The interactions and effects of sea lice on wild salmon

Sea lice distribution map

To understand the risk of interaction between farm origin sea lice and wild salmon smolts, information must be obtained on the distribution of both.

Two approaches will initially be taken to investigate spatial and temporal changes in sea lice distribution, and levels, along the west coast of Scotland. 

Analysis of publically available sea lice data

Industry data on regional sea lice numbers published by the Scottish Salmon Producers Organisation (SSPO) and data on sea lice treatments provided by the Scottish Environmental Protection Agency (SEPA), as a proxy for sea lice levels, is being statistically analysed to identify variation in sea lice levels over time and space.  Initial results from this analysis have already been published (Murray & Hall, 2014).  Differences in treatment rates were found to be associated with fish biomass, and also with local hydrodynamic conditions e.g. the flushing rate of the loch.  However the association with flushing rate was not always the same, i.e. high loch flushing rates were associated with lower treatment requirements in some areas, but higher in others.  This may reflect sea lice either being removed from a loch or being brought into a loch with water movements.  On the other hand, low flushing rates may also result in sea lice numbers building up within a loch.

The images on the page show sea lice monthly treatment rates by region and a map of Scotland showing prevailing ocean current (thick dashed line) and coastal currents (thin dashed lines), as well as regions used in analysis.  Regional variation in dependence suggests that oceanographic processes at larger spatial scales play a significant role in determining lice dynamics.  A possible explanation is flushing of lice away from island sea lochs but into mainland sea lochs, in line with prevailing currents.

Sea lice dispersal modelling

A better understanding of sea lice dispersal in the marine environment would help highlight sources of sea lice as well as where they may end up.  Management practices to reduce sea lice could be adjusted in these areas, potentially resulting in wider ranging effects.  The Scottish Government have developed a sea lice dispersal model for Loch Linnhe, on the west coast of Scotland, to help understand sea lice infestation dynamics and this will be refined and expanded during the current project.

During a previous Scottish Government funded project, a bio-physical model was developed for Loch Linnhe to describe the dispersal of sea lice in the marine environment when released from any specified location such as a salmon farm.  A POLCOMS hydrodynamic model (implemented and run by the Scottish Association for Marine Science (SAMS)), was coupled to a particle tracking model, with the particles being given biology and behaviour of sea lice.  This included parameters to describe their position in the water column, time to develop through to infective stages, and a fixed rate of mortality.  In this model, the focus was on sea lice dispersal based on water movements in the surface layer, as the majority of sea lice have been shown to occur in the upper layers of the water column.  Wind measurements were therefore also included in the hydrodynamic model, as wind can be a major influence on water movement in surface layers.  Tidal and freshwater inputs were also added.

The model predicts the distribution pattern and transport distances, farm connectivity and sea lice population mixing, following release from a fixed point.  The model was assessed by comparing the model estimations of relative sea lice numbers in different areas with sea lice numbers in plankton samples and from sentinel fish in those same areas.  A good degree of agreement was found between field observations and ranked model predictions.  In the current project, the model will be refined and will incorporate elements such as dispersal of sea lice in deeper water layers, and the effects of changing salinity on sea lice mortality levels.

The copepodid map above is an example of a sea lice dispersal model output. Sea lice were continually released for 14 days from various points within the loch and their movements followed for up to two weeks of their plankton life cycle. To allow all sea lice to become infective copepodids (which takes approximately 5 days) the simulation ran for 19 days. The final model output shows the relative copepodid sea lice distribution over the 19 day simulation.  This output can change depending on wind and hydrodynamic conditions during the model simulation.

The previous sea lice dispersal model was developed for Loch Linnhe but it is planned to take the particle-biology element of the model and couple it to the hydrodynamic  Scottish Shelf Model.  This would allow sea lice dispersal to be modelled on a much larger scale, predicting where lice originating from different regions may end up along the coast, and identifying potential “hot spots”, or those areas giving rise to “hot spots” through dispersal of lice from one area to another.