The aquaculture industry is constantly making efforts to improve fish welfare while maintaining the ethically sustainable farming practises. This work presents an enhanced tank environment designed for testing and developing novel combinations of technologies for analyzing and detecting behavioral responses in fish shoals/groups. Regular cameras are combined with event cameras and a scanning sonar to comprise a sensor suite that offers a more detailed and complex way of fish observation. The modified tank environment is designed to simulate the prevailing conditions on-site at cage based farms, particularly in terms of lighting conditions, while all tank systems and sensors are hidden behind specially designed enclosures, providing a "clean" environment (open arena) less likely to impact the fish behavior. The proposed sensor suite will be tested and demonstrated in the modified tank environment to benchmark its ability in monitoring fish, after which it will be adapted for use in a more industrially relevant situation with open cages.

Digital twins and relevant concepts are being applied in a wide variety of ways, and they are of most use when an actual real-world physical system or process (a physical twin) is changing over time and when measurement data correlated with this change can be captured. In this work, a digital twin model was implemented for real-time monitoring of aquaculture net cage systems, which is notoriously challenging because of several difficult-to-measure properties, such as forces on and deformation of the flexible netting structures, waves and flow field alterations around the cage and complex stiffness behaviour of the mooring elements made by fibre ropes. These properties were set to be adaptable according to the resultant outputs, such as cage responses and mooring loads that were continuously compared with the measurement data obtained from remote monitoring sensors. In this way, real-time sensor data were assimilated into the numerical simulation model for representing the actual net cage system. No dedicated sensors were used for fish monitoring, but the fish behavioural responses to current, wave and cage deformation were modelled according to relevant field observational data. A wireless sensor network has also been tested for the digital twin implementation, which was found to be suitable for practical uses in fish farms.

The shift towards salmon farming in more exposed locations has been an industry-wide trend for the last decade. Moving fish farms to locations with high water currents and waves can improve production by providing more stable temperatures and water quality, as well as reducing the negative environmental impacts of fish farming. This study investigates how waves affect the behavior of salmon from the same group, reared at different locations within a fish farm in standard circular sea cages. Using echosounders, DO (dissolved oxygen), temperature sensors and ADCP (Acoustic Doppler Current Profiler), we show that salmon avoid waves, swim below them and maintain their normal behavior. We also show that salmon behavior is related to the exposure of the cage, in the farm layout, to waves and currents. An integrated numerical model of fish and flexible sea cages is used to simulate the fish behavior under waves and currents and it able to reproduce the observed fish distributions in general.

The goal of the study was to compare open and closed transports of adult Atlantic salmon in terms of water quality and stress (blood chemistry and white muscle biochemistry). The study comprised of two phases: (i) live transport from a commercial farm by vessel (open system) and vehicle (closed system) to flow-through laboratory tanks and (ii) after post-transport recovery, simulated live transports in open and closed systems. The stress reaction the salmon experienced during transfer from fish farm to laboratory tanks was severe. Some delayed mortalities were observed possibly related to extreme acidosis. The fish were left to recover for 65 h before the simulated open and closed transports were carried out. By then, they had not yet fully recovered to baseline levels. The simulated transports did not cause excessive stress reactions relative to the partially recovered fish. However, deteriorating water quality during closed transport eventually affected fish behaviour where fish welfare could be questioned. The main finding was that crowding and transport due to commercial transport overshadowed stress-related results obtained during the laboratory study. The results may also be viewed as effects of repeated fish handling operations, a typical feature of salmon farming.

Sea lice infestations have been a major problem for the global salmon farming industry for several decades. To date, few studies have addressed the measurement of lice abundance in plankton samples and a standardized method to quantify sea lice larvae in water samples is still lacking. This study aims to: (1) evaluate the methods used to detect sea lice larvae based on published data and (2) to determine experimentally the volume of filtered sea water needed to obtain precise estimates of sea lice larvae abundance at different lice densities. Twenty-eight publications were reviewed with particular attention to sampling method and depth, total filtrated volume, analysed volume and nauplii and maximum copepodite densities. Moreover, plankton samples were obtained in and around salmon farms to evaluate the optimal water volumes required to estimate sea lice larvae abundance. This study provides a sampling and analysis strategy for quantifying larval sea lice in plankton samples from a cost/benefit point of view. Quantification of sea lice larvae in the plankton communities would be more precise than indirect methods used today (i.e adult sea lice attached on salmonids), and suitable for validation of modelling tools predicting the spatiotemporal dispersal of lice and, hence, the risk of infestation of salmon farms.

Due to increasing demand for salmon and environmental barriers preventing expansion in established sites, salmon farmers seek to move or expand their production to more exposed sites. In this study we investigate the effects of strong currents and waves on the behaviour of salmon and how they choose to use the space available to them. Observations are carried out in a site with strong tidal currents and well mixed water. Using video cameras and echo sounders, we show that salmon prefer to use the entire water column, narrowing their range only as a response to cage deformation, waves, or daylight. Conversely, salmon show strong horizontal preference, mostly occupying the portions of the cage exposed to currents. Additionally, waves cause salmon to disperse from the exposed side of the cage to the more sheltered side. Even when strong currents decrease the amount of available space, salmon choose to occupy the more exposed part of the cage. This indicates that at least with good water exchange, the high density caused by limited vertical space is not so aversive that salmon choose to move to less desirable areas of the cage. However, the dispersal throughout the entire available water column indicates that ensuring enough vertical space, even in strong currents, would be beneficial to salmon welfare.

Salmonid aquaculture, producing nearly 3 million tons per year, has expanded across temperate seascapes around the globe in recent decades. Cage technologies used to farm salmonids are thought to have changed in both size and location in coastal environments, yet remarkably little data exists to explain these major developments. Using satellite images from Google Earth, we mapped and measured sea-cages and their positions to quantify changes between historical and current use in major production regions. While cage numbers have remained similar across the major producing nations from 2005 to 2020, cage diameters have increased by 27-87%, which has resulted in large increases in farm sizes (total surface area enclosed by cages per site) of 61% in Chile, 84% in Scotland, 212% in the Faroe Islands and 221% in Norway. Cage type has changed from predominately square cages within steel platforms to plastic, circular arrays over the past 15 years, with notable exceptions such as Chile and the Pacific coast of North America. While farms in the top producing regions in 2020 were slightly further from the coast compared to 2005, salmon farming remains an activity that occurs close to the coast with farms on average sited just a few hundred metres from the nearest land. Distance to the nearest neighbouring farm increased for Norway, Chile and the Faroe Islands, but decreased for Scotland and the Atlantic coast of North America. Changes in farm technology used and their location have environmental and production consequences, with larger farms producing more waste at an individual site, bigger cages impacting the dissolved oxygen available to fish within them, and proximity of farms to their neighbours influencing the risk of disease incidence and spread. Characterising cage technologies in use across the major farming regions may allow for management practices to be critically analysed to improve fish welfare and production and reduce farm biosecurity threats and environmental impacts.

The characteristic current flow field around a 55 m deep full-scale stocked conical Atlantic salmon sea-cage equipped with a 10 m permeable skirt was studied experimentally using acoustic Doppler velocimeters and profilers. The weakest current speed was inside the cage at 6 m depth and the highest reduction downstream was recorded behind the shielded volume. Downstream of the cage the reduction in speed became little to non-existing at 22 m depth, probably due to the decreasing diameter of the cage with depth. To reduction in current speed through the cage was compared with estimated reduction from theoretical expressions. The results compared reasonably well downstream of the shielded cage, while the reduction inside the cage was higher than the estimates. The difference in current flow field behind a conical cage compared with a cylindrical cage may have implications for the dispersal of waste, feed pellets and microorganisms from the cage influencing the benthic impact of the farm.

Most Atlantic salmon mariculture operations use open sea cages for the grow out phase. The ultimate fate and effects of the effluents and the possibilities of disease transfer between fish farms are major concerns for farmers, governance and the general public alike. Numerical model systems applied to studying and managing effluents and disease transfer in mariculture must realistically resolve the hydrodynamics in the vicinity of the fish farms. In the present study, the effects of the aquaculture structures on the current patterns were introduced in the ocean model system SINMOD. The drag parameters for the ocean model were determined by comparing the simulation results from the ANSYS Fluent ® software suite and SINMOD in an idealized channel setting with uniform currents. The model was run for a number of realistic scenarios in high horizontal resolution (∼30 m) with sea cages influencing the flow field. Comparisons between extensive current measurements and the simulation results showed that the model system reproduced the current local current field well. By running simulation scenarios with and without the effects of the sea cages on the flow field, it was possible to assess the importance of such effects for numerical dispersal models and aquaculture environment interactions simulations and hence for assessment of environmental impacts.

Measurements were performed inside and in the wake of a commercial salmon sea cage. The key instrumentation included the following: sea bottom-mounted acoustic Doppler current profilers providing continuous concurrent flow velocity and turbulence information about the water columns; vessel-mounted acoustic current profilers mapping the flow pattern around the wake of the cage in a selected incoming flow; a microstructure profiler measuring the fluctuations in vertical shear in the dissipation range; an acoustic Doppler velocimeter measuring the velocity inside the sea cage; dissolved oxygen sensors and echosounders measuring the distribution of fish inside the cage. The measurements have performed with stocked and emptied sea-cage. The results showed simultaneous strong flow reductions in the wake near the cage and high turbulence in the upper part of the water column, both of which were generated by the sea cage. Measurements inside the cage showed that although the schooling fish reduced the flow, there was no evidence that they generate secondary radial and vertical flows.

Disease, pest control, and environmental factors such as water quality and carrying capacity limit growth of salmon production in existing farm areas. One way to circumvent such problems is to move production into more exposed locations with greater water exchange. Farming in exposed locations is better for the environment, but may carry unforeseen costs for the fish in those farms. Currents may be too strong, and waves may be too large with a negative impact on growth and profit for farmers and on fish welfare. This study employed two major fish monitoring methods to determine the ability of Atlantic Salmon (Salmo salar) to cope with wavy conditions in exposed farms. Echosounders were used to determine vertical distribution and horizontal preference of fish during different wave and current conditions as well as times of day. Video cameras were used to monitor shoal cohesion, swimming effort, and fish prevalence in locations of interest. The results indicate complex interacting effects of wave parameters, currents, and time of day on fish behaviour and vertical distribution. During the day, hydrodynamic conditions had stronger effects on vertical distribution than during the night. In weak currents, fish generally moved further down in taller waves, but stronger currents generally caused fish to move upwards regardless of wave conditions. Long period waves had unpredictable effects on vertical distribution with fish sometimes seeking deeper water and other times moving up to shallower water. It is unclear how much the cage bottom restricted vertical distribution and whether movement upwards in the water columns was related to cage deformation. In extreme cases, waves can reach below the bottom of a salmon cage, preventing fish from moving below the waves and cage deformation could exacerbate this situation. Farmers ought to take into consideration the many interacting effects on salmon behaviour within a cage as well as the potential for cage deformation when they design their farms for highly exposed locations. This will ensure that salmon are able to cope when storms and strong currents hit at the same time.

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.

Impacts of aquaculture fish cages and their biomass on the local water current field is investigated with extensive, full scale field measurements. This is important for the fish welfare as well as for waste transport from open fish cages. The relationships between the current flow rate at various locations close to fish cages are examined at an aquaculture marine site close to Froya, Norway. This is an area with a complex current pattern due to the bathymetry, rocks and islands, coast current and strong wind conditions. An array of ADCPs (Acoustic Doppler Current Profilers) measured currents. Moreover, CTD and separate temperature profiler instruments were made in order to analyze flow stratification. Two different measurement periods are analysed with 1) small fish size (0,2-0,4 kg) and 2) large fish size (1 kg-5 kg). The field measurements indicate some impact from the biomass on the measured current flow at distances from 90 to 320 m around it. The results are supported by results from the numerical ocean model SINMOD (SINtef MODel).

Understanding dissolved O2 flux in marine cages, and how individual fish respond to and experience such variation, is critical to optimizing growth and production performance of farmed salmon. We used a high resolution environmental monitoring system to create a 3-dimensional map of a commercial marine cage with respect to salinity, temperature and dissolved O2 through time, while also tracking the oxygen experience of 4 individually tagged Atlantic salmon. Despite all of the dissolved O2 measurements at the reference site being physiologically suitable for maximum growth, 1 in 4 of the recordings collected within the cage were below dissolved O2 levels known to reduce feed intake and growth. Recorded dissolved O2 in the cage ranged from 26 to 90% saturation with a high degree of vertical, horizontal and temporal variation. Poorest dissolved O2 conditions consistently occurred at night in the central and down-current cage positions. Dissolved O2 levels experienced by individual fish ranged from 30 to 90% saturation, with variation within 5 minute intervals as large as 32 percentage points. These results expand the current body of knowledge on environmental variability in marine cages, and provide valuable insights to aid farm managers in focusing mitigation and monitoring efforts when and where they are most needed.

This study investigated the three-dimensional flow structures in an ellipsoid, closed sea fish cage. The results are presented using computational fluid dynamics (CFD) simulations and experimental measurements. Experimental residence time distribution (RTD) measurement and CFD simulation are the best methods to study the hydrodynamics of inflow systems. Three-dimensional numerical simulations of the flow and transport characteristics of the system were conducted using a Reynolds-averaged Navier-Stokes equation approach and the results were compared to the measurements performed using acoustic Doppler velocimetry techniques. The objective of the investigation was to characterize the flow field generated in an ellipsoid, closed tank. The flow in the enclosed volume is driven by four inlets pipes integrated into the wall of the cage. The focus is on the turbulent structures and undesirable flow patterns that lead to reduced self-cleaning efficiency and a lower quality habitat for the fish through phenomena, such as recirculation zones or low velocity areas. Correlations between CFD and the experimental data confirm the adequate reproduction of hydrodynamic conditions and reinforce the predictive capabilities of numerical models as tools to simulate field scale closed containment systems or to optimize existing and future aquaculture designs. The simulation of aquaculture-like particles demonstrates that almost 100% of particles with a diameter ranging between 1μm and 3000μm are removed during a maximum of two hydraulic retention time (HRT) cycles. Smaller particles are removed via the upper-side outlets and larger particles are removed via the bottom outlet.

Critical swimming speed (Ucrit) has traditionally been measured in relatively small swimming tunnels with 1 fish alone. However, both increased flume lengths and swimming in groups are known to improve performance. Atlantic salmon Salmo salar farming is currently expanding to more exposed locations, which necessitates guidelines for limits in water current peak velocity to secure animal welfare. A large swim tunnel system was therefore developed which allowed for swimming trials at relevant stocking densities of Atlantic salmon comparable to the conditions in exposed sea cages. Ucrit was measured in groups of 3 size classes (small post-smolts, large post-smolts, adults) in this new swim tunnel and compared to measurements from individual fish in a smaller swim tunnel. Ucrit (in cm s–1) increased with size and was significantly lower in the smaller setup. Our results suggest that Ucrit can be used as a maximum current velocity tolerance threshold in exposed aquaculture, where longer periods above this value would be detrimental to the welfare and physio logical function of the fish. This study represents a starting point in obtaining suitable current velocity profiles for farmed Atlantic salmon during the on-growing phase in sea cages.

The effects of juvenile Atlantic salmon (Salmo salar) on flow and turbulence in a circular tank were investigated. Three fish sizes were studied (37.5 g, 82.5 g and 218 g) with between 268,050 and 318,000 fish in a 15 m diameter by 4 m deep tank (mean stocking densities of 15.3 kg m(-3), 35.6 kg m(-3), and 79.4 kg m(-3)). Flow in the enclosed tank was driven by the inflow from the water supply system and a degassing system. Velocities were measured using acoustic Doppler velocimetry with and without fish present. Dissolved oxygen was also measured, and the turbulent transport of dissolved oxygen calculated from eddy correlation. The average water velocity was reduced by 15% at low and medium stocking densities, and 57% at high stocking density. Turbulent kinetic energy, turbulence intensity, and turbulence dissipation rates were higher with fish than without. Fish altered the distributions of mean velocity, turbulence and oxygen, and increased the turbulent transport of oxygen. Vertical distributions of turbulence were consistent with echo-sounder derived fish distributions.

This paper presents the full-scale measurements of the deformation and current reduction of a large-scale fish sea cage submitted to high currents. Pressure tags were used to measure the cage deformation and the vertical displacement of the bottom ring, while an Acoustic Doppler Current Profiler (ADCP) and Acoustic Doppler Velocimeter (ADV) were used to measure the current reduction. The results show a reduction of 30% of the cage volume for current velocity above 0.6 m/s. The measured current reduction in the cage was 21.5%. A simulation model based on super-elements describing the cage shape was applied, and the results show good agreement with the cage deformations. Also the current flow measurements show the interaction between the sea cage and the bathymetry chart.

Smoothed particle hydrodynamics (SPH) simulations are often initialized on a regular Cartesian grid. Ten SPH kernels were analyzed numerically with respect to their ability to correctly reproduce a uniform density distribution. Three types of kernels were identified. The number of neighboring particles required to represent the density with an accuracy of 0.1% was obtained for the 10 kernels in 1, 2 and 3 dimensions. The effect of random particle displacements from grid nodes on the error was investigated. The results of the study are important for correct application of initial conditions in SPH simulations.

Positioning of sea cages at sites with high water current velocities expose the fish to a largely unknown environmental challenge. In this study we observed the swimming behaviour of Atlantic salmon (Salmo salar L.) at a commercial farm with tidal currents altering between low, moderate and high velocities. At high current velocities the salmon switched from the traditional circular polarized group structure, seen at low and moderate current velocities, to a group structure where all fish kept stations at fixed positions swimming against the current. This type of group behaviour has not been described in sea cages previously. The structural changes could be explained by a preferred swimming speed of salmon spatially restricted in a cage in combination with a behavioural plasticity of the fish.