Dear Scottish Ministers: If you don’t see this commentary, I hope that one of your colleagues will ensure you receive a copy.
Against the background of the Salmon Interaction Working Group and the proposed SEPA risk assessed framework, I want to tell you about the sprat eye-maggot. This is a parasitic copepod (as are sea lice). They are a member of the order siphonostomatoida (as are sea lice). They infest fish in the marine environment (as do sea lice), specifically, the European sprat. This is one of about 130 species of parasitic copepods that are found in British waters.
The sprat eye-maggot has a large number of development stages, some free living and some parasitic (as do sea lice) The two naupliar stages and the first copepodid stage are free living (as are sea lice). The copepodid stage must grasp hold of a host fish before it can moult into 4 chalimus stages (just like sea lice). The parasite is then mobile (like sea lice) and usually invades the fish’s eye (unlike sea lice) where it matures and produces egg strings (just like sea lice).
The host fish, the sprat, swims in large shoals. The younger fish (1-2 years old) tend to come inshore to breed, whilst older individuals (up to 7 years) tend to breed offshore. The following link is to a video that illustrates how close the shoals of sprat come to shore. This was filmed around West Bay outside Bridport in Dorset – https://www.youtube.com/watch?v=LKd85AymwSc
Spawning generally takes place at night. Important spawning areas include the English coast, such as seen in the video and in areas west and north of Scotland.
There has been research on how the sprat eye-maggot infests the host fish (just as with sea lice). The work was conducted in Norway around the Oslo Fjord. The researchers sampled over 15,000 fish and found that infestation rates were about 3.5% in the inner part of the fjord compared to 0.3% in the outer fjord. The same areas were also sampled for larval parasites in the water using plankton tows (just like for sea lice). In general, larvae were identified in the inner fjord but not in the outer. The times when larvae were identified tended to be around the times sprat breeding occurred so when the host fish was present. More importantly more larvae were caught at night however, numbers of larvae were still not large. On one day, 119 larvae were caught during the day whilst 710 were caught at night.
It turns out that the larvae of the sprat eye-maggot are light sensitive (just like sea lice), however whilst sea lice are attracted to the light, sprat eye-maggot larvae are attracted to the dark. By coincidence, the dark is exactly when sprat congregate in shallow waters to breed.
What is thought to occur is that infested sprat migrate inshore with the shoal at the same time the sprat-eye-maggot breeds and produces eggs. These are released and the eggs, if there are no potential hosts available, will only develop so far before entering a resting stage when they are attached to rocks or vegetation. When the parasite receives the right environmental cues – such as the disturbance of many fish as well as there being no light, they migrate to the surface, where they encounter the shoals of potential hosts to infest.
The adult sprat eye-maggot do not release large quantities of eggs into the water column in the hope of finding a host by chance. The opportunity of finding a host is greatly increased by their specific life strategy. Sea lice are undoubtedly the same.
Researchers have so far been unable to find large quantities of sea lice larvae in the water column and this is simply because they are not there. Like the sprat eye-maggot, sea lice have a specific strategy for finding host fish. Farming salmon, the hosts, does not change this strategy. The whole narrative of wild fish infestation from salmon farms is based more on conjecture than fact. This is because the narrative has been conceived by fish biologists, not by parasitologists with specialist knowledge of parasite ecology.
SEPA’s risk assessed framework assumes that sea lice larvae are dispersed in the water column by wind and currents. No parasite leaves finding a host to chance in this way as they are far too specialised. There are other aspects of parasite ecology that have been simply ignored because the significance is not understood or has been rejected because it doesn’t fit in the accepted narrative. The importance of sea lice is because of the relevance to salmon farming but they are not unique. Whilst the sprat eye maggot is not one of the family Caligidae, rather another family of copepods, there are 559 species of Caligidae copepods including 162 Lepeophtherirus spp. of which salmon lice are just one.
Dear Mairi Gougeon and Mairi McAllan, I would hope that you might consider pushing the stop button on the introduction of further sea lice regulation, so there can be a proper discussion about the impacts of sea lice on wild fish, rather than accept a narrative that will be eventually proven to be wrong.
There has yet never been a proper discussion about the science. In response to a FOI request, I was told that the science was discussed during SIWG however the chairman reliably informs me that discussion of the science did not take place during SIWG because it was considered too contentious. There must be agreement of the science before SEPA can progress towards a workable framework.
Anthropogenic: In the last issue of reLAKSation, I referred again to the annual report from Norway’s Scientific Committee for Salmon Management (VRL) which states that the greatest anthropogenic threats to wild salmon are from fish farming.
The report includes the following illustration of the risk from each anthropogenic factor that the committee investigated.
The graph shows that in the Committee’s opinion, the greatest threat to wild salmon in Norway is from escaped farmed salmon and salmon lice, whilst the least threat comes from overexploitation and agricultural pollution.
I have asked VRL more than once why the killing of over 100,000 adult breeding fish by both commercial and sports fishermen is considered much less of a threat than the estimated deaths of 30-40,000 smolts to the future of Norwegian wild salmon stocks. I have never received an answer.
Actually, these 100,000 plus dead salmon aren’t actually considered a threat to wild salmon stocks by VRL because they are considered acceptable exploitation and it is only overexploitation that is considered a threat. Of course, a less blinkered view might consider any exploitation to be a threat and that these 100,000 plus dead salmon actually represent overexploitation.
What makes consideration of the VRL assessment of threats to wild salmon is that scientists at England’s Centre for Environment, Fisheries and Aquaculture Science (CEFAS) have also published a paper on threats to wild salmon in England in April this year (https://doi.org/10.1007/s11160-022-09714-x ). This includes a graph that follows the format of that produced by VRL in Norway:
Salmon farming features on this graph but its position and the level of uncertainty about its threat to wild salmon are very different to that from Norway. Of course, there is no salmon farming in England, but it could be argued that smolts heading to their feeding grounds might interact with Scottish salmon farms. However, any impact is inconclusive.
What is interesting is that the authors of the paper also consider the threats dependent on various regions of England and Wales.
The areas are (a) Northeast, (b) South, (c) Southwest and (d) Northwest. It is only in the Northwest that salmon farming is considered to be a possible threat due to smolts from these rivers heading north along the west coast. Even so, the threat is still low and uncertain. Elsewhere salmon farming is considered one of the lowest threats.
Wild salmon in English and Welsh rivers are in severe decline so if it is not salmon farming causing this decline then other factors must be at play. The ones that stand out are that the authors consider any exploitation to be a threat, not overexploitation. They also include bycatch which does not feature in the Norwegian graph. However, the one threat that really jumps out of the graph is predation and again this is not considered by VRL. A quick search of the literature does highlight that both goosanders and seals do impact Norwegian salmon stocks but clearly VRL do not consider their inclusion as a potential threat as being important.
Could it be that the availability of large research grants in Norway for investigation of the interactions between wild and farmed salmon may influence the view that the salmon farming is the greatest threat to wild fish in Norway. After all, if salmon farming is not a threat, then extensive research would be no longer required. Alternatively, when all the research is focused on salmon farming then equally it might be considered the biggest threat
Finally, it is interesting that noise pollution features on the CEFAS graph. I have argued previously that seismic surveying which takes place continually in the North and Norwegian Seas may interrupt salmon’s navigational capability. Research scientists in Scotland have yet to show any interest in such possibilities. Possibly, they are too focused on salmon farming to have time for any other potential threats. After all, it is now some time since Marine Scotland Science drew up their list of 12 threats to wild salmon, yet how many have been investigated since?