What research reveals about underwater aquaculture lights and fish growth
Fish growth in aquaculture is not simply a matter of feed quality and water temperature. Light plays a fundamental role in regulating the biological processes that govern how fast fish develop, when they reproduce, and how efficiently they convert feed into body mass. For offshore fish farmers and aquaculture operators, understanding the science behind underwater lighting is increasingly the difference between a productive farm and one that underperforms season after season.
This article builds from the ground up, starting with what aquaculture lighting systems are and how they function, moving through the biology that makes light so consequential for fish, and arriving at practical guidance for designing a species-optimized lighting strategy. Each section builds on the one before it, so by the end you will have a clear, working understanding of how to apply aquaculture photoperiod lighting principles to real farming conditions.
What Underwater Aquaculture Lights Are and How They Work
Underwater aquaculture lights are submersible luminaires designed to deliver controlled light at depth within fish pens, cages, or enclosures. Unlike surface-mounted floodlights, which lose intensity rapidly as light passes through water and disperses, underwater units position the light source directly within the environment where fish are held. This direct placement ensures that light reaches the fish at the intensity and spectrum intended, rather than what survives the journey through the water column.
The core technology behind modern aquaculture lighting systems is LED. LED sources offer precise spectral control, meaning operators can select specific wavelengths of light rather than broadcasting a broad, uncontrolled spectrum. They also consume significantly less power than traditional incandescent or halide alternatives, which matters considerably in offshore environments where power supply is constrained and operational costs are closely managed.
Aquaculture grow lights are typically deployed on weighted frames or brackets that hold them at a fixed depth within the pen. The depth of placement is not arbitrary: it determines which layers of the water column are illuminated, how uniformly light is distributed across the pen, and how the fish respond. For example, in Atlantic salmon farming, lights are commonly suspended at depths between five and ten metres to create a light field that influences fish behaviour and hormonal cycles throughout the pen without concentrating fish at the surface.
- Submersible LED units rated for continuous operation at depth, typically to IP68 standards or beyond
- Power supply systems designed for marine environments, often with waterproof cabling and corrosion-resistant connectors
- Mounting hardware that positions lights at the correct depth and prevents movement in tidal or wave conditions
- Control systems that allow operators to set photoperiod schedules, intensity levels, and spectral output
Why Light Matters to Fish Biology and Behavior
Fish are photosensitive organisms, meaning that light directly influences their physiology through multiple biological pathways. The most important of these is the neuroendocrine system, which uses light signals received through the eyes and, in some species, through photoreceptors in the brain itself, to regulate hormone production. These hormones govern reproductive cycles, appetite, stress responses, and growth rates.
The relationship between light and fish biology is mediated primarily through the hormone melatonin. In darkness, melatonin production increases, signalling to the fish that it is night. As light exposure increases, melatonin is suppressed, triggering the hormonal cascades associated with daytime activity and, critically, growth. This is why photoperiod, which is the duration of light exposure within a 24-hour cycle, is one of the most powerful tools available to aquaculture operators for influencing fish development.
A useful analogy is the effect of daylight on seasonal plant growth. Just as longer days in spring trigger flowering and growth in many plant species, longer photoperiods stimulate specific biological responses in fish. Atlantic salmon, for instance, are highly sensitive to photoperiod because in their natural environment, day length signals the seasons and determines when they transition from juvenile parr to migratory smolt. Aquaculture lighting exploits this sensitivity deliberately, using extended photoperiods to prevent premature sexual maturation or to accelerate smoltification on a controlled schedule.
Light also governs feeding behaviour directly. Most commercially farmed fish species are diurnal or crepuscular feeders, meaning they are most active and most willing to feed during daylight hours or at dawn and dusk transitions. Extending effective daylight through aquaculture photoperiod lighting can therefore increase the number of hours per day during which fish actively feed, with direct consequences for growth rate and feed conversion efficiency.
How Light Spectrum and Intensity Influence Fish Growth
Photoperiod, as introduced in the previous section, determines how long fish are exposed to light. But spectrum and intensity determine the quality of that exposure and how deeply it penetrates the water column. These three variables work together, and understanding how each one operates independently is essential before combining them in practice.
Spectrum: Which Wavelengths Reach the Fish
Water is not optically neutral. Different wavelengths of light are absorbed at different rates as they pass through the water column. Red wavelengths are absorbed rapidly, often within the first two to three metres. Green and blue wavelengths penetrate significantly deeper, with blue light reaching the greatest depths in clear water. This means that the spectral composition of light changes with depth, and a light source that appears white at the surface may deliver a predominantly blue-green spectrum by the time it reaches fish held at ten metres.
For aquaculture operators, this has two practical implications. First, the spectral output of the luminaire must account for what the water column will absorb before the light reaches the fish. Second, different species have different spectral sensitivities, and matching the light spectrum to the species being farmed improves the biological effectiveness of the lighting system. Salmon, for example, have strong sensitivity to green light, which aligns with the wavelengths that penetrate naturally through the water column in their native habitats.
Intensity: How Much Light the Fish Actually Experience
Light intensity at fish depth is measured in lux. The threshold intensity required to suppress melatonin production and trigger the biological responses associated with extended photoperiod varies by species, but in Atlantic salmon it is well established that relatively low intensities are sufficient. Research in this area consistently shows that even modest underwater light levels can produce measurable neuroendocrine responses, which means that intensity management is about achieving the biological threshold consistently, not about maximising brightness.
Excessive intensity can be counterproductive. Very high light levels can cause avoidance behaviour, driving fish away from illuminated zones and reducing the uniformity of light exposure across the pen. A well-designed aquaculture lighting system targets the minimum effective intensity distributed as evenly as possible throughout the pen, rather than creating bright focal points that fish actively avoid.
Applying Aquaculture Lighting to Real Farming Conditions
The principles of spectrum, intensity, and photoperiod covered above provide the theoretical foundation. Translating them into operational practice requires accounting for the specific conditions of each farm: water clarity, pen geometry, species requirements, and the seasonal variation in natural light that the artificial system must complement or override.
Water clarity is the first variable to assess. In clear oceanic water, blue-green light penetrates deeply and a single well-positioned light unit may illuminate a large volume effectively. In turbid coastal water, where suspended particles scatter and absorb light rapidly, multiple units positioned at different depths may be necessary to achieve consistent coverage. Operators who design their lighting system based on clear-water assumptions and then deploy it in turbid conditions will consistently underperform because the light is not reaching the fish at the intended intensity.
Pen geometry determines how many units are required and where they should be placed. A large circular pen requires a different deployment configuration than a smaller square cage, and the depth at which fish are held during different seasons affects which depths need to be illuminated. For example, salmon in cold winter water tend to hold deeper in the pen, which means lights positioned for summer conditions may not reach the fish effectively in winter without adjustment.
Seasonal natural light is the context within which artificial aquaculture photoperiod lighting operates. In high-latitude farming regions such as Norway, Scotland, or the Faroe Islands, natural day length varies dramatically across the year. In midsummer, natural photoperiods may already exceed the threshold needed to suppress melatonin, making supplemental lighting unnecessary. In midwinter, artificial lighting must compensate for very short natural days to maintain the extended photoperiod that prevents early sexual maturation. Effective lighting programs are therefore seasonal programs, adjusted throughout the year rather than set once and left unchanged.
Common Aquaculture Lighting Mistakes and How to Avoid Them
Building on the application principles above, it is worth addressing the errors that most frequently undermine aquaculture lighting programs in practice. Many of these mistakes stem from applying general lighting logic to a biological problem, without accounting for the specific ways fish perceive and respond to light.
Treating Photoperiod as a Fixed Setting
The most common mistake is configuring a photoperiod schedule at the start of a production cycle and leaving it unchanged. Natural light levels change continuously across the year, and the artificial photoperiod needs to be managed relative to the total light exposure the fish experience, not as an absolute fixed duration. An operator who runs lights for sixteen hours per day in midsummer, when natural light already provides eighteen hours of daylight at high latitudes, is adding little biological value and consuming power unnecessarily. The same sixteen-hour schedule in midwinter, when natural daylight may be only six hours, delivers a genuinely extended photoperiod with measurable biological effect.
Ignoring Depth Distribution of Fish
A second frequent error is assuming that fish are uniformly distributed throughout the pen at all times. In practice, fish depth distribution changes with water temperature, feeding schedules, time of day, and sea conditions. Lights positioned to illuminate the upper half of a pen will not influence fish that are holding near the base of the cage during cold periods. Monitoring fish behaviour and depth distribution, and adjusting light positioning accordingly, is an operational discipline that directly affects the consistency of results.
Selecting Spectrum Based on Surface Appearance
Operators who select aquaculture grow lights based on how the light looks at the surface, rather than on the spectral output that will reach the fish at operational depth, frequently find that their systems underperform. A light that appears warm white above water may deliver a narrow, blue-shifted spectrum at depth due to differential absorption. Selecting luminaires based on their spectral output at depth, matched to the known spectral sensitivity of the target species, produces more reliable biological results than selecting based on surface appearance or lumen output alone.
Building a Species-Optimized Underwater Lighting Strategy
With the foundational concepts, mechanics, and common pitfalls now established, the final step is assembling them into a coherent, species-optimized lighting strategy. This is not a single decision but a framework of interconnected choices that must be revisited as production conditions change.
Begin with the biology of the target species. Different species have different photoperiod sensitivities, different spectral preferences, and different intensity thresholds. Atlantic salmon require extended photoperiods to prevent early sexual maturation and to control smoltification timing. Rainbow trout respond similarly but with different threshold values. Sea bass and sea bream, farmed extensively in Mediterranean conditions, have different seasonal light requirements shaped by their natural habitat. A lighting strategy that performs well for one species may be entirely inappropriate for another, so species-specific biological requirements are the non-negotiable starting point.
From that biological foundation, define the photoperiod target for each phase of the production cycle. Determine the natural day length at the farm’s latitude for each month of the year, then calculate the artificial extension required to reach the target photoperiod. This gives a month-by-month lighting schedule that accounts for seasonal variation rather than treating the year as uniform.
Next, select luminaires based on the spectral output required at operational depth, given the water clarity conditions of the specific site. Specify the number of units and their placement depths based on pen geometry and the expected depth distribution of fish across seasons. For offshore installations operating in demanding sea conditions, the physical durability of the lighting equipment is as important as its optical performance. Equipment that fails in winter storms, precisely when extended photoperiod lighting is most critical, defeats the purpose of the investment entirely.
Finally, build in a monitoring and adjustment protocol. Measure light levels at fish depth periodically, observe fish behaviour and depth distribution, and be prepared to adjust intensity, depth placement, or photoperiod schedule as conditions change. A lighting strategy that is actively managed throughout the production cycle will consistently outperform one that is installed and left to run without review.
Sabik has supplied aquaculture lighting solutions to offshore fish farms across demanding marine environments for over two decades, drawing on the same engineering discipline that underpins its marine aids-to-navigation products. If you are evaluating lighting systems for an offshore aquaculture installation, contact Sabik’s technical team to discuss the specific requirements of your site and species.
