The physics of open-water shellfish aquaculture

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<ul><li><p>Review</p><p>making offshore shellfish developments more attractive. Here we review issues relating to the design and mechanics of shellfish</p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146</p><p>www.elsevier.com/locate/aqua-online</p><p>Available online at www.sciencedirect.com</p><p>Aquacultural Engineering 38 (2008) 145160* Corresponding author. Tel.: +64 4 386 0476.</p><p>E-mail addresses: c.stevens@niwa.cri.nz (C. Stevens), d.plew@niwa.cri.nz (D. Plew), ndh@dhi.com.my (N. Hartstein), fredriks@usna.edu</p><p>(D. Fredriksson).</p><p>0144-8609/$ see front matter # 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.aquaeng.2008.01.0064. Effect of the flow on the farm: structural mechanics and forcing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148</p><p>4.1. Mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148</p><p>4.2. Current measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150</p><p>4.3. Wave measurement techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151</p><p>4.4. Farm motion measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152</p><p>4.5. Remote sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153</p><p>4.6. Farm motion modelling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153</p><p>4.7. Mooring design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153</p><p>5. Flow-related environmental issues and associated technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1533. Comparable canopy systems . . . . . . . . . . . . . . . . . .2. Approaches to open-water shellfish aquaculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146</p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1471. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .currents) on the farm and (ii) the reverse perspective of the impact of the farm on the flow. Because these systems are relatively new,</p><p>we first examine similar systems, both natural (kelp beds) and man-made (floating breakwaters, fish farms). Techniques for</p><p>measuring both the local oceanography and the structural response are listed along with new approaches for measuring important</p><p>properties. A number of future applied research topics are identified as being a key to advancing the industry, including issues like</p><p>mooring design, vertical drag coefficients, wavecurrent interaction, stratification and influence on fauna.</p><p># 2008 Elsevier B.V. All rights reserved.</p><p>Keywords: Shellfish; Physics; Oceanography; Longline; Mussel farm; New Zealand; Waves; Currents</p><p>Contentslongline structures in relation to the offshore marine environment. Twomain facets are explored: (i) the effect of the flow (waves andAquaculture of shellfish species is expanding in many couThe physics of open-water shellfish aquaculture</p><p>Craig Stevens a,*, David Plew b, Neil Hartstein c, David Fredriksson d</p><p>aNZ National Institute for Water and Atmospheric Research, Greta Point, Wellington, New ZealandbNZ National Institute for Water and Atmospheric Research, Christchurch, New Zealand</p><p>cDHI Water &amp; Environment (M) Sdn Bhd, Wisma Perindustrian, Jalan Istiadat, Likas, 88400 Kota Kinabalu Sabah, MalaysiadDepartment of Naval Architecture and Ocean Engineering, U.S. Naval Academy, 590 Holloway Road, 11D Annapolis, MD 21402, USA</p><p>Received 8 November 2007; accepted 28 January 2008</p><p>Abstract</p><p>ntries. Limitations on, and competition for, inshore water space is</p></li><li><p>. . . .</p><p>. . . .</p><p>. . . .</p><p>. . . .</p><p>mussel is grown in parts of Europe, North America and</p><p>Scandinavia. New Zealand, the forth largest musselmussel farm design, which vary depending on the water</p><p>depth, hydrodynamics and the regional style. These</p><p>C. Stevens et al. / Aquacultural Engineering 38 (2008) 145160146producer, grows an endemic species Perna canaliculus.</p><p>A variety of methodologies have evolved to produce</p><p>shellfish in large quantities in inshore waters. Rafts and</p><p>longline techniques have been established in tandem</p><p>with mooring and harvesting approaches. Strategies</p><p>include surface and submerged longline farms and raft-</p><p>based structures (Fig. 1).</p><p>The continuous longline technique is becoming a</p><p>dominant farming style and can be used for any of the</p><p>main mussel species commercially grown around the6. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .</p><p>6.1. Future research emphases . . . . . . . . . . . . . . .</p><p>Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . .</p><p>References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .</p><p>1. Introduction</p><p>Shellfish aquaculture has a long history, with many</p><p>early societies extending their wild harvesting techni-</p><p>ques through the introduction of accessible artificial</p><p>substrata and transplanting of important species. This</p><p>early development occurred because shellfish larvae</p><p>have a free-swimming stage. Hence, provided there is a</p><p>suitable supply of juveniles and nutrients and that the</p><p>environment is not too extreme, a harvestable crop will</p><p>result. Given these determinants, provision and devel-</p><p>opment of a suitable substrate is a major factor over</p><p>which some control can be exerted. Here we review the</p><p>physical factors associated with open-water shellfish</p><p>aquaculture. There is inevitably a local bias. However,</p><p>the New Zealand industry is a useful template as it is</p><p>well established but still expanding and one that place a</p><p>high value on environmental sustainability.</p><p>Here we define offshore as being exposed to</p><p>substantial oceanic conditionsmainly in the form of</p><p>exposure to large waves and storms. As there have been</p><p>few shellfish developments to date in this environment</p><p>we consider open water shellfisheriesthose that are</p><p>at some distance from shore and require reasonable</p><p>infrastructure. This broader category includes develop-</p><p>ments in large bays and coastal inlets.</p><p>Many inshore techniques for sustaining shellfish</p><p>have evolved through individual ingenuity and adapta-</p><p>tion. These tend to have regional idiosyncrasies. Within</p><p>South East Asia the main species grown is Perna viridis</p><p>and open-ocean aquaculture is being trialled in a</p><p>number of countries including Malaysia, Indonesia,</p><p>Thailand, Cambodia and the Philippines. The largest</p><p>producer in the world China (FAO, 2004) grows</p><p>predominately Mytilus galloprovincialis. This species</p><p>is also grown across much of Europe, the Russian</p><p>Federation, Brazil and Australia (Gosling, 2003; Buck</p><p>et al., 2006). Mytilus edulis also known as the blue. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156</p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156</p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158</p><p>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158</p><p>have also been developed to deal with things like ice-</p><p>cover (Drapeau et al., 2006) and large tidal ranges</p><p>(France). In addition, recruitment is no longer left to</p><p>chance and collection of juveniles (spat), integration</p><p>onto the artificial substrate and harvesting are important</p><p>steps in the evolving methodological development.</p><p>Intensive farming of inshore locations highlights a</p><p>range of areas of potential usage conflict (Ridler, 1997):</p><p>aesthetic value, navigation, nutrient/phytoplankton</p><p>depletion, space allocation and the likelihood of</p><p>terrestrially sourced contamination. This combination</p><p>of issues had led to industry looking to offshore waters</p><p>for future expansion.</p><p>Moving into exposed offshore coastal/ocean waters</p><p>requires substantial investment in planning and infra-</p><p>structure. One only needs to look at the oil industry to</p><p>see the scale required and the importance of good</p><p>engineering for successful development of infrastruc-</p><p>ture in the ocean environment. Two complementary</p><p>facets to the engineering of such marine structures arise.</p><p>First, the effect of the flow on the structure controls the</p><p>structural survival and the environment the crop must</p><p>develop in. Second, the effect of the structure on the</p><p>flow is also important for correct assessment of</p><p>environmental impact, especially through its influence</p><p>on redistribution of waste and nutrient-depleted water.</p><p>Clearly, there is feedback between the two. Section 2</p><p>describes approaches to shellfish aquaculture in open</p><p>waters, Section 3 describes comparable canopy</p><p>systems, Section 4 describes the effect of flow on the</p><p>farm, Section 5 describes the environmental implica-</p><p>tions of such structures and then Section 3 synthesises</p><p>the findings.</p><p>2. Approaches to open-water shellfish</p><p>aquaculture</p><p>Currently, there are a number of different types of</p></li><li><p>C. Stevens et al. / Aquacultural Engineering 38 (2008) 145160 147world, e.g.Mytilus galloprovincialis, Mytilus edulis and</p><p>Perna canaliculus. Longline farms typically cover 3</p><p>100 ha (0.031 km2) and consist of up to 200 longline of</p><p>approximately 120 m in length, each consisting of two</p><p>parallel backbone ropes supported by buoys</p><p>(Fig. 1a). Culture ropes (line droppers) are looped</p><p>continuously from the backbone rope and can be up</p><p>to 5 km long depending on the water depth. Descending</p><p>fromeach end of the backbone ropes aremooring lines</p><p>connected to anchor blocks that are typically fixed into</p><p>the seabed. A number of countries (e.g. Canada,</p><p>Australia,) use a longline system consisting of individual</p><p>line droppers for the harvest ofMytilus galloprovincialis</p><p>orMytilus edulis (Fig. 1a), suspended from the backbone</p><p>towards the seabed rather than one continuous line</p><p>running along the length of the backbone. In South East</p><p>Asia (e.g. Malaysia, Cambodia) as well as using the</p><p>longline system Perna viridis is cultured offshore using</p><p>wooden poles imbedded into the seabed onto which the</p><p>mussels attach. In Cambodia these poles are also used to</p><p>hold strings on which seaweed is cultured similar to the</p><p>traditional French methods.</p><p>Fig. 1. Different styles of open ocean shellfish farm including (a)</p><p>surface longline, (b) submerged longline and (c) raft development. In</p><p>(a) and (b) the float-line combination is the backbone of the longline.The longline technique is becoming the dominant</p><p>approach to physically supporting a substantial crop</p><p>with the minimum of infrastructure. In open-ocean</p><p>areas with strong waves and currents a number of</p><p>countries are developing submerged mussel farm</p><p>culture. The degree of submergence of the crop lines</p><p>can be achieved through buoyancy control (e.g.</p><p>Norway, http://www.smartfarm.no) or through mooring</p><p>design (e.g. USA Grosenbaugh et al., 2002). Submer-</p><p>gence reduces visual impact as well as the degree of</p><p>influence of surface waves (Fig. 1b). It does make</p><p>harvesting more difficult but not prohibitively so, with</p><p>specialised vessels set up to manipulate the backbone</p><p>and strip and re-stock the line. A submerged longline</p><p>resembles that of a surface longline except that it is</p><p>suspended 510 m below the sea surface by cable</p><p>attached to large buoys sitting on the surface at each end</p><p>of the longline (Fig. 1b). Some approaches keep the</p><p>floats along the line sub-surface as well. Another open-</p><p>water technique is raft culture (Fig. 1c, also Blanco</p><p>et al., 1996), which in many respects is similar to</p><p>longline culture. As with some longline culture, ropes</p><p>are suspended from the raft. The rafts are square shaped</p><p>and usual vary between 100 m2 and 550 m2 in size.</p><p>Rafts are usually made of a series of wooden or plastic</p><p>cross frames. A range of other species can also be</p><p>matured in suspended culture including oysters (e.g.</p><p>Pilditch et al., 2001) as well as seaweeds (Romo et al.,</p><p>2001).</p><p>3. Comparable canopy systems</p><p>Offshore shellfish aquaculture physical science is</p><p>relatively new and little-studied so there are substantial</p><p>benefits to be gained from looking at other canopy</p><p>systems. Whilst the most obvious comparative system</p><p>are fish cage arrays (e.g. Cromey et al., 2002;</p><p>Fredriksson et al., 2005; Hartstein et al., 2006) there</p><p>are a number of other canopies that are more relevant in</p><p>size and outlay including, for example, kelp beds and</p><p>floating breakwaters.</p><p>Offshore natural kelp beds have a number of points</p><p>in common with open-water shellfish structures. They</p><p>are subject to offshore conditions whilst supporting</p><p>some type of suspended canopy. A major point of</p><p>difference is that in a kelp bed there are many</p><p>separate structural entities individually attached to the</p><p>seabed as opposed to the end-point attachment of a</p><p>longline. On the other hand crop densities are</p><p>similar with a kelp bed having around 0.01 individual</p><p>plants per m2 (Jackson, 1984) and a shellfish farm</p><p>holding between 0.01 and 0.1 elements (dropper)</p></li><li><p>4. Effect of the flow on the farm: structural</p><p>mechanics and forcing</p><p>4.1. Mechanics</p><p>The interaction between farms and hydrodynamics is</p><p>important from the perspective of structural survival</p><p>and wave attenuation. Large shellfish suspended culture</p><p>installations affect currents and waves (Plew et al.,</p><p>2005, in preparation). This is effectively a transfer of</p><p>energy from the flow to the farm structure. There are</p><p>two hydrodynamic driverscurrents and waves</p><p>(Fig. 2). Currents can be considered to be steady in</p><p>that their variability is generally slow enough that any</p><p>structure responds comparatively rapidly. Hence, tidally</p><p>driven currents which although they oscillate can be</p><p>C. Stevens et al. / Aquacultural Engineering 38 (2008) 145160148per m2. A key finding relating to plant survival is the</p><p>importance of wavecurrent interaction and co-effect</p><p>(Seymour, 1996; Seymour et al., 1989; Elwany et al.,</p><p>1995) whereby the role of currents that were in the</p><p>direction of the waves were crucial to survival. In</p><p>addition the identification of flow reduction within</p><p>kelp beds (Jackson and Winant, 1983; Rosman et al.,</p><p>2007) is fundamental for many aspects of shellfish</p><p>farm design. Stratification is an issue in shellfish</p><p>canopies (Plew et al., 2006) as it may serve to</p><p>separate the crop from nutrient-rich waters or confine</p><p>contaminant-laden waters with in the canopy. Jackson</p><p>(1984) showed how internal wave variations would</p><p>diffuse into a kelp canopy generating unpredictable</p><p>phase differences between outside and inside flow</p><p>variations.</p><p>Emergent vegetation canopies (i.e. reed beds) have</p><p>some parallels to a large dense shellfish farm. Nepf and</p><p>Koch (1999) showed how secondary recirculation</p><p>would transport a tracer vertically with speeds reaching</p><p>15% of the background flow. This requires the dropper</p><p>to penetrate well into the benthic boundary-layer. This</p><p>may have implications for nutrient transport at the</p><p>dropper scale and smaller. There are also sim...</p></li></ul>

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