The effect of swimming fish on the average velocity and velocity profile of a circular tank was studied. Working with different inlet diameters and flow rates, nine different impulse forces (configurations) were evaluated. Each configuration was tested with and without fish, and the effects of two different fish sizes were compared.
The velocity profiles in experiments with fish presented a considerable reduction in velocity in the centre of the tank near the outlet, which was a consequence of the increase in the kinematic eddy viscosity due to the turbulence introduced by fish swimming. A flattening of the angular velocity profile was observed in the central area of the tank, which had a radius of about 0.3 m (18% of the total volume of the tank).
A previous model proposed by Oca and Masaló (2013) was modified in order to better describe the distribution of velocities in the central volume of a tank with swimming fish. The proposed modification was based on Burgers¿ proposal for a bathtub vortex, which implies the determination of the parameter (1¿e¿ar2)(1¿e¿ar2), where r is the radius and the a values were experimentally obtained for each tank configuration, in which they increased with the impulse force.
The average velocities in the tank were proportional to the square root of the impulse force in experiments with and without fish. Experiments with fish presented lower average velocities, which imply higher tank resistance coefficients. At similar stocking densities (14.6 kg/m3), the increase in the tank resistance coefficients obtained with small fish sizes (154 g) were slightly higher than those obtained with bigger fish sizes (330 g).
The measurement of total fish biomass is an essential practice in the aquaculture management. The method commonly used which involves removing a sub-sample of fish from a tank, weighing it and extrapolating the result to the whole tank, carries a large error, is intense labor and causes great stress. Here, we tested a laser scanning method to estimate the total fish biomass from the total fish volume of a sole population (Solea senegalensis) in a tank. The ratio FB/FLV of fish biomass (FB), weighing the 100% of soles, versus the fish layer volume (FLV) measured by the laser scanning, is calculated. Different fish size (small and large) and stocking densities (very low, low, medium and high) were tested. To test the method in the worst conditions, in very low stocking density, fish were 3.0 g ± 1.1 (individual mean weight ± SD); but in low, medium and high stocking density fish were 234.0 g ± 84.6 (individual mean weight ± SD). The fish layer volume included the fish biomass and the interstitial water present among them, which can be estimated from the ratio FB/FLV. In medium and high rearing densities with larger fishthe ratio takes values very close to 1 (0.957 ± 0.021 and 0.967 ± 0.011) giving percentages of interstitial water lower than 5%. But in very low stocking density (0.4 kg/m2) with smaller fish (3.0 g ± 1.1), the ratioFB/FLV was much lower, giving a non-realistic percentage of interstitial water estimation. The low ratios obtained at very low stocking densities are due to the resolution of the image catching process, which is aggravated when working with small fish, since the error of a pixel from a digital image represents a larger percentage of error than with larger fish and higher stocking density. It should be noted that the coefficient of variation (CV) obtained was very low (in all cases lower than 7.2%) and decreased as the stocking density increased achieving the lowest value (1.1%) at high stocking density. The laser scanning has proven to be a useful tool to estimate the total fish layer volume of flatfish, and thus fish biomass, in an aquaculture tank with a usual grow-out stocking density for sole, reducing the labor involved and the stress commonly associated to manual sampling.
In sole (Solea spp.) culture, like in other flatfish culture, it is likely that a vertical gradient of dissolved oxygen (DO) occurs with the lower concentrations being at the tank bottom. This lower concentration at the tank bottom is a consequence of fish oxygen consumption and of the presence of the boundary layer. This fact generates lower DO concentrations in the near-bottom zone where soles are lying most of the time. The aim of this work was to study the hydrodynamic conditions that determine the oxygen gradient that occurs in the layer of water adjacent to flatfish.; Three flow rates were tested in a circular tank and in a rectangular or raceway tank. For each flow rate, water velocities, boundary layer thickness and Reynolds number were calculated. Results showed that the vertical gradient of dissolved oxygen diminishes when water velocity and Reynolds numbers (Re) increase. At the fish density used in this work (11.6 kg m(-2)), when Re decreased under 6000, a large increase in the DO gradient was observed. Guidelines are presented to determine in which situations, as defined by hydraulic parameters, Re >6000 is achieved and DO stratification avoided.; The present work shows that in raceways, the flow rate required to avoid DO stratification is higher than that typically needed to maintain water quality (oxygen, ammonia-nitrogen, carbon dioxide and suspended solids). In circular tanks, it can be easier to achieve velocities that are high enough to avoid stratification with low water inlet flow rates, by adjusting the area of the water entry orifices. (C) 2014 Elsevier B.V. All rights reserved.
The superior hydrodynamics of circular tanks over rectangular tanks are well known by aquaculturists; yet rectangular tanks are still widely used because of their easier handling. As a consequence, several studies have focused on the hydrodynamics of rectangular tanks, where rotating flow cells are generated by injecting water tangentially to the tank wall with the outlet placed in the center of the cell. In the present work, we evaluate the hydrodynamics in a rectangular tank with 4 rotating flow cells of 1m diameter, which is called multivortex tank. We also analyze baffle placement between two consecutive water inlets and the characteristics of the water inlet (flow rate and inlet velocity), which then allows us to determine the average velocities and the distribution uniformity of these velocities. The obtained results are compared with a circular tank having the same cell diameter and inlet configurations as the multivortex tank.
Baffle placement between two consecutive water inlets in the multivortex tank helps increase the average velocity, the uniformity of velocities and the symmetry in the rotating flow cells. In configurations without baffles, the combination of low flow rate and high impulse force present the lowest symmetry. Differences between intermediate and extreme cells were observed. In all configurations tested, velocities and uniformities were higher in extreme cells than in intermediate cells. In the circular tank the average velocities achieved were higher than in the multivortex tank with the same impulse force, but the uniformity of velocities were higher in the multivortex tank. Finally, the proportionality between the average velocity and the square root of the impulse force for a specific tank geometry has been corroborated in commercial scale models.
Aquacultural Engineering Society Superior Paper Award
Freshwater fish farms in Quebec are facing stringent phosphorus discharge limits of 4.2 kg P per tonne of fish produced. Most phosphorus in fish farm effluents is found in particulate form (uneaten food, feces,
etc.). Physical separation systems such as microscreens, filter beds, Cornell-type circular tanks and settling tanks have been proposed to remove solids from raceway and recirculation fish farm effluents but these
technologies are relatively expensive and labour intensive for small pond based production facilities, as mostly found in Quebec. A novel sediment retention system (SRS), consisting of a 1m3 truncated pyramid, was installed at the bottom of an earth pond, below a surface aerator. The objective of this study was to determine the feasibility of collecting and removing sediments by using the SRS and to determine its particulate phosphorus removal efficiency. Solids accumulated in the SRS were quantified and characterised weekly, for 10 weeks. Fish production, food supply, rain events and fish harvesting were also monitored over the course of the study period. The total solids (TS) accumulation rate in the SRS was, on average, 4.0 kg/d with a volatile solids fraction of 7.8% and the P accumulation rate was, on average, 12.4 g P/d. The P removal efficiency obtained with the SRS was 24% of the total P not taken up by fish (effluent P, PEFF). Assuming that 50% of the PEFF was in the particulate form, the removal efficiency of
the SRS was 47%. Lab-scale results correlated with the P mass balance calculations to show that sediments from earth ponds can play an active role in the sorption of soluble phosphorus. Approximately 30% of the P removed from the SRS was attributed to previous sorption/precipitation of soluble P into inorganic forms. The Langmuir model fitted the sorption isotherm of phosphorus onto earth pond soil and the maximum sorption constant obtained was 1.3mg P/g soil. Fish harvesting was identified as the main
external factor affecting sediments and phosphorus accumulation in the SRS. It was concluded that an SRS located under the aerator surface is a suitable and efficient strategy to collect and remove particulate
phosphorus generated from fish production in earth ponds.
Evaluating flatfish activity can be a useful tool for studying fish behavior and welfare. The aim of this
workwas to obtain a quantitative index for measuring flatfish activity using image analysis. Accordingly,
motor activity of a sole population was recorded by digital video for three nights, bearing in mind the
nocturnal lifestyle of the species. Subsequent image analysis was done by image subtraction of
consecutive frames. The result was a ‘‘difference frame’’ showing the changes in the image area due to
fish movement. Using these data, an image processing activity index (IPAI) was determined by
measuring the percentage of area altered due to fish movement and by taking this percentage as an
indicator of fish activity. Typical sole behavioral acts (take-off and surface swimming) were recorded by
direct observation during the same intervals in order to calibrate the IPAI. A direct observation activity
index (DOAI) was determined by weighting each kind of attitude according to its average duration.
Results obtained from image analysis (IPAI) were compared with results obtained by direct observation
(DOAI). A linear relationship between the two indexes was found with a correlation coefficient of
r2 = 0.80 for 92% of coverage area tank and r2 = 0.90 for 210% of coverage area tank. Thus, this digital
video-based index can be a highly reliable and accuratemethod for objectivelymeasuring activity levels
in sole with a low consumption of labor and time. Nevertheless, the application to farm conditions would
need further research and an accurate calibration for other species.
Menció d'Honor 2010 que atorga l'Aquacultural Engineering Society
Sedimentation and resuspension processes of aquaculture biosolids (non-ingested feed and faeces) are analysed using vertically oscillating grids as a source of turbulence in fluid tanks. An oscillating grid system consists of a container in which a grid is stirred vertically generating a well-known turbulent field that is function of amplitude and frequency of oscillation, distance between grid and measurement point, and mesh spacing of the grid. The grid used in this study had a mesh spacing of 1.2 cm, and was calibrated using different amplitudes (1, 1.5 and 2 cm), frequencies (from 1 to 6 Hz) and distances (2.4, 2.7 and 3 cm). After calibration, the turbulence needed to resuspend biosolids and to maintain them in the water column following different times of consolidation, and with biosolids of different origin, was analysed. It was observed that the turbulence needed to resuspend aquaculture biosolids increased with the time of consolidation. When the turbulence was decreased after a resuspension process, the next sedimentation of biosolids showed a hysteretic behaviour: turbulence needed to resuspend a fixed percent of biosolids from the tank bottom is substantially higher than that needed to maintain the same percentage suspended in the water column. Differences in resuspension behaviour of biosolids originated in different tanks were also observed.
The method provides useful information that can be compared with turbulence generated by fish swimming activity, in order to determine the culture conditions, which can promote self-cleaning conditions in a particular tank.