Effects of a shielding skirt for prevention of sea lice on the flow past stocked salmon fish cages
L.C. Gansel*1, K. Frank1, J. Birkevold1 and A.M. Lien1   1SINTEF Fisheries and Aquaculture                                                             
 Brattørkaia 17C, 7491 Trondheim, Norway
*email: lars.gansel@sintef.no
Introduction
Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) farming has been a success story in Norway. Salmon and Trout production has increased from the beginnings of the industry in the late sixties and early seventies to over 1.1 million tons in 2011 (FAO, 2012). Further growth of the salmon industry is dependent on solutions for a number of challenges.
One of the most pressing problems today is the infestations of farmed and wild salmon with salmon lice (Lepeophtheirus salmonis) (Costello, 2006). Salmon lice larvae are supposedly most prevalent within the first few meters from the water surface (Hevrøy et al., 2003). Preventing the upper part of the ocean water column from interacting with the water inside the fish cages, might therefore prevent salmon lice from entering fish cages, thus reducing lice infestations on farmed salmon. Impermeable skirts mounted around the upper parts of a cage might offer this desired shielding effect, and as a concept the use of a shielding skirt has already shown promising results (Næs et al., 2012). A shielding skirt might, however, not be a complete solution. Computational fluid dynamics (CFD) simulations have shown that some water might be pressed underneath the skirt, thus potentially delivering lice into the cage. Shielding the upper parts of a fish cage might have consequences for the water quality as in general water exchange is reduced. Model trials indicate that the circulation of the water inside the shielded part of the cage is strongly reduced due to the skirt (Lien and Volent, 2012), and consequently oxygen concentration might drop to levels dangerous for the fish (Stien et al., 2012). It is therefore necessary to carefully evaluate the overall benefit of the use of a shielding skirt in terms of reducing salmon lice infestation and potential negative effects from a reduced water exchange such as water quality.
 
Material and methods
The main goal of the experiments was to evaluate and understand the impact of the shielding skirt on the flow around and through a cage. All measurements were conducted at a stocked fish cage at a commercial Atlantic salmon fish farm (Salmo salar) at the coast of Middle-Norway close to Rørvik (64.838° N, 10.666° E). Fluorescent dye (fluorescein) was released in several locations inside of the cage and dye tests were conducted with and without a shielding skirt. A remote controlled drone with a digital camera (Samsung NX200) attached was used to obtain aerial images of the fish cage and its surroundings. Images were taken at a frequency of 1/15 Hz from a height of approximately 75m with a resolution of 5472 x 3648 pixels, resulting in an effective resolution of approximately 40 pixels per meter. The images were used to determine flow patterns around and through the fish cage within the first few meters from the surface by tracing the path of intense dye packages. Thus the analysis of aerial images is similar to widely used particle tracking methods. Current meters (NORTEK Vector)  were mounted in four locations on both sides of the shielding skirt to investigate i) the vertical flow close to the skirt and ii) differences in the horizontal flow across the net with and without shielding skirt. A current meter was used to monitor the ambient current.  
 
Results and discussion
Figure 1 shows an example of dye spreading during the tests. Dense packages of dye were tracked and flow patterns during the test were determined (upper image in Fig. 1). A shielding skirt can have a significant impact on the flow pattern through and around aquaculture net pen based fish cages. This study shows that an impermeable skirt can result in flow attenuation and deflection within the first few meters from the surface. Water was partly forced around the cage, thereby flowing along the impermeable skirt. The application of a shielding skirt lead to a longer retention time of water within the upper few meters of a fish cage compared with the retention time of surface water in the same cage, but without the skirt attached. Fish motion inside the cage seems to have an impact on the flow patterns of surface water inside a fish cage.
Water flowing towards a fish cage with shielding skirt is not only re-directed horizontally due to blockage caused by a shielding skirt, but also vertically. Thus, some surface water is pressed underneath the lower edge of the shielding skirt and enters the fish cage. Water blockage and resulting re-direction are associated with acceleration of water along the shielding skirt. This will cause a pressure drop behind the skirt, which can lead to vertical flow towards the surface.
Even though these effects can explain many of the findings presented in this study, specific causal connections and the importance of other influencing factors remain uncertain. Two seemingly similar tests resulted in different flow patterns. In both cases water was re-directed horizontally and vertically, but the ratio between dye being forced around the cage and dye entering the cage was very different. Many factors might influence the flow around and through fish cages and only few of them were monitored in this study. Even though some factors can be assumed constant throughout the experiments (e.g. fish biomass, net solidity), others might have been highly variable (e.g. flow conditions at locations around the fish cage that were not monitored and related effects from neighbouring cages, fish behaviour, cage and shielding skirt deformation) and could have influenced the results. However, the findings from this study give insight in some effects of an impermeable shielding skirt on the flow past stocked fish cages. It was shown that exchange of water outside and inside fish cages within the upper few meters is reduced when a shielding skirt is applied. The results might help to conduct better computational studies on specific effects of the use of impermeable skirts around net cages.
 
Literature
Statistics of the Food and Agriculture Organization of the United Nations, 2012. http://www.fao.org/fishery/statistics/global-aquaculture-production/en.
Costello M.J. (2006) Ecology of sea lice parasitic on farmed and wild fish. Trends in Parasitology (22), 475-483.
Hevrøy, E. M., Boxaspen, K., Oppedal, F., Taranger, G. L., Holm, J. C. (2003), The effect of artificial light treatment and depth on the infestation of the sea louse Lepeophtheirus salmonis on Atlantic salmon (Salmo salar L.) culture. Aquaculture  (220), 1-14.
Næs, M., Heuch, P. A., Mathisen, R. (2012), Bruk av «luseskjørt» for å redusere påslag av lakselus Lepeophtheirus salmonis (Krøyer) på oppdrettslaks, report, Vesterålen fiskehelsetjeneste.
 Lien, A. M., Volent, Z. (2012), Report: Deformasjon av not og Permaskjørt og krefter på fortøyning, SINTEF Fiskeri og Havbruk AS, ISBN: 978-82-14-05445-3 (in Norwegian)
Stien, L. H., Nilsson, J., Oppedal, F., Kristiansen, T. S., Lien, A. M., Folkedal, O. (2012). Skirt around a salmon sea cage to reduce infestation of salmon lice resulted in low oxygen levels. Aquacultural Engineering (51), 21-25, 2012.