Abundance and distribution of sea lice larvae in relation to the three-dimensional current flow patterns around and within salmon farms

The primary objective of this project was to assess if and to what extent the layout of salmon farms creates localized zones with high sea lice larvae aggregations. We investigated this by mapping the spatiotemporal distribution of pelagic lice larvae in relation to lice levels on the farmed fish and the current flow patterns around and within salmon farms on a micro scale. This challenging research problem has been tackled by using data from novel measurement techniques, such as 3D microscale turbulence sensors and genetic analysis, in order to assess the possible relationships between 3D microscale current flow patterns and lice abundance. A biophysical model has also been developed to promote our understanding of the influence physical parameters have on lice abundance in and around salmon farms. The main results are: -A complex 3D flow pattern has been measured near the cages, showing that the presence of salmon cages with fish changes the intensity and distribution of turbulence levels, and thus the mixing regime, in the wake of the cages. The use of skirts around cages will lead to a further increased level of turbulence. -A 3D numerical flow simulation has been performed to accurately model the current flow patterns through cages, thus enabling the flow structure (3D mixing, stagnant zone) to be determined. A tracking model has been developed to track the dispersion of micro-organisms (e.g. lice larvae, zooplankton) in turbulent flows. The results indicate that cage layout and the use of lice skirts increases mixing in the water column, which may alter the vertical distribution of micro-organisms within the farm -Plankton samples were collected from different depths and different positions near four salmon farming facilities through a large field measurement campaign. It was shown that the concentration of salmon lice larvae was higher near salmon farms compared to reference sites 1 km away from the farms, and that there was some retention of lice larvae within the farms. The density of lice larvae and plankton varied considerably between the different farms and with depths, but no clear trends were identified that could be unambiguously related to potential variation in flow patterns. This does not necessarily mean that there is no correlation between flow pattern and occurrence of lice larvae since the particle model predicts that this may occur, but more likely reflects that the relationship between flow pattern and lice density is complicated and that it is generally challenging to find such correlations with a realistic specimen sampling.

A novel molecular method has been used as a tool for determining the occurrence of lice larvae, and principles for reducing uncertainties related to sampling effort when collecting plankton samples for measurement of sea lice densities has been developed. There are no clear and obvious trends in the empirical data with respect to spatial variation in the occurrence of salmon lice that intuitively could be explained by variation in water currents. This do not necessarily mean that this relation doesn't exist, but merely reflect a highly complex situation that is difficult to understand with realistic biological sampling efforts alone. A 3D simulation of micro-organisms movement indicates that variation in currents could result in aggregations of lice larvae at specific locations and under realistic conditions. It would be reasonable to suggest that spatial reorganization of fish farms for reducing the turbulent mixing, the increase of distance among cages could be maybe recommended.

The objective of LiceRisk is to assess if and to what extent salmon farm layout creates local zones with high lice larvae concentrations. The work will generate new knowledge regarding whether optimal positioning of sea cages within farms may reduce lice infestation risk. The research outcomes will be useful for the development of functional and non-chemical mitigation measures. This will be advantageous as it will reduce the need for post-infection treatments against sea lice, which are challenging in terms of operation and fish welfare as well as environmentally hazardous. The project is highly relevant for the fish farming industry, management authorities and legislators as it address the following questions: -How do infective stages of lice larvae distribute around a farm and how does this relates to variation in current? -How does the abundance of sessile salmon lice vary among different cages and how do this relate to micro scale current pattern? -Can optimal spatial layout and positioning of cages within farms reduce lice infestation risk? The LiceRisk project consists of partners with different research background, including hydrodynamic, lice ecology, epidemiology and biophysical modelling. The project requires a novel and thorough integration, which will be organize in four work packages: -WP 1 will determine the 3D current flow and the flow microstructure in the vicinity of the farm (internal and peripheral areas). Also hydrographic parameters, wind magnitude and fish distribution will be monitored. -WP 2 will map the distribution of pelagic lice around and within farms. Plankton tows and light traps will be used the sample lice larva. -WP 3 will assess the relationship between the abundance of sessile lice on farmed salmon among cages within farms. -WP4 will develop a coupled biophysical model for assessment of how physical parameters influence the lice abundance around and within farms. Mitigative measures and recommendations will be defined.