh size) developed for a cod directed fishery with low flatfish catches, and showed a marked reduction in the catch of cod below 50 cm, with no differences for flounder and plaice catches. With cod stocks at critically low levels and relatively high discard rates of cod, plaice and flounder, the simple codend designs tested in this study could benefit the sustainability of the Baltic cod trawl fishery, but with some loss of commercially targeted sizes.

Experimental analysis of hydrodynamic forces on net panel in extreme waves

As fish farms move from nearshore region to open sea, understanding the hydrodynamic force on net structures in extreme waves plays a significant role in the design of offshore aquaculture structures. In this study, a series of experiments on different types of net panels in extreme waves were conducted in a wave flume. The dispersive focusing method with the constant steepness spectrum was adopted to generate extreme waves and the effects of the net solidity, the net material and the knot pattern of net panels were experimentally analyzed. The experimental results indicated that the force on the net panel in extreme waves was heavily dependent on the net solidity and the KC number, and it was slightly dependent on the net material. A concept of quasi-static forces was adopted to establish the wave force model to predict the forces on net panels in extreme waves, and then, a quadratic function describing the dependence of the drag coefficient on the solidity and the KC number was obtained. The results indicated that the wave force model could simulate the loading process of the submerged net panels in extreme waves very well.

A PANEL DYNAMIC ANALYSIS ON ENERGY CONSUMPTION, ENERGY PRICES AND ECONOMIC GROWTH IN NEXT 11 COUNTRIES

This study empirically examines the impacts of energy consumption and energy prices on economic growth for three data sets of panel groups of Next 11 countries within a multivariate panel framework over the period 1990-2013. For the entire panel group of Next 11 countries, it is obtained from the results of panel Autoregressive Distributed Lag estimation approach that, energy consumption has a positive and energy prices have a negative impact on economic growth, both in the long-run and short-run; and a unidirectional causal linkage between economic growth and energy consumption is confirmed by the results of panel causality tests. For the 6 of Next 11 net oil-importing countries panel group, it is found from panel Dynamic Ordinary Least Squares method that, economic growth is negatively influenced by both energy consumption and energy prices. The results of panel Autoregressive Distributed Lag estimation approach for the 5 of Next 11 net oil-exporting countries panel group portrays that, in the long-run, energy consumption negatively affects economic growth and energy prices positively influence economic growth, while in the short-run, economic growth is negatively affected by both energy consumption and energy prices.Keywords: Energy Consumption; Energy Prices; Economic Growth; Next 11 CountriesJEL Classifications: C33; C51; Q4; Q43DOI: https://doi.org/10.32479/ijeep.9880

A study of approach to assessing a functional relationship between solidity ratios for the rope/net part of midwater trawl

In this paper we have analyzed an approach to assessing the relationship between some characteristics of the net panels forming the rope/net cone of a fishing trawl. Using the mathematical relationship of solidity ratios for the rope/net panels, it is possible to analyze existing structures and, furthermore, reduce the time for selecting the prototype trawl. Such mathematical relationship allows faster calculation of new trawl designs which takes into account the production specifics of an enterprise manufacturing the trawl fishing gear. The use of mathematical relationships provides the opportunity to accelerate the process of trawl construction.

Loads and response on flexible conical and cylindrical fish cages: A numerical and experimental study based on full-scale values

This paper presents drag forces from uniform water current on two flexible net cage designs, commonly used in Norwegian fish farming, obtained through model-scale testing and numerical simulations. The primary focus is on the comparison between model tests and numerical simulations and performance of the numerical model for the different net cage designs. The two designs were a cage with a cylindrical main section with straight walls and conical bottom with a sinker tube weight system and a cage where both the main section and bottom has a conical shape. The latter cage has a central weight instead of a sinker tube. The effect of the governing parameters was explored by varying the design and loads of the weighting system. Both physical tests and numerical simulations revealed increased drag forces and reduced ability for deformation when increasing the load from the weighting system. Lower weight system load or weight system designs that allow for more deformation decrease the loads on the net cage. Although it was not a focus in the present work, fish welfare is dependent on sufficient available volume in the net cage, meaning that large reductions in volume to reduce drag forces at high current velocities may be undesirable. Comparisons between model scale tests and numerical simulations reveal good agreements for the highest weight system loads tested. Decreasing the weight system loads decrease the accuracy of the numerical simulations. This may be caused by larger deformations of the net cage when the bottom loads are reduced resulting in a larger number of net panels with small angles of attack relative to the incident current. Predictions of forces for low angles of attack may be less accurate than for larger angles of attack while there is a possibility of increased flow velocity reduction (wake effect) and flow deflection. The cages were tested and simulated for flow velocities up to 1.25 m/s (full scale value). Measurements of the flow velocity in the middle of the net cages revealed a higher velocity reduction than predicted with theory, with the exception of the highest velocity.

Biofouling Community Development on Commercial Cage Nets Submerged in a Cage Aquaculture Facility in the Red Sea

The colonization of biofouling organisms is a significant problem for the aquaculture installations which affect the economy of the fish farms and wellbeing of the cultured fish. In this study, the colonization of biofouling organisms on cage aquaculture nets submerged near to the existing floating cages was analysed for a period of one year from June 2017 to May 2018. Cage net panels were fitted onto an iron frame and submerged at 1 m depth near to existing aquaculture cages. The biofouling organisms found on the panels mainly consisted of macroalgae, bivalves, bryozoans, hydrozoans and ascidians. Some other epifauna like brittle stars, polychaetes, amphipods, crabs, gastropods were also occurred more occasionally on the nets. Recruitment of green algae and brown algae were observed from the first month of net panel exposure. Bivalves and ascidians were developed from the second month. Hydrozoans and bryozoans appeared on the nets after 7 months of panel deployment. Results also revealed that ascidians and bivalves were the dominant fouling organisms in most of the sampling months during this study period.

Evaluation of Alligator Gar Broodfish Collections in the Lower Mobile–Tensaw Delta, Alabama

AbstractIn Alabama, Alligator Gar Atractosteus spatula populations have experienced a decline in abundance and due to their limited distribution are classified as a nongame species of moderate conservation concern. Conservation management plans have included creel limits and stocking to augment a diminished population within the lower Alabama River. Our objective was to evaluate gill‐net efficacy on catch and the environmental variables associated with improving broodfish capture rates for hatchery propagation. Broodfish collections were initiated and completed over 75 sample dates between March 2005 and April 2015. Collectors utilized multifilament gill nets consisting of three mesh sizes (101.6, 127.0, and 152.4 mm) and three net lengths (22.9, 30.5, and 61.0 m). Gill nets (n = 569) expended 2,173 h of soak time, resulting in the capture of 101 individuals (TL range = 1,020–2,210 mm). Catch was highest for the smallest mesh size, but net panel length did not influence catch rates. Median fish lengths increased with larger mesh sizes. Catch declined more than sevenfold for net soaks of 1 h or more, and the proportion of successful gill nets declined threefold for longer soak times. Examination of environmental conditions during and prior to the setting of gill nets demonstrated that fish capture success was in part influenced by various aspects of tidal current and water temperature. Catch success was 68% when nets were set at water temperatures of at least 20°C and when at least 50% of net soak time occurred during rising tides. However, success was lower when nets were set at temperatures less than 20°C, capturing Alligator Gar in 44% of all soaks and 37% for those on falling or slack tides. Agencies involved in hatchery propagation programs should continually evaluate field sampling methods, use of collection gear, and environmental conditions to increase broodfish catch rates and improve staff efficacy.

Biofouling development in marine fish farm influenced by net colour, immersion period and environmental conditions

Biofouling poses low productivity and high maintenance in aquaculture. This study simulated the nets of fish cages to investigate biofouling composition and development as influenced by net colour, immersion period and environmental conditions. Yellow and green net panels were immersed for 60 days and sampled at 7, 15, 30, 45 and 60 days. Results revealed that biofouling composition was dominated by Bryozoan Bugula sp. (31%–38% cover), tanaids Tanaidae (64–146 individuals.100 cm-2) and nematodes Linhomoeidae and Oncholaimidae (142–189 individuals.100 cm-2). Biofouling development was observed in succession patterns of harpacticoid–nematode–amphipod–tanaid in yellow nets and amphipod–nematode in green nets. Water velocity within the cages significantly and positively influenced the total cover, biomass and the densities of amphipods, bivalves, polychaetes, fish eggs and bryozoans. Polychaetes, gastropods, fish eggs and macroalgae were present in green nets and absent in yellow nets at 7 days. Uniformity of biofouling assemblage in both net colours was observed beyond 7 days which may be largely explained by brown biofilm formation. The result of the study revealed the dynamics of biofouling in a marine fish farm and provides vital information for developing strategies in biofouling management to lessen its impacts.

A Lagrangian approach for the coupled simulation of fixed net structures in a Eulerian fluid model

A Lagrangian approach for the coupled numerical simulation of fixed net structures and fluid flow is derived. The model is based on solving the Reynolds-averaged Navier–Stokes equations in a Eulerian fluid domain. The equations include disturbances to account for the presence of the net. For this purpose, forces on the net are calculated using a screen force model and are distributed on Lagrangian points to represent the geometry of the net. In comparison to previous approaches based on porous media representations, the new model includes a more physical derivation and simplifies the necessary numerical procedure. Hence, it is also suitable for arbitrary geometries and large scale simulations. An extensive validation section provides insight into the performance of the new model. It includes the simulation of steady currents through single and multiple fixed net panels and cages, and wave propagation through a net panel. Different solidities, inflow velocities and angles of attack are considered. The comparison of loads on and velocity reductions behind the net with available measurements indicates superior performance of the proposed model over existing approaches for a wide range of applications.

Optimising targets for tsetse control: Taking a fly's-eye-view to improve the colour of synthetic fabrics.

The savannah tsetse flies, Glossina morsitans morsitans and G. pallidipes, are important vectors of Rhodesian human African trypanosomiasis and animal African trypanosomiasis in East and southern Africa. We tested in Zimbabwe whether robust, synthetic fabrics, and innovative fly’s-eye-view approaches to optimise fabric colour, can improve insecticide-treated targets employed for tsetse control. Flies were caught by electrocution at a standard target comprising a 1m x 1m black cotton cloth panel with 1m x 0.5m black polyester net panels on each side. Catches were subdivided by species and sex. Tsetse catches were unaffected by substitution of the black cotton with a blue polyester produced for riverine tsetse targets. Exchanging the net panels for phthalogen blue cotton to simulate the target routinely used in Zimbabwe significantly reduced catches of female G. m. morsitans (mean catch 0.7 times that at standard), with no effect on other tsetse catches. However, significantly greater proportions of the catch were intercepted at the central panel of the Zimbabwe (means 0.47–0.79) versus standard designs (0.11–0.29). We also engineered a new violet polyester cloth using models of tsetse attraction based upon fly photoreceptor responses. With and without odour lure, catches of females of both species at the violet target were significantly greater than those at standard (means 1.5–1.6 times those at standard), and typical blue polyester targets (means 0.9–1.3 times those at standard). Similar effects were observed for males under some combinations of species and odour treatment. The proportions of catch intercepted at the central panel of the violet target (means 0.08–0.18) were intermediate between those at standard and typical blue polyester. Further, the reflectance spectrum of violet polyester was more stable under field conditions than that of black cotton. Our results demonstrate the effectiveness of photoreceptor-based models as a novel means of improving targets to control tsetse and trypanosomiases.

Hydrodynamic performance of bottom trawls with different materials, mesh sizes, and twine thicknesses

Nowadays, the fishing industry faces several constraints such as continuous increase infuel price and decrease in fish stocks. Bottom trawl efficiency is mainly affected by the drag force of trawl net and sweep area during fishing operations. This study focuses on the drag of bottom trawl models constructed from traditional and new materials. A nylon net panel, a traditional material, was used in two model trawl nets, namely, trawl nets 1 and 4; the geometric shape ofwing net on trawl net 4 was a triangle. Considering the physical characteristics of the new materials such as Dyneema and nylon monofilament, net panels with a large mesh size and small twine stiffness were used, respectively. The trawl nets were named as trawl nets 2 and 3, respectively. The drag forces provided by the standard trawl (trawl net 1) were 57%, 56.2%, and 47% less than those of trawl nets 2,3, and 4 respectively. The drag coefficient of trawl net 1 was 40.68%, 30.35%, and 33.96% greater than those of trawl nets 2, 3, and 4, respectively. The results also show that these three trawl net models are geometrically efficient in terms of net mouth height than a standard trawl net. This indicates that the drag force of bottom trawl net with a new material was much lower than those made with a traditional material. The developed equations in this study provide the basis for a drag and geometry prediction model for a bottom trawl. The advantage of this predicted drag equations over previous net drag formulas is that it can be evaluated using only information available from a net drawing (mesh size and twine diameter), so that the net drag of bottom trawl net of a reasonably standard shape can be estimated at the design stage.

A novel method for modeling age and length selectivity of sockeye salmon as applied to the Bristol Bay Port Moller test fishery

This paper presents a novel approach for assessing sources selectivity in test fisheries using the Port Moller test fishery (PMTF) as a case study. The PMTF intercepts sockeye salmon (Oncorhynchus nerka) migrating to Bristol Bay, Alaska, to estimate run strength and timing. In 2011, the mesh size of gillnets used in the test fishery was decreased for half of the net panels to generate more accurate run estimates by correcting for greater selectivity of larger 3-ocean fish (fish that have spent 3 years in the ocean) relative to smaller 2-ocean fish (fish that have spent 2 years in the ocean). Here, we quantify two sources of age selectivity in the PMTF program, length selectivity parameterized by mean fish length (which should be corrected by the net mesh change) and length-independent selectivity, which we refer to as residual program selectivity (which would not be impacted by the net mesh change), both before and after the net change. Model parameters of selectivity show strong support length selectivity was eliminated, but residual program selectivity still existed after the reduction in net mesh size. Our results demonstrate the necessity of considering both vulnerability and accessibility to fishing gear when assessing selectivity in test fisheries.

Experimental analysis of the hydrodynamic force on the net panel in wave

The hydrodynamic force on net panels in waves plays a significant role in the wave-resistant design of net cages in the open sea. To investigate the hydrodynamic force on net panels in waves, a series of experiments about different types of net panels in waves were conducted in a wave flume. The effects of the net solidity, the net material and the knot pattern were experimentally analyzed in this study. A wave force model for the net panel was proposed based on the hydrodynamics of cylinder in waves. The experimental results indicated that the horizontal force on the net panel in waves was larger than the vertical force, and the horizontal force was heavily depended on the KC number; In addition, the horizontal force on the steel net was smaller than that on the nylon net, and the knot pattern had a negligible influence on the hydrodynamic force on the net panel in waves. The model was also compared with existing experimental measurements and previous numerical results. The comparison results indicated that the wave force model could calculate the wave force on the net panel accurately.

High contrast panels and lights do not reduce bird bycatch in Baltic Sea gillnet fisheries

Bycatch is a cause of mortality among marine mammals, sea turtles, fish and birds. For some species this mortality may be sufficient to cause population declines. The Baltic Sea is a global ‘hotspot’ for bird bycatch in gillnet fisheries and is globally important for wintering sea ducks, but no technical solution has been found yet to reduce bird bycatch in gillnet fisheries in the Baltic. Here, we report on trials conducted in the Baltic Sea to test whether two different gillnet modifications with visual stimuli can effectively reduce bird bycatch while maintaining volume of fish caught. We conducted paired trials of two types of visual stimuli attached to nets: 1) high contrast monochrome net panels and 2) net lights (constant green and flashing white LED lights). We measured the amount of fish and birds caught in standard nets and those modified with the visual stimuli. Neither of the two most commonly caught species, Long- tailed Ducks (Clangula hyemalis) and Velvet Scoters (Melanitta fusca), were deterred from lethal encounters with nets by either black-and-white panels or by steady green or flashing white net lights. Long-tailed Ducks were caught in larger numbers in nets equipped with flashing white net lights than in unmodified nets at the same location. Catch rates of commercial fish were not affected by net lights or net panels placed within the nets. Hence, while the deterrents that we tested successfully maintained fish catch, they failed to reduce bird bycatch and are therefore ineffective. We discuss likely avenues for future investigation of bycatch mitigation methods for gillnet fisheries, including species and location response to net lights, managed fishery closures, above-water distraction of birds and gear switching.

Numerical Study on the Heading Misalignment and Current Velocity Reduction of a Vessel-Shaped Offshore Fish Farm

Recently, the concept of a vessel-shaped fish farm was proposed for open sea applications. The fish farm comprises a vessel-shaped floater, five fish cages, and a single-point mooring system. Such a system weathervanes, and this feature increases the spread area of fish waste. Still, the downstream cages may experience decreased exchange of water flow when the vessel heading is aligned with the current direction, and fish welfare may be jeopardized. To ameliorate the flow conditions, a dynamic positioning (DP) system may be required, and its power consumption should relate to the heading misalignment. This paper proposes an integrated method for predicting the heading misalignment between the vessel-shaped fish farm and the currents under combined waves and currents. A numerical model is first established for the fish farm system with flexible nets. Current reduction factors are included to address the reduction in flow velocity between net panels. The vessel heading is obtained by finding the equilibrium condition of the whole system under each combined wave and current condition. Then, the Kriging metamodel is applied to capture the relation between the misalignment angle and environmental variables, and the probability distribution of this misalignment angle is estimated for a reference site. Finally, the requirement for the DP system to improve the flow condition in the fish cages is discussed.

Implementation of green manufacturing concepts: a case study of tea manufacturing industry in Sri Lanka

The objective of this study is to identify the potential implementation of green manufacturing concepts on medium scale tea industries in Sri Lanka. Five medium scale tea factories located in southern province were selected as the sample and case study approach was followed. The data were collected through regular factory observations and secondary sources including published reports and documents of the companies. Material and energy flow diagram was used as the main tool for the analysis. The result reveals that there are significant environmental issues such as large-scale raw material wastage, and high level of energy, water consumption and harmful emissions. This study suggests that energy consumption can be reduced by introducing new equipment, methods and systems such as installing variable speed motors, power factor correction capacitors, variable frequency drivers, replacing light emitting diodes (LEDs) with compact fluorescents, implementing well-planned production schedules, and fixing net metering system and solar panels.

Can the Dispersal Behavior of Halyomorpha halys (Hemiptera: Pentatomidae) Inform the Use of Insecticide-Treated Netting to Mitigate Homeowner Issues From its Fall Invasion?

Brown marmorated stink bug, Halyomorpha halys (Stål) (Hemiptera: Pentatomidae), is a serious agricultural pest and can be a significant nuisance when it invades human dwellings during its fall dispersal to overwintering sites. Methods informed by behavioral data to exclude or reduce its entry into buildings are needed. The temporal and spatial distribution of adults on an invaded building was assessed over multiple years, revealing its seasonal dispersal pattern and that its numbers varied by wall aspect. Moreover, its density was higher in recessed doorways than on associated walls, raising questions about its behavioral response to dark, contrasting surfaces. This response was evaluated using black, framed panels of deltamethrin-incorporated netting, non-treated netting, and an open frame with no netting, deployed in pairs on the wall of a private residence. More dispersing adults landed on panels of non-treated netting than on open panels, but there was no difference between panels with treated and non-treated netting. Adults remained on treated panels for less time than on non-treated panels, and most walked rather than flew from both. Adult male and female H. halys collected during the dispersal period were exposed to panels of treated and non-treated netting in a laboratory, using durations derived from field recordings. Exposures to treated panels intoxicated but did not kill them over a 7-d assessment period. The deployment of insecticide-treated netting, guided by the behavior of adult H. halys alighting on buildings, is discussed in relation to potential options to mitigate homeowner issues from this serious annual problem.

Experimental analysis of the hydrodynamic coefficients of net panels in current

The hydrodynamic loads on net panels were affected significantly by the net configuration and the Reynolds number. Six groups of net panels with different materials, net solidities and knot patterns were experimentally analyzed in this study. The concept of the cruciform element was proposed to calculate the hydrodynamic loads on net panels. The effect of the net solidity, the net material, the knot pattern and the Reynolds number on the hydrodynamic coefficients of net panels in current was comprehensively discussed. A formula, which was suitable for the net panels with high solidity, was proposed to estimate the hydrodynamic coefficients of net panels in current based on the theoretical and experimental analysis of the measurements. The results indicated that the relationship between the Reynolds number and the drag coefficient of the net panel was similar to that for the circular cylinder when the net solidity was low (0.190); with the increase of the net solidity, the influence of the Reynolds number on the drag coefficient of the net panel decreased. In addition, the net-grouping standard was discussed and a correction factor for modifying the drag-coefficient of the equivalent net model was proposed.

Investigating fish behavioural responses to LED lights in trawls and potential applications for bycatch reduction in the Nephrops-directed fishery

AbstractLight-emitting diodes (LEDs) have been tested in trawl fisheries to reduce the bycatch of unwanted species through behavioural stimulation. Previous studies used LED lights to either highlight escaping routes or increase the contact rate with square-mesh panels. However, phototactic responses (moving towards or away from light sources) to LED lights could also be exploited to separate species during the catching process. We investigated if either positive or negative phototaxis can be used to improve fish vertical separation from Nephrops (Nephrops norvegicus) in the aft section of a horizontally separated trawl codend. The aim was to increase the proportion of fish entering the upper compartment. We conducted two different experiments in front of the separation into compartments, inserting green LED lights in the upper and lower netting panel, respectively. Species vertical separation was analysed and compared in two identical trawls towed in parallel, one equipped with lights and one without. We obtained significant changes in vertical separation, but no clear species-specific phototactic response was identified. Neither of the light positions improved fish separation from Nephrops. However, the potential of LED lights as behavioural stimulators is confirmed, and a more mechanistic understanding of light and fish vision may improve the results of future applications.