Is one lamp per cage enough?
In today's cage systems, there is a lot of technical equipment that must be removed and reassembled when carrying out necessary operations in the cages, for example during lice treatment. These are time consuming and costly work operations. A relevant measure in this respect is to reduce the number of light sources in the cages. The company Oxyvision engaged the Institute of Marine Research (Havforskningen) to measure light scattering and intensity in a 160 m cage with respectively one large LED light (2500 W) and 4 small LED lights (600 W).
Since the early 1990s, the fish farming industry in Norway has used artificial light to control or delay sexual maturation and to increase the growth of salmonids. Such lights have mainly been based on metal halide technology with a light spectrum from 400 to 700 nm and with the greatest brightness in the blue-green part of the spectrum. In recent years, LED-based underwater lights where the composition of light colours can be more easily determined, and which have much lower energy consumption, have entered the market.
Early sexual maturation is a major potential problem in salmon farming that results in reduced growth and reduced meat quality, but manipulation of the natural light cycle using artificial light programs is routinely used to reduce the problem. In a study conducted by Porter et al. (1999) only 6.1% of salmon were sexually mature using an artificial light regime (additional light at night) compared with 61.5% sexual maturity in natural light. Use of high-intensity light from mid-winter and 4-6 months ahead can reduce the incidence of sexual maturation, while use of high-intensity light during the autumn has the opposite effect and can thus induce sexual maturation. Commercial testing with LED lights at Norwegian cage farms has shown 10% faster growth and 10% improved biological feed factor (Philips Lightning Aquaculture).
Several studies have shown that artificial lighting can also be used to influence the farmed fish's behaviour. Extensive studies at the Institute of Marine Research (HI) that combine the use of light sources in deep water with underwater feeding in cages have shown that this can contribute to increased swimming depth at night and thus reduced risk of lice infestation. Frenzl et al. (2014) showed that the level of infection of salmon lice is significantly lower in fish in experimental cages with underwater light that attracts salmon by placing a light source at a depth of 10 m compared with light at a depth of 1.5 m. Recent observations show that in order to achieve a lice-reducing effect with deep light, it is important to attract salmon some distance away from the halocline (transition between brackish water and seawater).
Other experiments have shown that salmon respond differently to different light colours (wavelengths), but that it is sensitive to even weak amounts of light and follows weak light sources that move up and down the cage (Wright et al. 2015). Normally, such an attraction works better in the dark period of the day. During the day, the natural light at the surface is much stronger than the artificial lights so that the effect of underwater light is reduced. The use of blue light (range 420 - 560 nm) to guide salmon away from the surface layer with high lice density showed little effect on the fish's distribution in the cages in a recent project (FHF 901456).
Artificial light regime is routinely used in Norwegian cage systems where the main purpose is to reduce / prevent premature sexual maturation in fish. Several light sources are used in each cage, usually 4 - 6 pieces, to ensure good light coverage in the entire cage volume. Indicative effect for achieving adequate lighting is 1 - 2 W / m2 cage surface. The light sources are usually located at a depth of 3-5 m. Many lights together with other components in the cages, such as hiding places for wrasse, sensors for measuring water quality and camera, are pointed out to be an obstacle to general operation of the cages and to involve significant extra work when carrying out work operations such as lice treatment. Therefore, there is a general desire that the number of light sources in the cages can be reduced if possible. The article describes a recent test at the Institute of Marine Research's experimental station at Austevoll where the brightness at varying distances in the sea from a single strong light source was compared with the brightness from 4 correspondingly weaker light sources (Kristiansen et al. 2020).
The test at Austevoll Aquaculture Station consisted of comparing the brightness of a powerful 2500 W lamp (Aqualux 2500 W) with a normal 600 W lamp (Aqualux 600 W) (see also picture 1). The test was carried out around midnight 28 - 29 May 2020 on behalf of Oxyvision, which has developed the LED-based underwater lamps. The candles were hung at a depth of 5 m and the light was measured horizontally from the lamps at a distance of 1-2-3-4-5-10-15-20 m.
A Li-Cor quantum sensor light meter (Li-193SA, Li-Cor, Lincoln, NE, USA) was used which registered photon flux rate (microE / m2 / s) within the range 400 - 700 nm. To provide more comprehensible values, the measured readings are converted to "illuminance" which is visible light for people with unit lux (lumen / m2) using conversion factor 55 (as for sunlight). This is not 100% correct since salmon and humans have somewhat different light sensitivity for different wavelengths so that the brightness will not be experienced the same, and that the lights have a different composition of wavelengths than sunlight, but the relative differences between the lamps will still be correct. At the time of measurement, there was extremely much zooplankton in the sea which led to reduced light scattering.
Results and discussion
The results from the light measurements at 5 m depth 1 - 20 m from the 600 W lamp and the 2500 W lamp are shown in Table 1 and Figure 1.
Furthermore, so-called modelled value is calculated which expresses the degree of light attenuation with increasing distance in air (the brightness from a spherical light source decreases with the square of the distance, E = I / d2). Particles and bubbles in the water will, however, scatter the light in all directions so that we must expect the light to weaken faster with increasing distance than this "best case" model.
At a distance of 5 m, the measured brightness was approx. 5 times stronger from the large lamp compared to the smallest lamp, while the corresponding ratio was approx. 10 times stronger brightness 15 - 20 m from the lights. The large lamp (Large-measured) gave almost as much light at a distance of 20 m as the smallest (Small-measured) at a distance of 10 m.
Due to a great deal of zooplankton in the sea, the brightness deteriorated more than had been the case in clear water (Figure 1). The lights are most used in winter when the turbidity in the water is usually lower than it was during the test and it is assumed that the measured amount of light becomes more similar to modelled values when the lights are in use in late autumn and early winter.
The measurements showed that the brightness was approximately equal at a distance of 20 m from a large lamp and at a distance of 10 m from a small lamp. By a simple calculation, one can calculate the area with brightness above 10 lux:
Large lamp (2500 W): 3.14 x 202 = 1256 m2, Small lamp (600 W): 3.14 x 102 = 314 m2 (1256 m2 / 4 = 314 m2). This indicates that a large lamp (2500 W) will be able to replace four small lamps (á 600 W) and provide sufficient light intensity / coverage in the cages.
By placing a strong light centrally in the cage, the light effect on the fish at the outer edge of the cage will probably be weakened by the fish stock closer to the light source and thus experience less light than with 4-6 lamps scattered in the cage. At the same time, the fish will be able to experience significantly more light by swimming closer to the lamp, so it is not possible to say for sure whether the light effect on the fish stock will be greater or smaller. The brightness 1 m from the 2500 W lamp was found to correspond to strong sunlight at a depth of 2 m in the sea, and it is also conceivable that the fish will avoid such bright light and that there may thus be a different distribution of the fish in the cage.
Measurements carried out at the Institute of Marine Research show that one strong underwater light can give approximately the same amount of light in cages as four correspondingly smaller lights. Breeders who have tried one light in the cages have good experiences, but further documentation is needed to achieve a sufficient professional basis for concluding the effect on sexual maturation and growth. Several issues need to be considered further when using a simple light source, in particular:
• will there be greater shadow effects from the fish stock?
• will the fish be distributed differently in the cages and thus affect the light and shadow effect?
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