Look­ing at the cage re­al­i­ties

Fish Farmer - - Norway -

HOW will ma­rine aqua­cul­ture de­velop over the next few years? As the num­ber of avail­able shel­tered sites dwin­dles and the need to com­pete with tourism and other sea users in­ten­si­fies, al­ter­na­tives must be stud­ied, in­clud­ing land­based tank farm­ing and offshore cages. I think we will see de­vel­op­ments in both of these di­rec­tions.

Off­shores sites of­fer some ad­van­tages over more shel­tered sites; a com­par­i­son of phys­i­cal pa­ram­e­ters is given in Ta­ble 1. Gen­er­ally, offshore sites pro­vide bet­ter con­di­tions for the fish, although the ef­fects of in­creased wave ac­tion on the stack and phys­i­cal struc­ture need to be taken into ac­count. Also, offshore cages are more dif­fi­cult to op­er­ate, re­quir­ing big­ger boats and well­trained staff.

In Nor­way, fish farm­ing has de­vel­oped from home-made wooden cages in shel­tered sites to steel or plas­tic cages in more ex­posed sites and third-gen­er­a­tion cages used in ex­posed ar­eas with waves up to 2m height. Lately, a fourth gen­er­a­tion of cages has been pro­duced for the open sea, which can tol­er­ate waves of up to 4m in height.

How big should cages be? Ex­per­i­ments Hel­ge­land Plast have done show that fish in 22m di­am­e­ter cages give 20-30 per cent bet­ter growth than those in 16m cages, but they found no dif­fer­ence in growth be­tween fish in 22m and 33m di­am­e­ter cages. This sug­gests that 22m di­am­e­ter cages are the op­ti­mal size.

Nets are the weak points in offshore cage sys­tems. The float­ing struc­ture or col­lar is al­ways well-built, as much is known about the strength of the steel or other ma­te­ri­als used in con­struc­tion; a lot less is known about the design and strength of nets.

Amer­i­can com­pany, N.E.T. Sys­tems Inc’s Ocean Spar sys­tem has a ver­ti­cal spar which sup­ports the sus­pended net, al­low­ing waves to pass through with lit­tle move­ment of the mesh.

Another new open sea struc­ture, Arc­tic Farm, de­signed by Offshore Design A/S in Nor­way al­lows the net to be tight­ened at a depth of 5m, which helps pre­vent it from mov­ing and hurt­ing the fish. One such unit is in use in Nor­way. The cost of this sys­tem is about NOK 3.5 mil­lion (£307,000) for six cages mea­sur­ing 18x18m.

Hy­daq A/S have made and sold seven sys­tems in Nor­way. These have been ap­proved for use in seven me­tre waves and use chains and ropes to sup­port a float­ing net kept sus­pended by the moor­ings.

One of the more well-known sys­tems is the Aquasys­tem 104 barge which had its nets dam­aged in a storm last win­ter.

Hel­ge­land Plast have de­vel­oped and tested their offshore cages in waves with a sig­nif­i­cant wave height of up to 4m. These cages are made from three cir­cu­lar HDPE pipes.

Ta­ble 2 com­pares the cost per m3 of the var­i­ous sys­tems, as­sum­ing 15m deep nets where the depth is not de­ter­mined by the design.

Blak­stad (1988) in­ves­ti­gated the economics of farm­ing salmon in in­shore and offshore cages. His re­sults are sum­marised in Ta­bles 3 and 4, and show that prof­its are slightly higher in offshore cages. This gives an in­cen­tive to salmon farm­ers who may soon have no other choice but to ex­pand into offshore sites if they want to in­crease pro­duc­tion.

Bjorn Myrseth, Ma­rine Har­vest A/S, makes an ap­praisal of some ex­ist­ing sys­tems, their rel­a­tive costs and cien­cies” effi

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