ABSTRACT This study explores the average flow field inside and around stocked Atlantic salmon (Sa... more ABSTRACT This study explores the average flow field inside and around stocked Atlantic salmon (Salmo salar L.) fish cages. Laboratory tests and field measurements were conducted to study the effects of biofouling and especially fish behaviour on the flow patterns around and through fish cages. Currents were measured around an empty and a stocked fish cage in a fjord to verify the results obtained from laboratory tests without fish and to study the effects of fish swimming in the cage. Fluorescein, a non-toxic, fluorescent dye, was released inside a stocked fish cage for visualization of 3-dimensional flow patterns inside the cage. Atlantic salmon tend to form a torus shaped school and swim in a circular path, following the net during the daytime. Current measurements around an empty and a stocked fish cage show a strong influence of fish swimming in this circular pattern: while most of the oncoming water mass passes through the empty cage, significantly more water is pushed around the stocked fish cage. Dye experiments show that surface water inside stocked fish cages converges towards the center, where it sinks and spreads out of the cage at the depth of maximum biomass. Furthermore, the converging surface water swirled in the direction of the swimming fish. In order to achieve a circular motion, fish must accelerate towards the center of the cage. This inward-directed force must be balanced by an outward force that pushes the water out of the cage, resulting in a low pressure area in the center of the rotational motion of the fish. Thus, water is pulled from above and below the fish swimming depth. The laboratory tests with empty cages agree well with field measurements around empty fish cages, and give a good starting point for further laboratory tests including the effect of fish-induced currents inside the cage to document the details of the flow patterns inside and adjacent to stocked fish cages. The results of such experiments can be used as benchmarks for numerical models to simulate the water flow in and around net pens, and model the oxygen supply and the spreading of wastes in the near wake of stocked fish farms.
ABSTRACT Salmon fish farmers are seeking methods to reduce infestations of salmon lice. The highe... more ABSTRACT Salmon fish farmers are seeking methods to reduce infestations of salmon lice. The highest concentrations of salmon lice copepodites are typically found in the upper meters of the water column and preventing this water from passing through the salmon cages has been proposed as a way to reduce salmon lice infestation. One simple implementation proposed by the industry is to place a permanent tarpaulin skirt around the upper part of the sea cages. Currently, there is little evidence that a skirt will block the entrance of surface water to the cages and thereby reduce infestation. However, this pilot study shows that putting a tarpaulin skirt around a full scale commercial sea cage may seriously decrease the oxygen saturation levels available for the fish inside the skirt. Further investigations into the effectiveness of skirts as a means of reducing the exposure of farmed salmon to sea lice must therefore take precautional measures.
ABSTRACT Aquaculture is expanding all over the world. The limitations are mainly related to locat... more ABSTRACT Aquaculture is expanding all over the world. The limitations are mainly related to location, water flow, escapees and fish health. The present status of flow hydrodynamics within and around sea-cages is reviewed in this paper, providing a framework for understanding the spatial and temporal variability of key environmental parameters within and outside sea-cages. The paper presents contemporary experiments on drag forces on net panels, model-scale cages, the biological effects of fish, fish movements and fouling as the major topics. It includes also a presentation of different theoretical studies as an attempt to simulate experiments. The accumulated experimental results are sorted out following a gradually increasing scale from the cruciform as a basic element of a net, via net panels to a net cage with solidity and velocity as the main parameters, while the effect of fish and fouling are discussed only at the full-scale level. The compilation is important to understand issues related to the design and mechanics of net cages, taking into account fish behavior in relation to future engineering development within the field of hydrodynamics in aquaculture cage farming. Improved knowledge of water movement through aquaculture cages is critical for future development of efficient and sustainable aquaculture, including a shift toward more exposed locations.
This study comprised the results of three different seawater trials using unique combination of t... more This study comprised the results of three different seawater trials using unique combination of techniques to study protease digestive efficiency and growth performance quality to illustrate the effects of light regimes and vaccine types in Atlantic salmon (Salmo salar L.). Fish with higher growth had higher trypsin (T) and chymotrypsin (C) specific activities with higher T/C ratio or slope T/C ratio [calculated from the regression between trypsin (y) and chymotrypsin (x) specific activities] in the pyloric caeca. The T/C ratios indicated fish growth rates over a period of 1-2 months, while the slope T/C ratios indicated fish growth rates at sampling. Adaptation period for adjustment to the new environment of continuous light was 70 days, indicated by the differences in trypsin specific activities and the crossing of slope T/C ratio regressions following with the changes in growth rate directions between the control and the treated group. Vaccine types affected fish vertebral growth, and additional continuous light enhanced the impact of vaccines on fish growth during springtime, indicated by differences in slope T/C ratios. Continuous light stimulated fish growth during winter to spring, when the natural day length was short, without significantly changing white muscle and oocyte qualities in the fish of about 500 g, except for significantly increased white muscle RNA concentration. Continuous light also reduced fish growth rate later during summer, when the natural day length was long, by precedently decreasing the T/C ratio in late spring. Interestingly, plasma levels of free lysine related to tryptic digestion were correlated with trypsin specific activity levels. Continuous light caused higher levels of most free amino acids (FAA) involved in nitrogen metabolism, higher incorporation of essential FAA for protein synthesis, and higher protein turnover rate (free hydroxyproline levels) in both plasma and white muscle. However, continuous light did not affect higher protein content, intracellular buffering capacity and RNA levels in the white muscle of the fish of about 1 kg, probably due to limitation of FAA available for protein synthesis. It is therefore suggested that enhancing fish growth by continuous light stimulation should be accompanied by increasing availability or content of dietary protein (and probably minerals), which in turn would improve the quality of fish growth performance through increasing fillet protein concentration, strengthening vertebral growth, and delaying oocyte development.
Host-parasite interactions are moderated by the environmental conditions of the interaction mediu... more Host-parasite interactions are moderated by the environmental conditions of the interaction medium (e.g. air or water). Encounter rate and the time available for a parasite to make physical contact with a host are both influenced by fluid dynamics, yet how they interact is poorly known. Here, we tested whether current velocities altered the initial attachment and post-settlement survival of an ecto-parasitic copepod (Lepeophtheirus salmonis) on Atlantic salmon. Current velocities strongly influenced attachment; infestation levels were 2.5 and 1.3times higher in moderate than high and low velocity currents, respectively, while current velocities did not affect post-settlement survival. An interplay between a reduced host-parasite encounter rate in a low velocity current and reduced contact time in a high velocity current likely explains this result. Initial parasite attachment position was influenced by an interaction between current velocity and swimming behaviour, likely due to different fin positioning by fish in flows of different velocities. Our results imply that rapid swimming by salmon migrating out of coastal waters, usually described as adaptive against predation, could also be adaptive against parasitism. Infestation rates were also highest at the typical swimming speed of farmed salmon in coastal fish farms, which may be a hitherto unrecognised factor contributing to L. salmonis epidemics.
ABSTRACT This study explores the average flow field inside and around stocked Atlantic salmon (Sa... more ABSTRACT This study explores the average flow field inside and around stocked Atlantic salmon (Salmo salar L.) fish cages. Laboratory tests and field measurements were conducted to study the effects of biofouling and especially fish behaviour on the flow patterns around and through fish cages. Currents were measured around an empty and a stocked fish cage in a fjord to verify the results obtained from laboratory tests without fish and to study the effects of fish swimming in the cage. Fluorescein, a non-toxic, fluorescent dye, was released inside a stocked fish cage for visualization of 3-dimensional flow patterns inside the cage. Atlantic salmon tend to form a torus shaped school and swim in a circular path, following the net during the daytime. Current measurements around an empty and a stocked fish cage show a strong influence of fish swimming in this circular pattern: while most of the oncoming water mass passes through the empty cage, significantly more water is pushed around the stocked fish cage. Dye experiments show that surface water inside stocked fish cages converges towards the center, where it sinks and spreads out of the cage at the depth of maximum biomass. Furthermore, the converging surface water swirled in the direction of the swimming fish. In order to achieve a circular motion, fish must accelerate towards the center of the cage. This inward-directed force must be balanced by an outward force that pushes the water out of the cage, resulting in a low pressure area in the center of the rotational motion of the fish. Thus, water is pulled from above and below the fish swimming depth. The laboratory tests with empty cages agree well with field measurements around empty fish cages, and give a good starting point for further laboratory tests including the effect of fish-induced currents inside the cage to document the details of the flow patterns inside and adjacent to stocked fish cages. The results of such experiments can be used as benchmarks for numerical models to simulate the water flow in and around net pens, and model the oxygen supply and the spreading of wastes in the near wake of stocked fish farms.
ABSTRACT Salmon fish farmers are seeking methods to reduce infestations of salmon lice. The highe... more ABSTRACT Salmon fish farmers are seeking methods to reduce infestations of salmon lice. The highest concentrations of salmon lice copepodites are typically found in the upper meters of the water column and preventing this water from passing through the salmon cages has been proposed as a way to reduce salmon lice infestation. One simple implementation proposed by the industry is to place a permanent tarpaulin skirt around the upper part of the sea cages. Currently, there is little evidence that a skirt will block the entrance of surface water to the cages and thereby reduce infestation. However, this pilot study shows that putting a tarpaulin skirt around a full scale commercial sea cage may seriously decrease the oxygen saturation levels available for the fish inside the skirt. Further investigations into the effectiveness of skirts as a means of reducing the exposure of farmed salmon to sea lice must therefore take precautional measures.
ABSTRACT Aquaculture is expanding all over the world. The limitations are mainly related to locat... more ABSTRACT Aquaculture is expanding all over the world. The limitations are mainly related to location, water flow, escapees and fish health. The present status of flow hydrodynamics within and around sea-cages is reviewed in this paper, providing a framework for understanding the spatial and temporal variability of key environmental parameters within and outside sea-cages. The paper presents contemporary experiments on drag forces on net panels, model-scale cages, the biological effects of fish, fish movements and fouling as the major topics. It includes also a presentation of different theoretical studies as an attempt to simulate experiments. The accumulated experimental results are sorted out following a gradually increasing scale from the cruciform as a basic element of a net, via net panels to a net cage with solidity and velocity as the main parameters, while the effect of fish and fouling are discussed only at the full-scale level. The compilation is important to understand issues related to the design and mechanics of net cages, taking into account fish behavior in relation to future engineering development within the field of hydrodynamics in aquaculture cage farming. Improved knowledge of water movement through aquaculture cages is critical for future development of efficient and sustainable aquaculture, including a shift toward more exposed locations.
This study comprised the results of three different seawater trials using unique combination of t... more This study comprised the results of three different seawater trials using unique combination of techniques to study protease digestive efficiency and growth performance quality to illustrate the effects of light regimes and vaccine types in Atlantic salmon (Salmo salar L.). Fish with higher growth had higher trypsin (T) and chymotrypsin (C) specific activities with higher T/C ratio or slope T/C ratio [calculated from the regression between trypsin (y) and chymotrypsin (x) specific activities] in the pyloric caeca. The T/C ratios indicated fish growth rates over a period of 1-2 months, while the slope T/C ratios indicated fish growth rates at sampling. Adaptation period for adjustment to the new environment of continuous light was 70 days, indicated by the differences in trypsin specific activities and the crossing of slope T/C ratio regressions following with the changes in growth rate directions between the control and the treated group. Vaccine types affected fish vertebral growth, and additional continuous light enhanced the impact of vaccines on fish growth during springtime, indicated by differences in slope T/C ratios. Continuous light stimulated fish growth during winter to spring, when the natural day length was short, without significantly changing white muscle and oocyte qualities in the fish of about 500 g, except for significantly increased white muscle RNA concentration. Continuous light also reduced fish growth rate later during summer, when the natural day length was long, by precedently decreasing the T/C ratio in late spring. Interestingly, plasma levels of free lysine related to tryptic digestion were correlated with trypsin specific activity levels. Continuous light caused higher levels of most free amino acids (FAA) involved in nitrogen metabolism, higher incorporation of essential FAA for protein synthesis, and higher protein turnover rate (free hydroxyproline levels) in both plasma and white muscle. However, continuous light did not affect higher protein content, intracellular buffering capacity and RNA levels in the white muscle of the fish of about 1 kg, probably due to limitation of FAA available for protein synthesis. It is therefore suggested that enhancing fish growth by continuous light stimulation should be accompanied by increasing availability or content of dietary protein (and probably minerals), which in turn would improve the quality of fish growth performance through increasing fillet protein concentration, strengthening vertebral growth, and delaying oocyte development.
Host-parasite interactions are moderated by the environmental conditions of the interaction mediu... more Host-parasite interactions are moderated by the environmental conditions of the interaction medium (e.g. air or water). Encounter rate and the time available for a parasite to make physical contact with a host are both influenced by fluid dynamics, yet how they interact is poorly known. Here, we tested whether current velocities altered the initial attachment and post-settlement survival of an ecto-parasitic copepod (Lepeophtheirus salmonis) on Atlantic salmon. Current velocities strongly influenced attachment; infestation levels were 2.5 and 1.3times higher in moderate than high and low velocity currents, respectively, while current velocities did not affect post-settlement survival. An interplay between a reduced host-parasite encounter rate in a low velocity current and reduced contact time in a high velocity current likely explains this result. Initial parasite attachment position was influenced by an interaction between current velocity and swimming behaviour, likely due to different fin positioning by fish in flows of different velocities. Our results imply that rapid swimming by salmon migrating out of coastal waters, usually described as adaptive against predation, could also be adaptive against parasitism. Infestation rates were also highest at the typical swimming speed of farmed salmon in coastal fish farms, which may be a hitherto unrecognised factor contributing to L. salmonis epidemics.
The use of submerged artificial light has been extensively used in salmon cages to delay or inhib... more The use of submerged artificial light has been extensively used in salmon cages to delay or inhibit sexual maturation, but guiding fish vertical distribution could be a secondary benefit of this farm practice. Studies have shown that submerged artificial light lures caged salmon and permits schooling from daylight hours to continue through the night. Most behavioural manipulations to guide and distribute salmon across depths within sea-cages at a production-scale have focused on fixed cage modifications. Creating more dynamic environmental conditions via mobile light may alter salmon swimming depth trade-off decisions compared with stationary lighting. Previous work suggests attraction of salmon to stepwise light movements in coastal sea-cages, though no knowledge exists on continuous moving lights or how light movement performs in stratified fjord environments. Whether light movement opposed to fixed light positioning can impel caged salmon in vertically stratified conditions to swim at depths normally not selected is yet to be explored. If successful, guiding salmon in this way has diverse applications in the salmon culture industry, most importantly in potential parasite reduction and poor water quality avoidance. We investigated a) whether salmon were attracted to vertically moving lights, b) how the speed of movements affected this attraction, and c) how the thermocline and halocline affected these responses. To determine salmon responses to moving light we used submerged light sources in three commercial-scale cages at a fish density of 5 kg m–3. Lights were hauled up and down via a winch between 0–13 m depth at several speeds. This allowed assessment of the capabilities of a range of light velocities to vertically move salmon. Fish depth distribution was inferred from echo sounders and swimming behaviour was observed via underwater cameras also attached to the winch moving the lights. Further, we explored the environmental variables potentially contributing to fish swimming behaviour from a vertically profiling CTD measuring temperature, salinity, and oxygen between 0–13 m within a cage using a separate winch. Preliminary results suggest that salmon follow vertical light movement up to a certain speed and that there is a trade-off between attraction and stratified temperature conditions. Complete results and analysis will be presented.
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Papers by Frode Oppedal
We investigated a) whether salmon were attracted to vertically moving lights, b) how the speed of movements affected this attraction, and c) how the thermocline and halocline affected these responses. To determine salmon responses to moving light we used submerged light sources in three commercial-scale cages at a fish density of 5 kg m–3. Lights were hauled up and down via a winch between 0–13 m depth at several speeds. This allowed assessment of the capabilities of a range of light velocities to vertically move salmon. Fish depth distribution was inferred from echo sounders and swimming behaviour was observed via underwater cameras also attached to the winch moving the lights. Further, we explored the environmental variables potentially contributing to fish swimming behaviour from a vertically profiling CTD measuring temperature, salinity, and oxygen between 0–13 m within a cage using a separate winch.
Preliminary results suggest that salmon follow vertical light movement up to a certain speed and that there is a trade-off between attraction and stratified temperature conditions. Complete results and analysis will be presented.