26 DECEMBER 2015 • WORLD AQUACULTURE • WWW.WAS.ORG netting porosity represented slightly fouled netting, as clean nets typically have a porosity of 0.83 for containment nets and 0.95 for predator nets. A 1:15 scale model array of six (3 rows × 2 cages/row) circular cages (Fig. 3) was deployed in a flume tank at the Fisheries and Marine Institute of Memorial University in St. John’s, Newfoundland, Canada. Cages were arranged to a cage spacing of 0.91 times the cage diameter, where cage spacing is defined as the distance from outer edges of adjacent cages in the array. This cage spacing, which is commonly used in industry, is representative of a 100-m circumference cage placed within a 61-m (200-ft) grid. All cages were tied off to the sides of the flume tank to ensure minimal movement of cages during experimentation. Each cage had 4 kg of weight hung from the bottom weight collar to ensure nets maintained a cylindrical shape when exposed to currents; the amount of attached weight added was also determined by current velocities used for model testing. With 4 kg of added weight, the scale model velocity is approximately 50 percent of the full-scale velocity. Therefore, cage deformations observed on fullscale cages will occur at half the flow velocity for the scale model cages. Quantitative and qualitative measures of hydrodynamics. Small, neutrally buoyant drogue balls (Fig. 4) were released at several different locations within the cage array and in the wake of the cage array to determine local flow velocities. Balls were pushed down a hollow tube with a heavy chain to force their release at the desired depth. As drogue balls were carried downstream by flow, distance travelled and time of transit were measured to determine an average velocity. Average drogue ball velocity can be related to average flow velocity over the distance travelled by the ball. Drogue balls were released within cages at mid-depth for two current velocities (10 cm/sec and 30 cm/sec). Orange flagging tape was cut into 6-m strips and attached to sides and backs of cages within the array at multiple depths to visualize unsteadiness of cage wakes. Dye was used to visualize flow rates and dispersal patterns in two sequential cages. A 236-mL bottle of acrylic craft dye was mixed with 13.7 L of water in a bucket. Dye was simultaneously released in the up-current third of two sequential cages at three different incident current velocities. Dye-release trials were filmed from multiple angles, including the flume tank gallery to enable underwater observations. Flume tank depth was dropped by 0.5 m for dye release trials to facilitate easier image capture from the gallery. Clearance times were tested at 12.50, 18.75 and 25.00 cm/ sec. Cage clearance time was calculated as the time required to completely clear the dye color initially released in a particular cage. Results and Discussion In general, mean water velocity inside cages decreased exponentially with sequential downcurrent cage crossings (Fig. 5). The greater the incident water velocity, the lower the velocity reduction relative to incident water velocity. At an incident velocity of 12 cm/ sec, water velocity was essentially stagnant in the third row of cages, suggesting that that, as water velocity decreases, fish movement may become the more dominant force affecting water movement in a commercial cage. Velocity reduction within successive cages may have implications for infestations of ectoparasites such as sea lice, which have greater attachment potential at slower current velocities (Samsing et al. 2015). Clearance times decreased in a near-perfect exponential relationship with initial incident velocity (Fig. 5), within up-current and down-current cages (Fig. 7). Clearance time of the down-current cage required approximately twice as much time as the up-current cage, regardless of initial current velocity. This suggests that, considering fish biomass, stocking and excretion rate in cages, nutrient concentrations from fish excretion would be double that exiting the down-current cage relative to concentrations exiting the up-current cage, related to a two-fold reduction in clearance time. At initial velocities of 12.5 TOP, FIGURE 5. Exponential relationship of velocity reduction through sequential cages at incidental velocities of a) 12 cm/sec b) 17 cm/sec and c) 34 cm/sec. Means have n = 3 and ± v. MIDDLE, FIGURE 6. Simultaneous two-cage dye release at three different incidental current velocities. C2 indicates camera two and a different camera angle. BOTTOM, FIGURE 7. Clearance time relationships between the up-current and down-current cages. Clearance time of the down-current cage takes approximately twice the time of the up-current cage regardless of initial velocity.
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