Ending the confusion about salmon and sea lice?

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As an interested observer and admittedly firmly on the side of aquaculture and salmon farming in particular, I’ve always been puzzled by the sea lice argument.

Opponents of salmon farming blame salmon farms for producing the sea lice that infect and kill wild fish. They claim this is a major cause of the decline in wild stocks.

The first part of the puzzle is ‘where do the sea lice come from in the first place?’.

The 16 month old smolts that are put to sea certainly don’t have any sea lice. They have been raised from eggs in freshwater where sea lice do not exist. So, there are no sea lice on them when they are put to sea.

The sea conditions are likely to be pristine at the outset. In the case of Loch Duart, the loch has been fallowed for 12 months and sea lice cannot hang around for a year waiting for a salmon farm to be set up.

This rather leads to the conclusion that the initial problem is likely to be the reverse of what we are generally told i.e. that the initial infections are the result of wild fish infecting farmed fish.

This is logical. One of the ever-present vulnerabilities of farming of all types is that the concentration of livestock means that disease problems can be quickly magnified once they are introduced, as we know from foot and mouth and, more topically, Schmallenberg disease.

So, once wild fish have infected the farm stock, it’s easy to see why some people might think that sea lice concentration could make farm sites dangerous places for passers-by.

It is helpful therefore, that two research studies, one in Ireland and one in Norway, both over extended periods of time, have addressed the second part of the puzzle – the issue of ‘what happens next?’.

Both studies have now confirmed that sea lice are not the major factor in wild salmon mortality – and not a significant factor at all. The Irish study has concluded that sea lice account for no more than 1% of wild salmon mortality and the Norwegian study is in broad agreement. The Irish conclusions were published in the peer-reviewed ‘Journal of Fish Diseases’ and included the statement: –

“sea lice represent a minor and irregular component of marine mortality in the stocks studied and are unlikely to be a significant factor influencing the conservation status of salmon stocks”

The methods used to reach these conclusions seem to be quite sophisticated and the time taken over the studies (nine years in Ireland, eight years in Western Norway) cover the complete salmon life cycle more than adequately. You can see abstracts of the peer-reviewed papers published as a result of these two studies athttp://www.ncbi.nlm.nih.gov/pubmed/23298412 (Ireland) andhttp://onlinelibrary.wiley.com/doi/10.1111/jfd.12052/abstract (Norway) – contact[email protected] if you would like to see the full articles.


Not much has been heard from the opponents of salmon farming since these reports have been published. There seem to be two main types of opponent – activists funded by foundations funded by the so-called ‘wild’ Alaskan salmon fisheries, and fishing associations representing fly fishermen who for years conspicuously slaughtered more salmon than they could eat, lining up their catches in rows along the river banks. Today they release a percentage of their catch ‘sustainably’, allowing exhausted and damaged fish to continue their long and arduous journey upstream.

Perhaps these vested interests will find more to say at some stage – but it’s good that the sea lice argument has been put into its true perspective by objective and independent scientists.

Does that mean the debate is over?

Let’s wait and see…


Tim Mott