WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2015 27 cm/sec and 18.8 cm/sec, dye from the up-current cage entered the down-current cage before dye from the down-current cage had completely cleared. At 25.0 cm/sec, the majority of dye had cleared the down-current cage before dye from the up-current cage entered. At lower current velocities, where soluble nutrients from up-current cages enter down-current cages, increases in nutrient concentration will theoretically be greater than the two-fold increase expected by halving the flushing time. Dye patterns indicated an area of reduced flow directly behind cages, which was more pronounced at higher velocities. Dye exiting cages at the slowest velocity of 12.5 cm/sec traveled in a clump, whereas dye exiting cages at 18.8 cm/sec and 25.0 cm/sec showed a hornshaped pattern, with the horns originating at cage edges (Fig. 6). These results suggest that water exiting laterally from cages is directed by faster current moving around cage edges. In many commercial aquaculture settings, flow direction is subject to change based on tidal cycles, unlike testing in a flume tank. Ideally, if extractive species sensitive to high rates of nutrient delivery, such as blue mussels (Cranford et al. 2013), are placed in a locale that is in the lowvelocity area directly down-current from a cage or cages, they would obtain maximum benefit from the greatest relative particulate concentration traveling at the slowest relative velocity. Dye exiting cages at the slowest current velocity would occasionally be drawn below the level of the cage bottom (Fig. 8). This was not readily observable at higher current velocity, suggesting that a greater relative shearing force of faster water may have superseded turbulencerelated downwelling forces caused by cages. This suggests that, under some conditions, co-cultured extractive aquatic animals and plants could be placed below the level of the cage bottom and be able to intercept some loaded nutrients. Vortical and turbulent effects were observed more easily with flagging tape than with dye release (Fig. 9), given the inherently transient nature of released dye. Greater undulation and spiraling of flagging tape occurred at outer lower cage edges under the slowest incident velocity. Very little vortical movement was indicated from flagging tape attached to the middle of the cage or the cage edges at the two higher incident current velocities. These observations complement the findings of Turner et al. (2015), who used propeller current meters to measure mean velocities and turbulence at different locations in the wake of cage arrays. There were some notable study limitations. Neutrally buoyant balls released within cages will travel, in actuality, slower than the local flow velocity. However, differences between ball velocity and local flow velocity were assumed to be negligible for the purpose of this study. The research interest was concerned with relative differences in water velocity in different cages of the array. Largescale turbulent motions shed from cages may also affect the motion of neutrally buoyant balls, thus affecting their transit time and mean velocities. The exponential relationship of dye clearance times under different incident velocities were consistent with exponential rates of change of common engineering and physical phenomena, suggesting the data collected was a good representation of a real effect. At the qualitative level, flagging tape undulation and the lead front of dye plumes appeared to be a good proxy for current velocities. However, there was insufficient concurrent footage of adjacent drogue balls in the same video frames to validate current velocity estimates in post processing image analysis. Finally, as with most studies examining forces on fish cages, no fish were stocked into cages. Fish may have a major effect on water movement, so study results should be interpreted with caution. Nevertheless, a good understanding of hydrodynamic effects on cage arrays in the absence of fish is of value for comparison with studies that do include fish because this will help to differentiate the effects of fish from effects caused by cages. Conclusion Inner cage water velocities and cage clearance times decrease exponentially as water passes downcurrent through successive cages, with zones of theoretically greater nutrient concentration directly behind single and successive cages relative to current flow direction. Although current direction and the location of slow-flow areas at commercial sites may change with tidal cycles, this research suggests that location of proximate placement of co-cultured species matters and consideration of predominant current directions and near-field culture space should be prominent considerations for effective IMTA. (CONTINUED ON PAGE 28) TOP, FIGURE 8. Gallery view of dye plume exiting the down-current cage, during simultaneous two-cage dye release trial. Incident current velocity of the flume tank was 12.5 cm/sec. Pink dye was released in the up-current cage and brown dye was released in the down-current cage. BOTTOM, FIGURE 9. Flagging tape attached to the third cage in a 2 x 3 cage array. Tape in foreground is attached to the outer edge of the weighted (lower) collar. Tape in background is attached to the upper and bottom collar in the middle of the exiting plume.
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