24 DECEMBER 2015 • WORLD AQUACULTURE • WWW.WAS.ORG drag coefficients and change porosity (Klebert et al. 2013). Swimming fish can mix water, and provide additional barriers to incoming water, drawing water from above and below swimming depths, thereby promoting outward flow at the depth of maximum fish biomass (Gansel et al. 2014). The review by Klebert et al. (2013) provides an in-depth discussion of these effects. Current flow and direction is of interest for the placement of co-cultured species in openwater Integrated Multi-Trophic Aquaculture (IMTA) systems to optimize nutrient delivery. Shellfish and seaweeds are cultured adjacent to finfish in some IMTA systems to intercept organic particles and take up dissolved nutrients (Chopin et al. 2012). The concentration of particulates or solutes in the plume exiting a fish cage is a function of the mass loaded and the current velocity through the loading source. If a discrete volume, such as a cage, and the timeframe of interest are defined, a simple one-dimensional concentration model can be used to provide rudimentary estimates of mean near-field concentrations (Silvert and Sowles 1996, Troell and Norberg 1998, Reid and Moccia 2007, Middleton and Doubell 2014). Although mean concentration estimates produced in this manner are useful for providing general guidance, they do not provide detailed spatial information, nor details on concentration extremes or nutrient delivery rates, which affect the ability of cocultured species to take up and use nutrients (Cranford et al. 2013). Dissolved oxygen supply may also be affected by aquaculture structures. Oxygen depletion can occur in fish cages if the rate of biomass respiration exceeds the rate of supply through flushing (Page et al. 2005). Loading additional animals onto site lease areas for IMTA results in additional oxygen demand and current velocity takes on greater importance. Consequently how farm structures Influence of cage arrays on current flow can result in highly complex, near-field hydrodynamics and detailed knowledge of these effects are important considerations for numerous cage aquaculture management criteria. Current flow in and around aquaculture cages affects volumetric loading of nutrient waste (Silvert and Sowles 1996, Reid and Moccia 2007, Middleton and Doubell 2014), attachment potential of ectoparasites (Chang et al. 2011), therapeutant dispersal from in situ treatments and the rate of dissolved oxygen supply (Page et al. 2005). Predominant effects of water flow through cages are drag, velocity reduction and the generation of wakes and turbulence. Drag is produced as water flows through cage netting and around cage structures (Løland 1993), thereby reducing current velocity and increasing turbulence in adjacent waters. Drag and turbulence are affected by multiple factors: the ratio of net thread to space, referred to as solidity (Løland 1993) or porosity (Helsley and Kim 2005), flexibility of the cage (Lader et al. 2007, Moe et al. 2010), angle of attack (Løland 1993, Helsley and Kim 2005, Zhao et al. 2013), number of net panel crossings (Løland 1993, Zhao et al. 2013), lift (Le Bris and Marichal 1999), space between cages (Løland 1993, Helsley and Kim 2005), the drag coefficient (or roughness) of individual net threads (Løland 1993, Helsley and Kim 2005) and current velocity (Fu et al. 1989). Wakes can be generated at multiple scales ranging from individual net threads (Løland 1993, Helsley and Kim 2005) to vortical flow generated by the general obstruction of water flow caused by cages (Helsley and Kim 2005). Wake extension is a function of incident current velocity (Cornejo et al. 2014) and wake spread a function of distance travelled (Løland 1993). These parameters are seldom constant. Fouling of mesh can alter thread Effects of Circular Fish Cage Arrays on Current Dynamics: Implications for Near-field Velocity Reduction, Nutrient Concentrations and Cage Clearance Times A. Turner, J. Del Bel Belluz, S. Sprague, A. Byrne and G.K. Reid TOP, FIGURE 1. Workshop group: From left to right, Allie Byrne, Jordana Van Geest (co-leader), Hannah Bradford, Sarah Sprague, Gregor Reid (co-leader), Adam Turner, Adam Gray, Di Wan, Kurt Simmons and Justin De Bell Belluz. BOTTOM, FIGURE 2. Cross-sectional schematic of a single cage showing inner containment net and outer predator net.
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