SOS: Save our Science - Save our Salmon

This is a story that's been going for a long while. The concerns over the effects of fish farming on wild fish populations. Also it's nice to take the science outside for a change. This new research is quite interesting and was well reported. Although the actual findings still need a lot of work.

A sudden population crash in wild salmon during 2002 caused mass panic for the authorities in Canada. This coincided with closer monitoring of farmed salmon, and resulted in the blame being placed on parasites transferred from farmed fish. The phenomenon has drawn the attention of many researchers and a new study appears to show that although there is transfer of parasitic sea lice between farmed and wild salmon it cannot be responsible for the decline in population.

This study is unique in having access to the full records of all 17 fish farms in the Broughton Archipelago, Western Canada. This includes data on fish numbers, parasite infestation and treatments over the last 10 years. It also uses figures collected about the wild salmon populations.

Sea lice (Lepeophtheirus salmonis) Img source

The parasite causing all the concern is the Pacific variant of a species of sea louse (Lepeophtheirus salmonis) which typically infests a variety of fish, including the wild pink salmon (Oncorhyncus gorbuscha) and the farmed Atlantic salmon (Salmo salar). Adult wild salmon live in the ocean but return to where they were born, in rivers, during autumn every two years to lay eggs. Wild fish pass by fish farms as they migrate back to the rivers and transfer some of the sea lice they carry to the farmed fish. When the newborn wild fish (which weigh one fifth of a gram each) enter the ocean in the spring they also pick up lice as they pass the farms. These fish will return two years later to produce their own offspring. This results in two groups of wild salmon; those that spawn on even numbered years and those that spawn on odd years. It is unusual for young fish to act as hosts to sea lice, though infestation of adults is very common. The infestation of juveniles is blamed for the decline in salmon.

The farmed Atlantic Salmon (Salmo salar) Img source

Previous research has looked at the relationship between lice, farmed fish and wild fish without access to fish farm records. Infested and healthy fish were captured and studied separately over several weeks. Fish with lice were found to be more likely to die, however this may mean that lice favour fish which are already going to die, and does not prove that lice cause death of the host fish. If healthy fish are infested with lice under laboratory conditions very few of them die (less than 4.5%), however salmon act oddly in labs so the results may be inaccurate.

Much of this research involved mathematical analysis of fish farm records. They found that lice numbers are naturally highest towards the end of the year, boosted by lice from wild fish arriving in autumn and increased reproduction during winter (due to higher sea salt concentration). This natural pattern is strongly affected by delousing treatments on fish farms, causing a sharp drop in louse numbers over the following two months. They found a significant link showing that the more fish that return to spawn in the autumn, the more lice there are in the farms the following spring and the more lice there are on the young fish entering the ocean. The relationship between wild adults and resulting louse numbers has been altered by delousing. However, the statistics show that some of their findings are borderline and thus may be questionable.

Wild pink salmon (Oncorhynchus gorbuscha) Img source

Fish which carry lice are more likely to die but lice usually leave the fish before this happens. Additionally many of the fish show symptoms not associated with louse infestation. Since no full medical tests have ever been performed it may be possible that bacteria or viruses have been killing the fish. In the lab infested fish jumped more and preferred freshwater to seawater. Young fish generally shed their lice in a few weeks, only very severe infestations of very young fish were seen to be lethal. Only about 8% of deaths can be explained by parasitic lice.

I found the paper quite inconclusive. It definitely shows that fish farms are causing unnatural spread of sea lice. Whilst it also shows that they are probably not causing decline of the population, they may be contributing, they may also help to spread any viruses or bacteria between fish (which may not show up in their experiments), which would explain the association between the lice infested fish and increased death rates. This demonstrates that the sea louse (and fish farming) is not directly responsible for the severe decline in wild pink salmon, but much more work is needed to prove that fish farms are not affecting wild populations.