The interactions and effects of sea lice on wild salmon

Infection pressure studies

Once information on the risk of interaction between farm origin sea lice and wild salmon smolts  is obtained, then the next question is how the levels of lice in the environment translate into numbers actually infecting the fish.

Understanding infestation pressure is difficult due to the many variables affecting infestation, such as fish behaviour, ability of lice to attach, and effects of local environmental conditions on the viability of the sea lice. For example, sea lice do not survive well at salinities below 27-29.

Towed fish

In an initial attempt to understand infestation levels on salmon in relation to local environmental lice levels, salmon will be towed in cages at speeds and depths at which salmon swim.  Simultaneously, plankton samples will be collected.  Salinity and temperature measurements will be conducted at the cage to put the results into context. This may give some indication of infestation pressure as salmon travel through areas with different environmental lice levels and conditions, and whether levels of planktonic lice can be used to predict resulting lice on fish.  As more information becomes available from tracking studies on movement and behaviour of wild salmon smolts, this can be used to interpret and extrapolate results from fish towed on a continuous basis and in a given direction.  In addition sentinel fish in cages may also be deployed in future at different sites to assess infestation pressure in relation to local conditions.

In the images below, both cages have one end attached to metal frame.  The fish cage, with netted mouth (on left), and the plankton net contain flow meters outside and within to monitor flow.  A camera was deployed within fish cage to monitor fish. An oceanographic instrument measuring pressure, salinity and temperature was deployed on the structure.  Initial prototype had a tube attached to cod end of plankton net through which water pumped by onboard pump, with water samples collected at designated times for filtering and preservation.

Sea Trout sea lice infestation

The possibility of using sea lice levels on wild sea trout, as a proxy for infestation pressure on salmon will also be investigated.  This would represent wild salmonids in their natural environment, as opposed to fish in sentinel cages.  However, sea trout have been found to remain inshore for longer, close to natal rivers, as compared to wild salmon smolts which migrate relatively quickly away from rivers, into fjords and out to open sea.