Considerations on Cage Design

8/6/2019 Considerations on Cage Design

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Considerations on Cage Design

Philip S. Cruz

Cruz Aquaculture Corp.


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General Types of Cages

Fixed

Floating (also called gravity cage)

Wooden/bambooSteel

Plastic

Submersible


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Fixed Cage

Tilapia cages (Lemery, Taal)

Bangus cages (Dagupan City)


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Features

ADVANTAGES

Simplest and cheapest

to construct of all

cage types

DISADVANTAGE

Requires well-sheltered areas

Cage volumefluctuates with tide

Prone to rapid waterquality deterioration

Prone to damage fromflash floods

Requires frequent

repair


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Floating Cage: Wooden/Bamboo

Grouper cages

(Guimaras, Iloilo)

Bangus cages

(Ragay Gulf)


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Features

ADVANTAGES

Simplest and cheapest

to construct of

floating cages

DISADVANTAGE

Easily damaged by

rough weather

Maximum length and

width limited to 5-6

meters

Requires frequentrepair and

maintenance


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Typhoon damage, Taal Lake


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Floating Cage: Steel

Milkfish cages (Bolinao) Milkfish cages (Bolinao)

Tilapia cages (Taal) Grouper cages (Taal)


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Features

ADVANTAGES

More robust

compared to wooden/

bamboo cages

Can be constructed to

larger dimensions

DISADVANTAGE

Prone to typhoondamage when not

properly constructed High steel prices have

made steel cagesalready expensive to

construct Costly to maintain

(requires frequentpainting)


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Floating Cage: Plastic

Australian tuna cages

Taiwanese cobia cages


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Features

ADVANTAGES

Will easily tolerate 3

meter waves when

properly constructed

and moored

Long service life

Low maintenance

DISADVANTAGE

High capital cost


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Submersible Cage


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Submersible Cage

100-m3


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Features

ADVANTAGES

Can survive 5-7 m

waves

DISADVANTAGE

High capital cost, not

practical for low value

and medium value

fish species

Difficult to feed fish

equitably Difficult to clean nets


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Considerations on Cage Design

Cage shape

Cage size

Net depth Net mesh

Cage layout


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Effect of Cage Shape

Productivity increases with greater cage

surface area exposed to the water current

A rectangular cage is potentially more productive than a

square cage, which is potentially more productive than a

circular cage


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In terms of fish

behavior, a circular cage

is more suited forpelagic species

A square cage is

desirable for demersalspecies as it maximizes

bottom space


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In sites exposed to

strong tidal water

currents, small cagesshould be

rectangular in shape

to minimize feeds

from drifting outside

of cage

W

A

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Effect of Cage Size

The smaller the cage, the higher is the

water exchange rate, and hence the higher

is the productivity per unit volume


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Effect of Cage Size

In a square cage, the potential productivity

drops by 50% every four fold increase in

area 400kg

200

kg100

kg

A 1 x 1 m cage is potentially twice more productive than a

2 x 2 m cage, and four times more than a 4 x 4 m cage


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Effect of Cage Size

Smaller cages which are stocked more densely,tend to have higher FCRs due greater feedingcompetition, stress, and inevitable feed losses

during feeding frenzy


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Effect of Cage Size

Higher FCR in smaller cages is also partlydue to the more pronounced feedingdomination of bigger fish

Smaller cages demands more carefulfeeding management

Use of floating feeds is recommended for

small cages


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Effect of Net Depth

Assuming the same watervolume, a deeper net willsupport more fish than abigger cage

Deeper nets, however, will

tend to produce uneven fishsizes due to the limited

feeding area on the surface


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Effect of Net Depth

Deep nets (for gravity typecages) easily collapseunder strong currents

resulting to stress if notinjury to fish; use should belimited to calm water

The deeper the net, the more

difficult it is to change andinspect; maximum depthshould be limited to 8-10 m


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Effect of Net Depth

DEEPER NETS WILL REQUIRE:

heavier net weights (to minimize net

deformity)

greater cage buoyancy (to support

heavier net weights)

tapering cut (to reduce net drag)

thicker twine (for tolerance to greater

drag)

stronger cage frame (for tolerance to

greater torsional stresses)


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Effect of Net Mesh

The bigger the mesh size, the freer is the waterflow

To attain the maximum growth potential, it is

necessary to increase mesh size as the fishbecomes bigger

Ideally, at least three mesh sizes are used duringthe entire culture period, starting with 25-35K

and ending with 8-10K


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Effect of Net Mesh

Square mesh net orientation is more desirable

compared to diamond mesh orientation as it

maintains the cage shape better

Square mesh net orientation is also less prone to

clogging


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Effect of Cage Layout

Multiple cages should ideallybe positioned one afteranother in a straight line and

perpendicular to the currentflow for optimum waterexchange

This is especially so in areas

where water flow ispredominantly uni-directional.


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Effect of Cage Layout

In multiple cages adopting a chess board

arrangement, the compartments facing the

incoming water will tend to have higher

growth rates and biomass productivity


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End

Considerations on Cage Design

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