A fish cage net may be designed and manufactured with a specified depth, such as:
5 meters
8 meters
10 meters
15 meters
When the cage is installed in calm water, the bottom of the net may hang close to the intended depth.
Under strong current, however, the cage can become noticeably shallower.
The net does not necessarily lose material or permanently shrink. Instead, the flexible side panels are pushed downstream, while the bottom panel and weighting system move away from their calm-water positions.
Part of the net’s vertical depth is converted into:
Horizontal displacement
Diagonal panel length
Inward deformation
Bottom uplift
The result is a reduction in the cage’s effective vertical depth and internal volume.
This change can affect:
Fish swimming space
Stocking density
Water exchange
Feeding behavior
Net tension
Clearance from structures
Escape risk
Understanding current-induced shallowing helps farms select suitable net construction, weighting, cage geometry, and maintenance procedures.
Nominal depth is the specified dimension of the net under an agreed measurement condition.
Effective depth is the actual vertical distance reached by the cage in operation.
A net may be manufactured correctly while showing less effective depth under current.
A net panel does not behave like a rigid wall.
It can:
Bend
Stretch
Rotate
Bulge
Change mesh angle
This flexibility allows the cage to respond to water movement, but it also permits substantial deformation.
When water flows through and around the mesh, it applies drag to:
Twine
Knots
Seams
Borders
Fouling
The side panel is pushed in the direction of the current.
In calm water, a side panel may hang nearly vertically.
Under current, the lower part moves downstream while the top remains connected to the floating structure.
The panel then forms a diagonal or curved shape.
The net panel may still contain its full material length.
However, only the vertical component contributes to cage depth.
The remaining length is used in downstream displacement and curvature.
Imagine a side panel with a fixed material length.
When it hangs vertically, nearly all of that length contributes to depth.
When the bottom is displaced horizontally, the same panel forms a diagonal.
Its vertical reach becomes smaller.
A shallower cage does not automatically mean:
The roll was manufactured short
The net permanently contracted
Material is missing
The first question should be whether the cage is deforming under load.
As current speed increases, the hydrodynamic force acting on the net generally rises significantly.
The relationship is not simply one-to-one.
A moderate increase in current speed can produce a much larger increase in loading.
The current pulls the side and bottom netting away from their calm-water positions.
This can cause:
Greater side-panel inclination
More inward bowing
Increased bottom movement
Reduced vertical depth
A net with more open space allows more water to pass through.
A net with more blockage acts more like a solid barrier.
The effective open area depends on:
Mesh size
Twine diameter
Knot size
Fouling
Panel angle
When other factors are similar, smaller mesh usually places more twine and intersections within the same area.
This can increase obstruction and drag.
However, mesh must still be small enough to contain the cultured fish.
Thicker twine may improve abrasion allowance and structural capacity.
At the same time, it occupies more of each mesh opening.
This can increase water resistance and cage deformation.
A heavier net may contain thicker or more numerous twine elements.
It may also create more drag.
Stability depends on the complete balance between:
Net construction
Weighting
Cage frame
Current
Fouling
A lighter net may reduce some hydrodynamic resistance.
However, it may have less material available to resist:
Abrasion
Fatigue
Cleaning damage
Contact wear
The correct construction should match the operating environment.
Knots create thicker local obstructions than straight twine segments.
Across thousands of mesh intersections, they can contribute to overall resistance.
Knotted and knotless nets may therefore respond differently to current.
Knotless netting may have smoother junctions, but it remains a flexible flow-through structure.
It can still:
Bow
Incline
Lose effective depth
Experience border loading
A mesh opening is not hydraulically identical at every angle.
As the panel inclines or twists, the projected obstruction presented to the current changes.
Deformation can therefore influence further loading.
A current pushes the panel downstream.
The panel angle changes.
The changed angle alters water flow and load distribution.
This can create additional deformation until the cage reaches a new temporary equilibrium.
Strong current can pull the bottom net:
Downstream
Upward
Toward one side
This reduces the lowest vertical point of the cage.
The side wall may not remain a straight diagonal.
It can curve inward toward the cage center.
This reduces internal width as well as depth.
A small reduction in depth across a large cage can remove a considerable amount of swimming volume.
If side walls also bow inward, the total volume loss becomes greater.
A cage may look normal from the surface.
The floating collar can remain circular or rectangular while the underwater net volume has changed significantly.
Underwater observation is necessary.
Under directional current, a circular cage may develop:
A rounded upstream side
An elongated downstream section
Uneven internal volume
The bottom may also shift downstream.
In rectangular systems, panels and corners may move differently.
The underwater shape can become:
Slanted
Twisted
Uneven between corners
Sharp directional changes may concentrate stress at borders.
A rectangular corner connects two panels.
When current pushes both panels, their loads meet at the corner.
This can create high tension and uneven depth near the corner structure.
Weights, bottom ropes, and sinker tubes are used to resist uplift and maintain cage geometry.
They provide downward force against current-induced displacement.
If the weighting system is too light, the net bottom may move more easily.
Possible consequences include:
Reduced depth
Bottom-panel uplift
Increased folding
Net-to-net contact
Adding excessive weight can increase continuous tension in:
Border ropes
Attachment loops
Seams
Corners
It can also complicate lifting, cleaning, and maintenance.
A suitable total weight can still perform poorly if it is concentrated unevenly.
One side may remain deep while another rises.
Uneven weighting can distort the cage and overload local connections.
A properly designed sinker tube can distribute load around the cage perimeter.
It may help reduce local bottom uplift.
However, it must remain correctly positioned and separated from the net where required.
If the tube moves, tilts, or becomes partially detached, the cage may become:
Deeper on one side
Shallower on another
Twisted at the bottom
Inspect its full circumference or perimeter.
Ropes and attachments supporting the weighting system may:
Elongate
Slip
Wear
Change position
This can reduce effective tension and depth.
The main panel distributes current force over a large area.
Border ropes collect and transfer that load into:
Floating frames
Attachment points
Sinker systems
A shallowing cage often places increased demand on the borders.
The top edge may be fixed to the cage frame, but the first mesh rows can still angle inward.
This reduces usable space near the surface.
The lower perimeter is influenced by both:
Current drag
Downward weighting
The balance between these forces determines bottom position.
Wide attachment spacing allows larger unsupported sections to bow.
Closer suitable spacing can improve load distribution.
However, excessive tight attachment can create local abrasion.
When one loop or clip fails, the neighboring attachments carry more load.
The panel may sag, twist, or move downstream more strongly near the failed point.
If some connections are tighter than others, the cage may not deform uniformly.
One section can become shallow before the rest of the panel.
Seams are normally thicker and stiffer than the surrounding mesh.
They may restrict movement locally.
The net can bend or deform beside the seam instead.
A stiff vertical seam may divide the panel into sections that bow differently.
This can produce irregular cage depth and internal shape.
A horizontal transition may become a repeated bending line under current.
The upper and lower portions can incline at different angles.
A patch may differ in:
Mesh size
Twine diameter
Weight
Stiffness
It may resist or respond to current differently from the original panel.
A large repair may add local weight while also increasing drag.
This can alter the panel’s underwater geometry.
Marine growth blocks mesh openings and adds mass.
A fouled net can experience much greater drag than the same net when clean.
As more of the mesh becomes blocked, less water passes through.
More current force is then transferred into the net structure.
The cage is pushed farther downstream and inward.
Barnacles and mussels increase downward mass.
This might appear helpful for depth.
However, their blockage and drag can create much greater horizontal force, often increasing deformation overall.
Fouling adds both:
Downward weight
Horizontal resistance
The final cage shape depends on which effect dominates and how the load is distributed.
One panel may be heavily fouled while another remains relatively clean.
The heavily blocked side can deform more strongly.
The cage may become asymmetric.
Fouling organisms vary with:
Light
Temperature
Depth
Water movement
The upper and lower parts of the cage may not carry the same load.
Cleaning one section while leaving another heavily fouled creates different flow resistance across the cage.
The clean section and fouled section may move differently.
This can change local depth and panel curvature.
A cage may appear acceptable during slack water.
At tidal peak, fouled panels can bow severely.
Inspection should include stronger-flow conditions where safe and practical.
When current reverses, the cage shifts toward the opposite side.
Netting and attachments bend through another loading cycle.
The shallowest region may move around the cage.
The cage may become shallow in one direction, recover partially, and then deform in the opposite direction.
Repeated cycling can wear:
Twine
Knots
Borders
Loops
Seams
Waves can:
Lift the frame
Move the net
Change line tension
Alter bottom clearance
Combined current and wave loading can produce greater depth variation.
During strong weather, current and waves may temporarily push the cage far beyond its normal geometry.
After the event, the cage may not return completely if:
Attachments slipped
Weights moved
Netting stretched
Borders were damaged
Twine and mesh may elongate under load.
This does not always restore depth.
Extension may occur along diagonal or horizontal load paths, increasing downstream movement.
Diamond mesh changes shape as tension changes.
It may become:
Longer in one direction
Shorter in the other
The net can therefore lose vertical depth even without permanent material elongation.
A loosely installed net may:
Flutter
Fold
Move excessively
An overly tight net may transfer high loads directly into attachments.
Balanced installation is essential.
Strong initial tension does not make a flexible cage rigid.
It may instead increase loads on borders and frames when current pushes the panel.
Excess slack allows the net to:
Fold
Touch itself
Rise at the bottom
Reduce swimming space
Loose installation should not be used as a simple solution for current.
A flexible floating frame can deform along with the net.
If the frame changes shape, the net’s attachment geometry also changes.
This can amplify underwater volume loss.
The mooring system determines how the cage aligns with current and waves.
Poor alignment may expose a larger panel area directly to flow.
If the cage orientation differs from the design assumption, current may strike:
A broad flat side
A vulnerable corner
A heavily repaired section
This can increase shallowing.
Upstream cages disturb the water reaching downstream cages.
Downstream units may receive:
Lower average speed
Higher turbulence
Irregular directional flow
Their deformation patterns may differ even with identical netting.
A cage with nominal depth of 10M in 15M-deep water appears to have 5M bottom clearance.
Under current, the net may shift downstream and upward.
Clearance can change relative to seabed features or nearby equipment.
Fish may respond to a shallower cage by:
Crowding deeper remaining zones
Moving away from high-current areas
Changing schooling patterns
The available volume is not only smaller but also differently shaped.
When cage volume decreases but fish quantity remains constant, the effective stocking density rises.
This may affect:
Movement
Competition
Oxygen demand
Welfare
Feed distribution is usually planned for a certain cage shape and current pattern.
A deformed cage may cause feed to:
Drift differently
Concentrate in one area
Leave the cage faster
If one side of the cage bows inward, the remaining swimming space may be closer to high-flow regions.
Fish may need to expend more energy maintaining position.
Strong current may increase water renewal.
However, severe deformation and fouling can create:
Fast-flow channels
Sheltered zones
Uneven oxygen distribution
More current does not always mean uniform internal conditions.
When cage walls move inward, fish may have less space to avoid the net.
Crowding against netting can increase:
Scale damage
Stress
Contact with rough fouling
Some cage systems use separate predator nets.
If the inner net deforms outward or the predator net deforms inward, the clearance between them may decrease.
This can reduce the effectiveness of the separation.
When two net layers touch under current, vibration can cause repeated rubbing.
This may create wear even when each net is individually strong.
A deformed bottom or side panel may rub against the weighting system.
The contact can create long abrasion lines.
A panel that normally has sufficient clearance may reach:
Pipes
Brackets
Walkways
Mooring components
under strong current.
Possible methods include:
Depth sensors
Marked vertical lines
Underwater cameras
Sonar or suitable acoustic systems
Diver inspection
The selected method should match cage size, risk, and operating conditions.
The cage collar may look normal while the underwater net is severely deformed.
Inspection must include the submerged structure.
Cameras can show:
Panel inclination
Bottom uplift
Inward bowing
Contact with structures
Fish distribution
Compare footage during calm and strong-current periods.
Sensors at selected points can record how cage depth changes over time.
They may help connect deformation with:
Tide
Current speed
Fouling
Storms
Cleaning
A single bottom-center sensor may miss:
Uneven corner depth
Side-panel bowing
Sinker-tube tilt
Multiple points give a clearer shape profile.
A properly installed reference line may help indicate vertical position.
However, line stretch, angle, and current movement must be considered.
Diver inspection can confirm:
Weight position
Border condition
Attachment damage
Net-to-frame contact
Fouling severity
Safety procedures must govern underwater work.
The most useful inspection compares:
Slack water
Normal current
Strong current
This reveals both baseline geometry and operational deformation.
Shape observations are more meaningful when linked to environmental data.
Record:
Current speed
Direction
Tide stage
Wave condition
Fouling condition
Operators should define how much loss of:
Depth
Volume
Clearance
Shape
is acceptable for the cage design and cultured species.
A net order should also consider:
Expected current
Mesh blockage
Weighting system
Cage geometry
Attachment layout
A 10M net does not guarantee 10M operating depth under every condition.
Large or exposed farms may require engineering analysis of:
Drag
Net deformation
Mooring loads
Sinker requirements
Cage volume
The net should be selected as part of the whole system.
Removing fouling reduces:
Mesh blockage
Drag
Added weight
Uneven loading
A cleaned cage may recover part of its original underwater shape.
Aggressive cleaning can damage:
Twine
Knots
Borders
Repairs
Cleaning should restore flow without weakening the structure.
Cleaning only one side may reduce drag there while other panels remain blocked.
The cage may deform unevenly until cleaning is completed.
Check for:
Missing weights
Shifted sinker tubes
Worn ropes
Uneven tension
Damaged connectors
Depth control depends on the entire weighting system.
Extra weight increases load on:
Frames
Borders
Loops
Lifting systems
Changes should be evaluated carefully.
The root problem may be:
Uneven attachments
Fouling
Misalignment
Missing supports
Correcting these can be more effective than adding mass.
Strong current increases movement and contact pressure.
Rings, clips, and frames should have smooth surfaces to reduce abrasion during deformation.
Borders, corners, seams, and loops may require reinforcement.
However, overly stiff reinforcement can shift stress into adjacent mesh.
A repair that differs greatly in stiffness or stretch can alter local movement.
Compatible repair materials help maintain more uniform deformation.
Even if the cage returns to its original shape after the current decreases, repeated cycles can damage:
Twine
Knots
Borders
Loops
Long-term fatigue must be considered.
After strong-current or storm events, check:
Actual depth
Weight position
Border tension
Attachments
Contact wear
Permanent deformation
Records of current, fouling, cage depth, and repairs can help determine whether future nets need:
Different mesh
Modified twine construction
Stronger borders
Revised weighting
Better cleaning intervals
Operational depth results from the interaction of:
Net Mesh + Twine + Current + Fouling + Weighting + Borders + Attachments + Cage Frame + Mooring
Changing only one component may not solve the problem.
Before deployment:
✔ Confirm nominal net depth and measurement method
✔ Review expected current speed and direction
✔ Select suitable mesh and twine construction
✔ Verify border, seam, and loop specifications
✔ Confirm sinker-tube or weight design
✔ Check attachment spacing and cage-frame compatibility
During operation:
✔ Monitor fouling across all panels
✔ Observe cage shape during strong current
✔ Check whether the bottom shifts downstream
✔ Compare depths at the center and corners
✔ Inspect borders and attachment points
✔ Maintain clearance from frames and predator nets
✔ Record current, tide, and cage deformation
After severe loading:
✔ Check for shifted weights
✔ Confirm actual operating depth
✔ Inspect net-to-frame and net-to-net contact
✔ Examine borders, seams, loops, and repairs
✔ Look for permanent mesh distortion
✔ Update maintenance and operating records
Fish cage nets become shallower under strong current because a flexible net cannot remain perfectly vertical while water pushes it downstream.
Part of the panel’s material length is converted into:
Diagonal inclination
Horizontal displacement
Inward bowing
Bottom uplift
The effect becomes more severe with:
Faster current
Smaller or blocked mesh
Thick twine
Heavy fouling
Insufficient or uneven weighting
Weak attachments
Unstable cage geometry
The key principle is:
A Net’s Manufactured Depth Describes Its Material Dimension—Its Operating Depth Depends on How the Entire Cage Responds to Water Flow.
A more stable cage combines:
Suitable Mesh and Twine + Controlled Fouling + Balanced Weighting + Strong Borders + Even Attachments + Underwater Monitoring
At PL Fishery, we manufacture PE fish cage nets, aquaculture panels, reinforced borders, custom seams, attachment loops, repair twine, marine ropes, and customized netting products for coastal and offshore farming projects.
Need to purchase or customize fish cage netting for a defined cage depth, current environment, mesh size, twine construction, unit weight, border-rope design, sinker system, attachment layout, color, packaging, or inspection requirement? Contact PL Fishery with your cage dimensions and operating conditions, and our factory team can help prepare an appropriate netting specification before production.https://plfishery.com/
