Questions over sea lice impact

atch data does not s pport cla s that sal on ar s are eh nd decl ne n ld fish

debated, even though these studies showed that the impact on wild fish was minimal.

While attempts to determine the direct impact of sea lice on wild fish continue, the main thrust of the debate has centred on the analysis of catch data. Marine Science Scotland has indicated that rod catches have traditionally been used to assess the state of wild fish stocks in Scotland as they are the most comprehensive indicator of stock status and, in many cases, may be the only information available.

According to his memoir (dhe ongshoreman, 2004), the former head of the Freshwater Fisheries aboratory, Dr Richard Shelton, said that the first indication that wild fish in Scotland had been a ected by sea lice from salmon farms was in 1989, when the sea trout fishery in och Maree collapsed. dhe problems of this renowned fishery were then highlighted in a scientific paper published in 2006 by Butler and Walker using data collected from the rod catch in the loch (graph 1).

dhe graph helpfully highlights the possible cause of the collapse with the inclusion of an arrow showing when in 1987 salmon farming arrived in the ad oining och Ewe. dhat year, the rod catch for sea trout was about 600 fish but only two years later it had dropped to less than one hundred. dhe implication was that sea lice emanating from the new farm were to blame for the collapse.

Although the Sco sh government has been collecting catch data from all Sco sh fishery districts since 1952, the graph highlighting the och Maree collapse was not based on their data. nstead, the catch data was derived from that collected from guests of the och Maree Hotel.

dhe graph based on Sco sh government data paints a slightly di erent picture (graph 2). Eumbers of sea trout caught between 1987 and 1989 do show a fall but the catch from the whole of the Ewe System had shown even greater drops in numbers long before salmon farming arrived in the loch. arge falls in the fish catch had been observed in 1966 67, 1970 71 and 1981 82.

While it is clear that the overall trend for och Ewe och Maree is in a downward direction, it is worth noting that this trend did not begin with the arrival of salmon farming in the locality (graph 3). t seems that sea trout catches (and stocks) were already in decline when farmed salmon were first stocked in och Ewe.

s it possible that whatever prompted this decline continued to cause the demise of this fishery even a er salmon farming s arrival in the loch While the cause of the decline remains a mystery, the blame was laid at salmon farming s door.

dhere is another element to this sea lice puzzle that is never discussed, and that relates to the wild salmon of the Ewe System. When salmon smolts leave the River Ewe, they too must swim past the salmon farm to start their migration. dhey should be ust as likely to pick up sea lice as young sea trout and, therefore, if the sea trout fishery has su ered as a conse uence of salmon farming, the salmon fishery should also be showing a similar pattern.

dhe salmon catch data for the Ewe System is, however, not as might be expected (graph 4). What is apparent is that from 1952 until 2014 salmon catches have been not only consistent but show an upward trend. Salmon catches do not appear to have su ered at all over this time and certainly not following the arrival of salmon farming in och Ewe.

dhis new analysis of catch data does not support any claims that sea lice from salmon farms are the reason for observed changes in wild fish numbers, but what is clear is that the Ewe System appears to be a healthy salmon fishery despite the presence of salmon farming in the loch.

r Martin :a a