Background

Artificial lights are routinely used as a management tool in Atlantic salmon farming to produce constant photoperiods that reduce or prevent incidence of sexual maturation. Such sexual maturation would else lead to loss of production efficiency for the farmer and reduced welfare of the fish due to reduced osmo-regulatory capacity and often mortalities. Earlier research has shown that in spring-transferred (from freshwater to seawater) salmon, onset of light from January the first winter in the sea reduces the incidence of sexual maturation from levels of 5-50% to 0-5% and comparable effects are seen using lights during the second sea-winter. In autumn-transferred fish equivalent reductions are seen using lights during the first sea-winter, while no trial has addressed effects of lights during the second sea winter. It has further been hypothesised that related changes in growth was dependent on the light intensity and unpublished results from Scotland indicate comparable effects of light intensity on reduced incidence of sexual maturation. Up until today metal halide lamps has been the major light source, while use of modern and more energy saving L.E.D. (Light Emitting Diode) may prove beneficial. L.E.D. light sources may also be dimmed in comparison to direct on/ off characteristics of metal halide lamps and may be used in more intelligent manners in future aquaculture. The first goal of the project was to develop the use of high efficient L.E.D. light in preventing sexual maturation in sea cages.

Salmon aquaculture lack sustainability due to the increased lice infestation pressure on wild stocks of salmonids. During common topical delousing, welfare of farmed salmon is depressed, the environment is polluted by treatment chemicals, fish production performance is reduced and lice treatments are generally considered a critical production point. Keeping the salmon away from the surface layer will potentially reduce lice infestation. Submerged light sources attract salmon and may be used to manipulate the swimming depth and densities. However, use of artificial light in the autumn may increase unwanted incidence of sexual maturation. The second goal of the project is to reduce the impact of salmon aquaculture on the environment and wild fish through reduced lice infestation rates. This is to be reached by development of low intensity submerged L.E.D. lights during the autumn in two steps: (i) Test what light quality is needed to attract the salmon to deeper waters and (ii) test if this light quality increase the incidence of sexual maturation or may be used as generally intended.

Methods

Within the Cage Environmental Lab at IMR-Matre Atlantic salmon were divided among 13 cages of approximately 2000 m3 were exposed to experimental light conditions from January to June 2012. Treatments included artificial light of both metal halide and different intensities of L.E.D. compared to natural conditions and effect of incidence of sexual maturation and growth during the second sea-winter of autumn-transferred Atlantic salmon were investigated.

At the Cage Environmental Lab of IMR- Austevoll three replicate cages of 5 500 salmon were exposed to 7 different colours of L.E.D. lights combined with 3 to 6 intensities per colour. Measurements of swimming depth and densities were used to describe attraction and avoidance to the light qualities. The trial commenced in November 2012 and lasted until May 2013.

A general increase in appetite and growth was seen dependent upon intensity of the light used. These findings are in accordance with earlier indications. A positive relation between light intensity and harvest weight was measured.

In conclusion, L.E.D. light sources may be used as a tool to reduce unwanted sexual maturation and increase growth within sea cage farming of Atlantic salmon.

The test of attraction towards different light qualities is promising and results will be presented in 2013. Light clearly attracts the salmon to swim deeper and potentially reduce the infestation of sea lice.