The interactions and effects of sea lice on wild salmon

Introduction

Growing a sustainable salmon aquaculture sector and the conservation of wild Scottish salmon and associated fisheries are important objectives for the Scottish Government. It is also recognised, however, that, salmon farms increase the number of sea lice in the environment and that this may have an impact on wild salmon.

Existing studies in other salmon producing countries have shown that sea lice infestation of young salmon leaving the river (smolts) can affect the numbers of wild salmon returning to rivers. Many factors can influence the extent to which sea lice may affect salmon populations, such as the size of the fish, the distance and the areas through which salmon smolts migrate when heading to open ocean, the distribution of sea lice due to farms, sea lice control measures on farms and the status of wild salmon populations. Therefore impacts in one country or region do not necessarily apply to others.

Currently there is no information on the impact of sea lice from aquaculture on wild salmon in Scotland, however, Marine Scotland has started a ten-year programme of research to investigate any potential risk to wild salmon from sea lice in the Scottish coastal environment. It will complement and extend another project currently being undertaken by the Scottish Aquaculture Research Forum (SARF), looking at the scale of sea lice impact on numbers of wild salmon returning to spawn by collecting data from a network of sites in which survival of smolts treated with anti-lice chemicals will be compared with controls.

The project will examine the outward migration of salmon smolts using acoustic tracking in key coastal areas coupled with modelling of salmon movement patterns based on swimming behaviour in relation to tides and currents. In parallel, maps of sea lice distribution in coastal waters will be generated, indicating areas or high and low concentrations of the parasite. The maps will be based on sea lice data from farms, and modelling of the dispersal of sea lice by prevailing winds or currents. Information on the distribution of smolts and of sea lice will be combined to assess the risk of interaction and data will also be collected to estimate the proportion of the lice in the fish’s environment that can be expected to settle on the salmon. This information will be coupled with assessment of the effects different numbers of settled lice on the welfare of the salmon. In this way, it is hoped that any risk due to interaction and the impact of that interaction will be better understood.

Modelled predictions from this suite of studies will be compared with the evidence of impacts from smolt treatment experiments. The approaches and the data collected to look at sea lice distribution can also provide information on connections between different farming areas and how lice spread and establish between them. This, in combination with information on salmon movement and locations of lice, can be used to look at options for development and improved management of aquaculture and conservation of wild salmon stocks.