40 DECEMBER 2014 • WORLD AQUACULTURE • WWW.WAS.ORG Since the early 1980s, there has been an exponential growth in the quantity of seafood produced by aquaculture and, in the near future, this production will account for half the world’s global harvest of fish. With evergrowing demand, fish farms are trying to expand. Countering this expansion is growing concern, globally, on the potential environmental impact of larger aquaculture farms. As a result, many governments are becoming reticent to grant new concessions to aquaculture developments in coastal waters. Consequently commercial producers of marine finfish have been seeking alternative locations to coastal bays, fjords or other areas to locate larger net-pen farms. Moving further offshore appears to be the logical alternative, making farms invisible from the coast, reducing their ecological and environmental impact on coastal marine life and also enabling the installation of wider and deeper pens. Inevitably this expansion raises a number of new requirements: increased operating costs, on-site maintenance personnel, energy use and net-pen design. Existing pens may be unsuitable for the harsher conditions found in the open ocean, where mooring lines must resist large fluctuating forces and high induced drag generated by waves and currents. Exploiting wave action to provide autonomous wavepowered energy system for aquaculture farms is being explored currently (Meggitt 2014). Failure of mooring lines caused by overloading often leads to net loss, resulting in ‘ghost nets’ that can pose a threat to marine life and a navigation hazard. Furthermore, an increase in climatic variability seems likely to lead to an increase in ocean wave states. Considering future changes in environmental conditions, the compounding problems of increasing pen size and the lack of natural shelter in the open ocean, new farms must increase their capabilities to withstand degradation and major mechanical fatigue. The interaction of the net with water flow from waves and currents govern the resulting forces. This is an area where efforts can be The Impact of Net Density on Oceanic Aquaculture Pens Benjamin Levy, Heide Friedrich, John Cater, Richard Clarke and James P. Denier made toward optimizing the net structure (mesh/twine composition or ‘weave’) and the relative location of a pen within a concession. In addition to the mechanical action of water flow, there is the associated problem of bio-fouling, which can serve to reduce net porosity by over 50 percent, with direct consequences for fish health through a reduction in the rate of oxygen supply. Another major impact of biofouling is the resulting drastic increase of drag forces on the structure, increasing tension in mooring lines or towing cables. These combined factors have motivated a deeper study of aquaculture net drag and how it may be minimized. The considerations presented here must be taken together with certain practical issues such as the constraint of the mesh void size in relation to fish size and the need of the twine thickness of the mesh to support mechanical stress. Technical Aspects Within the Faculty of Engineering at the University of Auckland, we are undertaking a study of the impact of net density on flow dynamics and drag, where variations in the mesh void (i.e. the space between two consecutive twines) and twine thickness are being investigated. To better quantify the effects of design changes, a small-scale experimental investigation was conducted within our Fluid Mechanics Laboratory, where changes can be evaluated under well-defined and controlled flow conditions. Experimental data were acquired using a recirculating flume with a measured turbulence intensity of less than 3 percent. The flume cross-sectional area is 0.4 m by 0.4 m, with a length of 4 m and an operational flow rate between 2 and 50 cm/s. Under these controlled conditions, it has been possible to obtain detailed measurements of the flow dynamics for a range of models with varying mesh and twine dimensions. Rather than studying a section of a net, we manufactured FIGURE 1. Three net-pen models with varying mesh and twine dimensions. FIGURE 2. Flow regimes obtained from two models with the same mesh void size but differing twine thickness. The increase in twine thickness reduces the bleed flow velocity which results in greater velocity gradients that evolve into an unstable wake (lower image).
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