Background

There is a lack of sustainability in salmon aquaculture due to increased lice infestation pressure on wild stocks, as well as negative environmental impacts in the release of treatment chemical after topical de-lousing or from orally-treated fish. Welfare of farmed fish may also be depressed during common topical delousing.

An innovative method of de-lousing in a surface oil layer has been described but with variable results due to low surface activity of salmon. However, recent knowledge within CREATE have led to the development of techniques to increase surface activity, by temporarily removing surface access. Combining the past and present results will provide a fruitful future method to efficiently treat salmon for lice, with minimum impacts on fish and environment.

Alternative to treating the fish for the parasite, prophylactic management to reduce infestations would be beneficial. The infestative stage of the salmon lice is free-swimming, predominantly occupying surface waters. It has been indicated that keeping the fish away from the surface layer will potentially reduce lice infestation. Egersund Group has developed within CREATE a net roof solution to hold the fish away from surface layers while still providing surface access for the fish to re-fill the swim bladder, through a tube-like chimney structure called a ‘snorkel’. This innovative technology needs to be fitted to biological limits and optimal technological designs.

Methods

The IMR Tank Environmental lab was used for testing drugs in oil. The IMR Cage Environmental Lab was used for testing of jumping behaviour using oil floated on the surface, and of snorkel designs in relation to salmon behaviour, lice infestations, and production performance. SINTEF Fisheries and Aquaculture are using mathematical models to calculate forces in different designs, and will follow up by testing of down-scaled models in flume tank facilities in Hirtshals.

Results

Killing of sea lice dipping fish through oil layer containing treatment drug. Combinations of oil thicknesses and concentrations of cypermethrin (a commonly used lice treatment drug) were tested by dipping anaesthetized fish through the layer. The de-licing effects were less than 40% and consequently, there is a need to develop a more efficient drug before we proceed with further testing of the method at a larger scale.

Test of fish avoidance of oil layer at cage surface. Salmon groups (N=200-300) were submerged using a cage roof for two days. Immediate jumping behaviour after re-instating surface access was observed in five replicates with normal surface conditions, and compared to a treatment group where an oil layer (using  rapeseed oil) of 1 mm thickness was added. No significant differences in jumping were observed between the control and oil group.

In conclusion, no avoidance of the surface during re-filling of their swim bladder was observed when an oil layer was present.

Snorkels reduced lice infestations by ¾. In pilot set-ups, we tested if salmon held in modified cages were using the snorkels to re-fill swim bladders or overused them so that the oxygen conditions were degraded to detrimental levels. As such, we conducted a full-scale trial at the research sea farm at the Institute of Marine Research, Austevoll, Norway, during summer 2012. We used 18 000 salmon of 89 g divided between triplicate cages with surface access (control), and cages equipped with a snorkel-like chimney attached to a net roof positioned at 3 m depth. For every third week from May 29 to August 21, a random sample of 20 fish per cage were anaesthetized and number of lice and stage registered.

The mismatch of the fishes’ swimming depth from the copepodids’ resulted in reduced lice infestations by 66-86%. Lice infestation was reduced consistently over time and variable environmental conditions. Behaviour of the fish was observed to be normal. However, a potential drawback of this technique was a growth reduction of 33%, likely caused by under-feeding due to the biofouling on the net roof.

Thus, we are presently performing a follow-up trial (funded by FHF) at IMR-Matre monitoring the growth performance, together with description of the groups’ behaviour, in both control and treatment cages. Fine-tuning and development of this preventive technique towards larger industrial use will advance the salmon farming industry towards increased sustainability. This is the first, but not last, ecology-based prophylactics practice to mitigate parasitic problems within aquaculture.