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reLAKSation no 1025

Colour by numbers: According to Salmon & Trout Conservation’s Andrew Graham Stewart, the first salmon of the year (8lb, not liced) was landed on April 3rd from the River Halladale. This is the sort of information provided in the various fishing reports that appear every month in Trout & Salmon magazine. However, when it comes to the west coast and the fishing in Wester Ross, Mr Graham Stewart prefers to use these fishing reports to return to his favourite subject of salmon farming. In the latest issue, Mr Graham Stewart discusses how the Wester Ross Fisheries Trust has been re-established and congratulates them on their ‘refreshing uncompromising stance’ on salmon farming. Highlighting a 40 page review, available from their website, Mr Graham Stewart, mentions that ‘cumulative larval sea lice on salmon farms poses a growing threat to wild fish in Wester Ross.’  They say that there is more than enough scientific evidence demonstrating the threat for precautionary action to be taken. They ask how much worse does the situation for our sea trout and salmon have to get before adequate regulation for sea lice on salmon farms.

Perhaps, Mr Graham Stewart might like to consider another question about the situation for sea trout and salmon and say how much worse it must get before his fellow anglers stop killing these valuable fish for sport. According to the latest salmon and sea trout statistics for 2020, and at a time when angling was restricted by Covid, anglers still managed to catch and kill a total of 578 wild fish from rivers around the salmon farming areas. This includes 45 salmon (av weight 4.5 kg), 109 grilse (av weight 1.96 kg), 185 sea trout (av weight 900g) and 239 finnock (av weight 412g). I don’t have a problem with anglers killing these fish, but I do have a problem with the same anglers blaming salmon farming as the reason why there aren’t more fish for them to catch and kill.

Five of these finnock (av weight 360g) were caught and killed in the fishery district where Wester Ross Fisheries Trust have their office and where they carry out the sweep netting commissioned by Marine Scotland Science to investigate sea lice levels on wild sea trout. Although the new Wester Ross Fisheries Trust has only been operating for a matter of months, their first published piece of work has been to sample sea trout in the Flowerdale estuary in Loch Gairloch. In an article published on 2nd June titled ‘Are lice levels too high to safeguard wild salmon migrating through coastal waters around Wester Ross?’ I am not sure whether they answered the question, but they do suggest that if salmon smolts become infested with potentially lethal numbers of sea lice, they continue to swim until damage associated with sea lice infestation causes premature mortality, most likely due to becoming more easily caught by fish eating predators.

Salmon & Trout Conservation subsequently tweeted

“Recent monitoring by Wester Ross Fisheries Trust shows lethal numbers of sea lice on sea trout and underlines yet again why there must be a back-stop adult female sea lice max on all fish farms, subject to strict inspection and enforcement.”

However, in common with many comments made about scientific publications, it seems that many observers simply repeat the headline story without exploring the detail. Often the detail doesn’t actually support the theories and claims, especially when it comes to interactions with salmon farming.

Unlike most of the other fisheries’ trusts carrying out sweep netting surveys about sea lice, Wester Ross Fisheries Trust has sometimes published the details of each individual fish they caught. This level of detail should be mandatory, as should be the provision of more in depth catch data, because it is essential that sea lice levels on salmon farms can be related to information about local fish. There is little point imposing stringent lice controls on salmon farming if there is no impact on wild fish. The connection between the two needs to be shown. At present, the evidence is simply circumstantial.

The Wester Ross Fisheries Trust article includes a link to the spreadsheet detailing all the fish caught in their most recent sweep netting survey which took place on 30th April of this year. The notes say that in addition to the fish that were assessed, a further 29 sea trout smolts were caught and then released because they were assumed to have only recently entered the sea for the first time. Other assumptions were also made including that lice levels on the fish might have been higher because of past damage to the dorsal fin. They also recorded that they had seen fish leaping near the estuary which they suggest might be related to sea lice infestation. However, these observations can only be conjecture not science.

This sweep netting has been part of a scientific study for Marine Scotland Science, so let us stick to the science. The spreadsheet lists 24 sea trout ranging in length from 223 mm to 525 mm and from 70g weight to 1345g. This leads me to raise the first important point. Salmon & Trout Conservation tweeted that recent monitoring by Wester Ross Fisheries Trust shows lethal numbers of sea lice on sea trout. Actually, they are incorrect because the data doesn’t show that at all. This is because the data is invalid as Wester Ross Fisheries Trust have not followed the official protocol established by the Scottish Fisheries Coordination Centre, a group formed of Fisheries Trusts, Marine Scotland, Marine Scotland Science and SEPA.

The protocol states that the catch should be a minimum of 30 fish. Not 24. I would even argue that 30 is insufficient as clearly if the sample size is not big enough, then the results are meaningless. Even at the best of times, sweep netting is not an accurate guide to the state of a fish population, not least because the likelihood is that the fish caught will be the weak and sickly individuals. The strong, healthy fish are unlikely to be hanging around the estuary mouth or are adept at escaping the sweep net. This means that any sample is totally unrepresentative of the population as a whole.

I believe that this would still be unrepresentative even if the intention of the sampling was to measure the size distribution of the local fish population, where a normal type spread would be expected. However, in the case of parasites, as I have to now often mention, the distribution is aggregated meaning that a large number of parasites are found on a few hosts, and these are the fish that Wester Ross Fisheries Trust have caught. The majority of hosts (sea trout) carry either no or very few lice and certainly some local anglers report catching healthy sea trout free of lice. The April 30th sweep data shows that only one fish had no lice. The fish with the fewest carried four but these were adult lice. By comparison, the fish with the largest load – 818 lice – consisted mainly of the larval stages.

However, the most interesting aspect of the results was the figure obtained for each fish by dividing the total number of lice (seemingly any stage) by the weight. This produces a figure of lice/gram. In the case of these fish, the numbers range from 0 to 1.2 but most are in the range from 0.003 to less than 0.5.

Wester Ross Fishery Trust have calculated these numbers in order to relate them to Taranger’s level of risk to wild fish from sea lice. This comes from a paper by Taranger and others from 2014 in which they have modelled three levels of risk, and which in Norway are equated to the Traffic Light system. According to Taranger, if the lice per gram figure is greater than 0.3, then there could be up to 100% mortality of the fish. When the figure is between 0.2 and 0.3 then there is above a 50% risk of mortality whilst this reduces to 20% when the figures are between 0.1 and 0.2. If the number is below 0.1, then Taranger perceives no risk.

Unfortunately, the perception of these risks is flawed. Firstly, Taranger assumes that any sea lice present must come from a salmon farm. He ignores the reality that sea lice are present naturally in the environment, often at quite high levels. Secondly, Taranger’s risk assessment fails to take into account the aggregated distribution of sea lice and thus his model is not representative of a typical wild fish population.

However, the one aspect of this sweep netting that caught my eye is that like the Norwegian Traffic Light system, Wester Ross Fisheries Trust has for the first-time colour coded each of the 24 fish to give a visual interpretation of the risk from sea lice.

A summary of the findings is provided on the spreadsheet which states that the number of fish with the highest level of 0.3 lice/g is seven.

Yet, there are nine boxes assigned a red colour. This is because Taranger presents two measures of risk dependent of the weight of the fish. According to Taranger’s scale for larger fish (greater than 150g) larger fish have a lower lice threshold, although this doesn’t make sense as larger fish would be thought to have a higher tolerance of lice infestation than those which are much smaller. Wester Ross Fisheries Trust have used both sets of risk together rather than separate the fish into two size classes. This is why there is a difference between the number of red boxes and the number of fish with more than 0.3 lice/g. The way this is presented is confusing, so much so, that they have used the wrong colour in assigning one of the boxes.

What is clear is that if the fish are sorted into groups by weight, then the risk of mortality is much lower in the larger fish. In fact, 62% of all the fish are not at the highest level of risk.

When I start looking into any subject, I don’t necessarily know where it is going to take me. I find that this is the most enjoyable aspect of research. Looking through the Scottish Fisheries Coordination Centre’s netting protocol, something else attracted my attention. However, first I would just mention that the purpose of SFCC is to provide a mechanism for fisheries managers to standardise data collection. The members are all the fisheries trusts and the management is a mix of Marine Scotland, Marine Scotland Science, SEPA, fishery boards and fishery trusts.

The section of the netting protocol under sea lice sampling states:

“Record fork length (to the nearest mm) and wet weight (to the nearest g). Given that it may be logistically unfeasible to weigh fish under certain conditions, for example in strong winds, recording wet weight may be considered as optional. However, it is strongly encouraged that wet weight be recorded if at all possible to further strengthen the validity of the data, for example by allowing the calculation of condition factor and the number of lice per gram of fish.”

I had to read this more than once.

What the protocol states is that after the fish is captured, it must be anaesthetised and then placed into water against a light background so that the lice can be counted and recorded. For each fish, the number of lice present, and their life stage is recorded. The protocol recommends the use of a binocular microscope, magnifying glass or hand lens.

It is not difficult to imagine that this is not an easy job requiring patience and accuracy, yet despite having to count possibly hundreds of tiny larval sea lice, the protocol says that recording the wet weight of the fish may be considered as optional.

Clearly, the wild fish sector like to rely on Taranger to assess their perception of risk and this requires two things – the number of lice and the weight of the fish. How can weighing the fish be considered optional?

In 2017, Wester Ross Fisheries Trust netted at Flowerdale on the 27th April. In total they recorded the details of 32 sea trout, but they only weighed 13 of the fish. The weights of the remaining 19 were estimated. Clearly, even a slight inaccuracy in the estimation of the weight can change the perceived level of risk to wild fish.

I am at a loss to understand why obtaining an accurate weight of each fish is not mandatory.  After all, the wild fish sector is using this data as evidence for the imposition of greater controls on salmon farming. I can understand why the local fishery trusts are happy with such a protocol, but I fail to understand why Marine Scotland Science and SEPA have not demanded the most stringent data possible, especially as the Scottish taxpayer, through MSS, has been paying fishery trusts to conduct this sweep netting on their behalf.

Finally, Wester Ross Fisheries Trust have headlined the article about this work ‘Are lice levels too high to safeguard wild salmon migrating through coastal waters around Wester Ross?’ This work does not answer that question for two reasons.

Firstly, the life cycles of salmon and sea trout are very different and whilst sea trout are perceived to be a proxy for salmon, the truth is that they are not, something that even Taranger recognises, and secondly, this sample is simply not representative of the local sea trout population.

There has been a reliance on Taranger’s 2014 paper for the assessment of the risk to wild fish from sea lice, not just for sweep nets but for the Norwegian Traffic Light system.

I will delve deeper to look at some of the science behind Taranger’s recommendations in a future reLAKSation.