UK - For years salmon farming has been the target of environmentalists and angling groups that blame sea lice from the farms as the cause of sea trout population decline. Taking a closer look at the data, Dr Martin Jaffa, from Callander McDowell, explains why he thinks salmon farms are not to blame.
During his presentation at Aquaculture UK this week, Dr Jaffa explained that many groups claim sea trout catches are down on the west coast of Scotland due to presence of salmon farms and therefore sea lice. However there are other differences between the east and west coasts not just the prescence of salmon farms.
For example, said Dr Jaffa, the east and west coasts have completely different sized river systems which obviously perform in different ways.
Digging through the data has also shown that some research actually shows increases in catch numbers on the west and a decrease on the east.
The loch Maree and loch Ewe river systems, once known for their huge sea trout catches, was apparently the first place where the problems of sea lice came to attention.
The initial study study showed sea trout catches declined in the 1980's and the only area change the researchers saw was that there was now farming in the loch.
However, taking a closer look at the data it can be seen that the fishery was already in decline before salmon farming even started in the loch.
In other data sets, salmon catches in the loch are included (which were left out of other reports). This salmon catch data showed that salmon, which were coming down the same stretch of river as the sea trout did not experience a decline in numbers and actually an increase.
This of course brings to mind the question of why if sea trout declined due to apparent sea lice did salmon numbers not decline also?
The reason of course, said Dr Jaffa, is that it probably has nothing to do with sea lice.
Similarly, in other parts of Scotland there are examples of areas where salmon farming takes place but wild fish catches have increased, such as the river Lochy and where there have been huge sea trout losses but before any salmon farming takes place.
It can also be seen that the migratory route of many juvenile salmon pass all the salmon farms on the west coast as they head north and their catches are increasing.
Yet the trout which migrates locally and does not go past the salmon farms are in decline.
Sea lice from salmon farms therefore do not look to be the reason for the declines. They may be impacting slightly on numbers but this is on fisheries that are already declining.
No one knows exactly why these losses are happening but it could be due to a number of things including changes in weather, breeding, disease, predators, river systems. Any of these could be the real cause.
Dr Jaffa also made the point that after 40 years of studies, the word 'probably' is still used to define the relationship between sea lice from farms and wild catch.
We therefore need to find the real answers, Dr Jaffa concluded.