When fish farmers select marine netting, they usually focus on mesh size, twine strength, abrasion resistance, and service life.
Light transmission is often overlooked.
Yet every cage net also acts as a light filter. Its color, mesh opening, twine diameter, depth, cleanliness, and surface fouling influence how much sunlight reaches the fish inside.
That matters because light is not merely needed for visibility. It is an environmental signal that affects fish behavior, feeding rhythms, biological clocks, stress responses, growth, and pigmentation. Research shows that fish can respond differently to changes in light intensity, spectrum, photoperiod, and background brightness, although the ideal conditions vary greatly among species and life stages. (Frontiers)
A marine net therefore does more than contain fish.
It partly shapes the visual environment in which they live.
Light transmittance describes the proportion of incoming light that passes through a material or structure.
For a marine cage, the net does not behave like a completely solid shade cloth. Light can pass through the mesh openings, while the twine absorbs, reflects, or scatters part of it.
A simple practical relationship is:
Light transmittance and shading rate move in opposite directions.
If a structure transmits 70% of the incoming light under a defined test condition, its nominal shading effect would be approximately 30%.
However, this is only a simplified description. The amount of light experienced by fish in an actual sea cage also depends on:
Sun angle and cloud cover
Water clarity
Cage depth
Suspended sediment
Phytoplankton
Net color
Twine thickness
Mesh opening
Fouling level
Cage deformation
Nearby structures
For this reason, a laboratory shading value should not automatically be treated as the exact underwater light level inside every cage.
A clean net with large openings allows more direct and scattered light through than a dense net made with thick twine.
As the net becomes darker, thicker, or more heavily fouled, more light may be absorbed or blocked.
Several characteristics are especially important.
Larger openings generally leave a greater proportion of unobstructed area for light and water to pass through.
Smaller mesh contains more twine per unit area, so it usually creates more visual obstruction.
Two nets may have the same nominal mesh size but different light behavior if one uses substantially thicker twine.
Thicker twine reduces the open area and casts stronger local shadows.
Dark netting tends to absorb more visible light, while lighter materials may reflect and scatter more of it.
Color also changes the visual background perceived by the fish. Studies in cultured fish have shown that background brightness and color can influence pigmentation, behavior, and physiological responses, but the direction and size of the effect are species-specific. (ScienceDirect)
Algae, hydroids, barnacles, mussels, and sediment gradually cover the net and reduce its open area.
A net that originally provided moderate shading may become much darker after weeks or months underwater.
This means its light-transmission performance is not fixed for its entire service period.
There is no universal “best shading rate” for marine fish.
Some species naturally inhabit bright surface waters. Others live near reefs, under structures, in turbid estuaries, or at greater depths where light is weaker.
Light requirements can also change between larvae, juveniles, and market-size fish.
A light level that supports feeding and growth in one species may increase stress or reduce performance in another. Studies of fish rearing environments indicate that light intensity and spectrum can influence endocrine activity, digestion, antioxidant responses, behavior, and growth, but results cannot be transferred automatically between species. (Frontiers)
This is why net selection should begin with the cultured species and production stage—not with a fixed claim that a certain percentage of shade is always ideal.
Most cultured fish depend on visual information to some extent.
They may use light to:
Locate feed
Recognize other fish
Maintain schooling behavior
Avoid collisions
Respond to predators
Synchronize daily activity
Regulate rest and feeding periods
If the cage environment becomes excessively dark, visually feeding fish may find pellets less efficiently, especially when water is turbid or feeding occurs at depth.
Reduced feed detection can result in:
Slower feeding response
More uneaten feed
Uneven access to feed
Greater size variation
Lower feed efficiency
However, excessive brightness can also be problematic. Very intense or inappropriate light may produce avoidance behavior, disturb normal activity, or contribute to physiological stress in sensitive species. Research on early-stage fish has linked unsuitable light conditions with stress-related and oxidative responses, reinforcing that more light is not automatically better. (Frontiers)
The practical goal is therefore not maximum light transmission.
It is a stable light environment suited to the fish’s biology and the farm’s feeding method.
Fish respond not only to the amount of light but also to the daily light–dark cycle.
Light is received by the eyes, brain, and other light-sensitive tissues and helps regulate circadian rhythms. These rhythms influence feeding behavior, activity, hormone secretion, metabolism, and rest.
The pineal system and melatonin are important parts of this regulation. Research has shown that light intensity and wavelength can alter biological signals associated with daily rhythms and growth-related processes. (Frontiers)
A cage that becomes much darker because of dense fouling may change the timing and strength of the light signal received by the fish.
This does not necessarily mean that every fouled net will immediately reduce growth. It means that changing light conditions can become one more environmental stressor when combined with poor water exchange, low oxygen, crowding, or high temperature.
Fish skin color is controlled by pigment cells, including melanophores and other chromatophores.
These cells can alter the distribution of pigment in response to:
Genetics
Diet
Stress
Hormones
Health
Social environment
Background color
Light intensity and spectrum
Many fish can adjust their appearance to match the brightness of their surroundings.
In darker environments, pigment may disperse in melanophores, making the body appear darker. In brighter environments, pigment may aggregate, producing a lighter appearance. The exact response varies by species and may occur over different timescales.
Research on leopard coral grouper, for example, found that background brightness was associated with differences in melanophore area and skin darkening. Other controlled studies have similarly shown that rearing-background color can influence commercially important skin coloration. (MDPI)
For high-value species sold partly on appearance, uneven or excessive darkening can reduce perceived quality even when the fish remain edible and otherwise healthy.
It would be misleading to blame every skin-color problem on cage shading.
Pigmentation can also be strongly influenced by:
Dietary carotenoids
Genetics
Disease
Handling stress
Stocking density
Water quality
Social hierarchy
Temperature
Salinity
Capture and transport
The color of surrounding structures
Nutrition is particularly important in species whose market color depends on pigments that cannot be produced in sufficient quantities by the fish.
Therefore, changing net color or shading should be viewed as environmental management—not as a substitute for proper feed, health management, or stock selection.
At exposed tropical farms, fish may experience strong surface sunlight for many hours each day.
If the cage offers very little visual shelter, sensitive species may swim deeper, avoid bright areas, or crowd beneath walkways and equipment.
Possible consequences include:
Uneven use of cage volume
Localized crowding
Increased contact between fish
Greater competition for shaded areas
Altered feeding distribution
More pronounced stress responses
Providing some shade may help create a more comfortable visual environment, particularly for species that naturally shelter around reefs, vegetation, or deeper water.
However, shade should not block ventilation, water exchange, access, or routine observation. It should also be evaluated against weather conditions and cage stability.
Heavy shading may also produce disadvantages.
Farm workers may find it harder to observe:
Feeding response
Abnormal swimming
Skin lesions
Mortality
Predator entry
Net damage
Uneaten feed
Very dark cage conditions can make underwater cameras less effective and may reduce the visibility of feed to visually oriented species.
If the darkness results from fouling rather than intentional shade, the problem is even more serious because fouling also reduces water flow and increases hydrodynamic drag.
A clean, deliberately selected shading system is therefore very different from an old net that has become dark because it is clogged with biological growth.
A black net is not automatically equivalent to a particular shade percentage.
Likewise, a green or white net does not guarantee a fixed level of transmission.
Two nets of the same color can transmit different amounts of light because they have different:
Twine diameters
Mesh dimensions
Strand structures
Surface finishes
Knot sizes
Hanging ratios
Levels of fouling
Color describes appearance. Shading rate describes measured light reduction under specified conditions.
When light control is important, farms should ask for test information or conduct site measurements rather than relying only on visual judgment.
The percentage of open area provides a useful first indication of how much light and water might pass through a clean net.
Open area is affected by mesh geometry and twine dimensions.
A net with:
Large mesh and thin twine generally has more open area;
Small mesh and thick twine generally has less open area.
However, open area is not identical to optical transmittance.
Twine can reflect and scatter light, while sunlight reaches the net from different angles. Water also absorbs different wavelengths as depth increases.
Open-area calculations are therefore useful for product comparison, but underwater light measurements provide a more realistic view of cage conditions.
The top of a cage and its lower sections do not receive the same amount of light.
Water absorbs and scatters sunlight. Red wavelengths usually diminish faster with depth, while blue-green wavelengths penetrate farther in clear marine water.
At the same time, suspended sediment, plankton, and dissolved substances can dramatically change underwater visibility.
A net that produces comfortable conditions near the surface may have a much stronger darkening effect at depth.
This matters in deep cages where fish can choose different swimming levels.
If a farm evaluates light only at the surface, it may miss the environment experienced by fish lower in the cage.
A net’s light transmission can change substantially during the production cycle.
The usual progression is:
A clean net enters the water.
A microbial film forms.
Algae and small organisms attach.
Fouling fills part of the mesh.
Sediment becomes trapped.
Light and water transmission fall.
Farmers may first notice the net becoming darker.
But at the same time, drag, weight, and the difficulty of cleaning may also be increasing.
This is why a reduction in cage brightness should trigger an inspection. The cause may be seasonal sunlight, water turbidity, or intentional shading—but it may also indicate advanced net fouling.
In some species, managing background brightness may help maintain a preferred market appearance.
For example, farmers may try to avoid conditions that cause excessive darkening in species valued for bright red, silver, or pale coloration.
Still, this requires controlled testing.
A shade level that improves visual quality in one fish may:
Reduce feeding activity in another;
Increase crowding in another;
Produce no measurable effect in a third.
Studies on fish pigmentation consistently show that responses depend on species, rearing background, light conditions, and physiological regulation. (MDPI)
The safest approach is to run a small-scale comparison and measure results rather than relying on a universal marketing claim.
A useful light-management trial should record more than fish color.
Farmers can compare:
Underwater light intensity at several depths
Feeding response
Feed consumption
Growth rate
Feed-conversion performance
Size uniformity
Swimming depth
Crowding behavior
Mortality
Skin brightness or color values
Net fouling
Dissolved oxygen
Water temperature
Water clarity
Photographs should be taken under standardized lighting and camera settings. Otherwise, apparent differences in skin color may be caused by photography rather than the fish.
For commercially valuable pigmentation, objective color measurement is more reliable than casual visual inspection.
Light should be considered together with containment strength, water flow, and durability.
Determine whether the fish naturally prefer bright, shaded, shallow, deep, clear, or turbid environments.
Larvae and juveniles may respond differently from larger fish.
Do not compare mesh size alone. A thick-twine net may block more light than a finer net with the same opening.
Consider how the cage background may affect fish behavior, visibility, and pigmentation.
The selected net should remain cleanable, and inspections should account for the gradual loss of open area.
Light conditions in clear offshore water may be very different from those in a turbid bay.
Do not assume that maximum transparency produces maximum growth or that darker netting always improves skin quality.
The correct choice is usually a balance.
A separate shade cover can provide more controllable surface shading than relying entirely on the side net.
Its advantages may include:
Easier replacement
Adjustable coverage
More predictable shade
Protection during very bright periods
Creation of shelter zones
However, poorly designed covers can create wind loading, interfere with access, block observation, or become unsafe during storms.
They should therefore be designed as part of the cage system, not attached casually.
Creating a partially shaded zone may also allow fish to choose between light conditions instead of forcing the entire cage into one environment.
It is tempting to promote a single figure such as 30%, 50%, or 70% shade.
In reality, that number is not meaningful without context.
The appropriate level depends on:
Fish species
Growth stage
Site latitude
Season
Water transparency
Cage depth
Feeding method
Net cleanliness
Production objective
Desired skin appearance
The same nominal shading material can also create different underwater conditions at noon, in cloud cover, during a plankton bloom, or after heavy fouling.
Shading rate should therefore be treated as one adjustable production parameter—not a fixed guarantee of faster growth.
A net or cover that creates a desirable visual environment but restricts water movement may do more harm than good.
Adequate water exchange is necessary for:
Oxygen supply
Waste removal
Temperature regulation
Feed distribution
Overall cage conditions
Cage systems depend on permeable net enclosures that allow water to pass through while retaining the stock. (FAOHome)
Any attempt to alter shading should therefore be evaluated together with mesh openness, fouling, current speed, oxygen levels, and cage deformation.
A marine net is usually marketed according to strength, mesh size, dimensions, and material.
For some aquaculture applications, its optical performance deserves attention as well.
A suitable net should help the farm achieve several objectives at once:
Secure fish containment
Adequate water exchange
Manageable hydrodynamic load
Suitable underwater visibility
Stable feeding behavior
Acceptable fish appearance
Practical cleaning and observation
No single property should be optimized while ignoring the others.
A highly transparent net that is too weak is unsuitable.
A very dense net that creates ideal shade but blocks water is also unsuitable.
Good net design is a balance between biology and engineering.
Light transmittance is an overlooked characteristic of marine aquaculture nets.
The combination of net color, twine diameter, mesh opening, water depth, turbidity, and biofouling determines how much light reaches the cage interior.
That underwater light environment can influence feeding, biological rhythms, behavior, stress, growth, and skin pigmentation. However, these effects are highly species-specific, and fish appearance is also shaped by genetics, nutrition, health, and handling.
For this reason, shading rate should not be treated as a simple product slogan.
It should be managed as part of the complete farming system.
The most effective approach is to match the net to the species, measure real underwater conditions, monitor growth and pigmentation, keep the mesh clean, and test changes on a controlled scale.
A marine net does more than keep fish inside.
It also helps determine what the fish see—and the environment in which they grow.
