Because of all the variables involved, there is not a simple answer to this question. Selection of anchor size, type and weight is empirical and based on experience. Factors to consider are, geographic location of the cage. Is is situated in open water or in a secluded bay? Does it have to weather storms or is it located in the mid latitudes where the weather is generally mild? What sort of bottom will the anchor be situated in? Soft mud, firm sand, rock, coral etc..? How deep is the water the cage is situated in? How big is the cage?

What will be the forces exerted on the cage? Prevailing wind? Maximum strength expected? Currents, especially tidal currents. The force exerted on a cage by current can be considerable and probably outweighs wind and wave forces. Tidal currents reverse every six hours, roughly. If you use a single anchor, this will mean that the anchor will be overturned and have to reset every six hours, not a good scenario. A single anchor will also not prevent the cage from rotating and entangling its ground tackle. If you intend to use a concrete block, bear in mind that the SG of concrete is roughly 2.3, so a concrete block loses almost half its weight underwater. My recommendation, if you can afford it, would be to set a pattern of anchors around the cage, at least three, preferably four, bearing in mind the prevailing forces exerted on the cage in order to determine positioning of the anchors. You could use a heavier anchor to counter the larger force, and smaller ones elsewhere. Your rodes (anchor tether lines) should have a length at least three times the water depth from the point of attachment, which should be one of the rings, not the center of the bottom (see cage rotation), and preferably four or more, local conditions will determine the feasibility. Anchors hold better if the force exerted on them is parallel with the bottom, so I would use a length of heavy chain at each anchor. Attach the chain to the anchor and attach the rode to the other end of the chain. This will also keep the rode off the bottom and away from any possible chafe points. I would probably not use all chain rodes because, while that would be a more reliable setup, the weight may be detrimental to your cage and a full chain rode does not provide more holding power than a rope rode with an appropriate length of chain at the anchor. Use a nylon rode that provides some stretch. Do not use zero stretch lines. Use thimbles where you attach rope rode to metal shackles. Conditions will determine feasibility.

Stay away from anchors that have moving parts or hinges. These are less reliable for your application. I would recommend an anchor type that was specifically developed for mooring oil rigs in the North Sea. It is called a Bruce or claw type anchor. They come in various weights and any reputable marine supplier will be able to get them for you and will be able to advise you on the specification, provided you have the answers to the questions I have posed ready for them. These anchors are simple and reliable and have a good record. But, again, it depends on the bottom. If the bottom is rocky, sometimes a fisherman anchor is a better choice. As you probably understand by now, there is a lot more to anchor performance than just the weight.

Good luck, and let me know if you have any further questions.

Parameters are : - ϕ : friction angle between sea-bed and the dead-mass bottom (generally 45°), - δ : density of dead-mass (in kg/m3), - k : Safety coefficient for sliding (generaly 1.5).

Mass is :

M = (k F δ)/[g ( δ - ρw) tan( ϕ )]

δ recommended is : 2650 kg/m3 (2100 kg/m3 is considered unsufficient)

As indicater by George de Neef, there is no simple answer, as a proper design depends on a number of aspects:soil properties, cage characteristics and environmental conditions and economic design life of the cage and the risk percent to encounter higher loadings. I put below a list of relevant publications, I used in a recent advisory study. Happy industrious learning.

References:

Ágústsson G., 2004. Design considerations and loads on open ocean fish cages south of Iceland, M.Sc. thesis, University of Iceland, June 2004

Aarsnes J. V., H. Rudi and G. LoLand, 1990. Current forces on cage, net deflection, Engineering for offshore fish farming. Thomas Telford, London, 1990

Berstad, A.J.and Heimstad, L.F.(2015), “Numerical Formulation of Sea Loads to Impermeable Nets”, VI International Conference on Computational Methods in Marine Engineering MARINE 2015, Rome Italy

Berstad, A.J., Heimstad, L.F. and Walaunet, J.(2014) “Model Testing of Fish Farms for Validation of Analysis Programs.” Proceedings of the ASME 2014 33rd International Conference on Ocean, Offshore and Artic Engineering OMAE2014, (June 8-13 2014), San Francisco, California. OMAE2014-2464

Berstad, A.J., Walaunet, J. and Heimstad, L.F (2012), “Loads from Currents and Waves on Net Structures.”Proceedings of the ASME 2012 31st International Conference on Ocean, Offshore and Artic Engineering OMAE2012, (July 1-6, 2012), Rio de Janeiro, Brazil. OMAE2012-83757.

Berstad, A. J., Tronstad, H., Ytterland, A. (2004), “Design Rules for Marine Fish Farms in Norway.

Calculation of the Structural Response of such Flexible Structures to Verify Structural Integrity.” Proceedings of OMAE2004 23rd International Conference on Offshore Mechanics and Arctic Engineering June 2004, Vancouver, Canada. OMAE2004-51577

Berstad, A. J. and H. Tronstad (2005a), “Response from current and regular/irregular waves on a typical polyethylene fish farm” , Maritime Transportation and Exploitation of Ocean and Coastal Resources. Eds. C. Guedes Soares, Y. Garbatov, N. Fonseca. 2005 Taylor & Francis Group London. ISBN #: 0 415 39036 2.

Bessonneau J.S., Marichal D. 1998. Study of the dynamics of submerged supple nets (application to trawls), Ocean Engineering, 25(1), pp 563-583

Beveridge M. C. M., 2004. Cage Aquaculture, 3rd Ed., 2004 by Blackwell Publishing Ltd

Beveridge, M.C.M.B. 1996. Cage aquaculture, 2nd Edn. Fishing News Books, Oxford, p. 346.

Blendermann W., 1987 .An analysis of the hydrodynamic forces on cables and nets, Applied Ocean Research, 1987, Vol. 9, No. 4

Bratteland E., ed., 1988. Advances in Berthing and Mooring of Ships and Offshore Structures, Kluwer Academic Publishers.

Burrows R., R.G. Tickell, D. Hames & G. Najafian, 1997. Morison wave force coefficients for application to random seas, Applied Ocean Research, 19 (1997) 183-199, Elsevier Science.

Cairns, J. and Linfoot, B.T. 1990. Some considerations in the structural engineering of sea cages for offshore fish farming. In: - Acquaculture Engineering, Technologies for the future, Thomas Telford, London, pp. 63-77.

Carson, R.M. 1988. Engineering analysis and design of cage systems for exposed locations, in: Aquaculture Engineering:Technologies for the Future. Ing. Chem. Eng. Symposium series No. 111, pp. 77-96, EFCE Publication Series No. 66, Institution of Chemical Engineers, Rugby, UK.

Chalia s., 2014. A Numerical Simulation of Full 3D Model of Net-Cage Structure - Mooring Line Tension, International Journal of Current Engineering and Technology, Vol. 4, No. 4 (Aug 2014), http://inpressco.com/category/ijcet

Chen H., and Christensen E.D., 2015. Methods for current and wave interaction with fish cages: A

review. In Christensen, E. D., Sumer, B. M., Schouten, J-J., Kirca, Ö., Petersen, O., Jensen, B., ...

Burchard, H. (2015). D5.3 Interaction between currents, wave, structure and subsoil., MERMAID mermaidproject.eu, FP7 Theme [OCEAN.2011-1] “Innovative Multi-purpose off-shore platforms: planning, design and operation”

Cheslav B., B. Colbourne, N. Bose, W. Raman, 2009. Aquaculture Net Drag Force and Added Mass, Aquacultural Engineering 41, pp. 14–21.

Cifuentes C. and M. H. Kim, 2014. Dynamic Analysis for the Global Performance of An SPMFeeder-Cage System Under Waves and Currents, China Ocean Eng., Vol. 29, No. 3, pp. 415 – 430

Cremer M. C., H. P. Lan, and J. Chappell, 2008. Engineering Manual: U.S. Soybean Industry OCAT Offshore Ocean Fish Culture Cage, U.S. Soybean Export Council

Dept. of Agriculture, Food and the Marine, 2016. Protocol for Structural Design of Marine Finfish Farms, April 2016, Ireland

Gansel L. C., D. R. Plew, P. C. Endresen, A. I. Olsen, E. Misimi, J. Guenther, Ø. Jensen, 2015.

Drag of Clean and Fouled Net Panels – Measurements and Parameterization of Fouling, PLOS ONE

10(7):e0131051. doi:10.1371/journal.pone.0131051

Huguenin J. E., 1997. The design, operations and economics of cage culture systems, Aquacultural Engineering I6 (1997) 167-203

ISO 16488:2015, 2015. Marine finfish farms - Open net cage - Design and operation, International Standards Organization, 15 July 2015.

Kumar V., and Karnatak G., 2014. Engineering consideration for cage aquaculture, IOSR Journal of Engineering, Vol. 04, Issue 06 (June. 2014), V6 pp. 11-18.

Marine Scotland, 2015. A Technical Standard for Scottish Finfish Aquaculture, The Scottish Government, June 2015, Edinburgh

Mochet C., F. Paquereau, N. Decultot, 2014. Mooring Integrity: Forensics Programs on Used Mooring Connectors Lessons Learned, Present Practices & Future Developments, Deep Offshore Technology International Conference, Aberdeen, Scotland, October 2014., Paper DOT-2014-79

Norwegian Standard NS 9415.E:2009, 2009. Marine fish farms - Requirements for site survey, risk analyses, design, dimensioning, production, installation and operation, Norwegian Ministry of Fisheries

and Coastal Affairs, 10 Nov. 2009, published April 2010, Standards Norway

Rudi H., J.V. Aarsnes, and L.A. Dahle, 1988. Environmental forces on a floating cage system, mooring considerations. In: Aquaculture Engineering: Technologies for the Future. Ing. Chem. Eng. Symposium series No. 111, pp. 97-122, EFCE, Publication Series No. 66, Institution of Chemical

Engineers, Rugby, UK.

Sarpkaya T., and Isaacson M., 1980, Mechanics of wave forces on offshore structures, Van Nostrand Reinhold Co., N.Y., USA.

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Turner R., 2000. Offshore mariculture: Mooring system design, in: Muir J., Basurco B.,- Eds., Mediterranean offshore mariculture. Zaragoza: CIHEAM, 2000. p. 159-172 (Options Méditerranéennes: Série B. Etudes et Recherches; n.30

Zhao Y., T. Xu, C. Bi, G. Dong and S. Liu., 2012. The Numerical Simulation of Hydrodynamics of Fishing Net Cage, in: Zheng Jinhai, ed., Hydrodynamics - Theory and Model, Chapter 12, InTech, March 2012, http://www.intechopen.com/books/hydrodynamics-theory-and-model