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Production process
Contents:
  1. Sea lice blamed for major fall in salmon tonnage - BBC News
  2. Optimal Salmon Lice Treatment and Tragedy of the Commons
  3. Disturbs salt and water balance
  4. Salmon lice
  5. Current research:

The adult parasites continue to feed on the slime, skin and blood of the salmon. The host fishes lose their appetite and grow poorly, and their susceptibility to secondary diseases increases. The more parasites infest the salmon the more serious are the consequences for the health and wellbeing of the fish and thus the economic success of the farm. Salmon lice do not only infest farmed salmon but also fishes living in the wild but they spread rapidly in farm environments because of the ideal living conditions that prevail there, offering them a generous diet.

In desperation they sometimes then clutch at even the thinnest straw to gain at least partial control of the situation.

Salmon eating sea-lice effect Scotland’s fish exports

Scientists at the University of Maine and from the DFO examined, for example, whether mussel cultures might be of help to the salmon farms by filtering the parasite larvae out of the water. In summer some farmers treated their salmon with hydrogen peroxide H2O2 in well boats. This worked well, proved to be effective and was also kind to the environment but the method is only suitable for low levels of parasite infestation and certainly not on all farms. In addition, the treatment is complicated and relatively expensive since several cubic metres of H2O2 are required to achieve the same effect as conventional therapies that only use one kilogram of Ivermectin or Diflubenzuron.

These chemicals are not permitted in Canada, however. This perhaps explains why salmon farmers are putting so much hope on the cunner. The fishes can measure up to 25 cm, are of varied colouring, but usually reddish brown or green, and have been shown in laboratory tests to be capable of freeing salmon of salmon lice using their tiny, sharp teeth. If they can also prove their worth as lice pickers in net pens it would give salmon farmers an environmentally friendly method of combating lice without having to use chemicals.


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Although a scientific study carried out by MacKinnon in the year doubted the efficiency of the cunner the study with 30 wrasse and 2, salmon was not able to show any significant reduction in salmon lice this could have been due to the strong infestation of the net material.

If sufficient fouling organisms are available it is presumably easier for the small fishes to feed on them than to swim from salmon to salmon searching for lice. In order to further clarify this and other issues Cooke Aquaculture is carrying out tests in Back Bay.

Sea lice blamed for major fall in salmon tonnage - BBC News

Among other things they want to find out how many cunners are needed to delouse the salmon in the cages effectively. Norwegian salmon farming is already one step further: Farming facilities have been using wrasse for years in the context of the National Action Plan against Salmon Lice on Salmonids. All the farmers in a certain region are obliged to start fighting the lice as soon as infestation numbers in the cages exceed a certain critical value.

Already the early pioneers of salmon farming had to cope with salmon lice. At that time the cages were often even dragged into river estuaries because the parasites cannot tolerate freshwater and fall off the salmon. Light can also be used to fight the louse: in deeper water layers and after weeks of darkness infestation rates are considerably lower. Adding glucans polysaccharides and vitamins to the salmon feed presumably also scares off some of the salmon lice.

Optimal Salmon Lice Treatment and Tragedy of the Commons

It is not known exactly how this works but in experiments salmon fed in this way suffered 25 per cent less lice infestation. Intensive work is also being carried out to find vaccines that might protect the salmon from lice infestation, but it is likely to take years for them to become available. Wrasses are mostly seen among the public as environmentally friendly and economically efficient biological weapons for fighting salmon louse, despite the fact that they are not always as efficient as would be necessary. In winter, for example, their hunger decreases noticeably.

Some farmers also fear that they transmit salmon diseases such as furunculosis, or that they eat the salmon feed, or could themselves be eaten by large salmon. But these are only some of the reasons why their use in Norway is currently still relatively hesitant. The main reasons are to be seen in the difficulties involved in obtaining the cleaner fishes which generally have to be put into the cages in a ratio of 2 to 3 wrasses for salmon. Added to this is the fact that, in order to prevent transmission of diseases, the fishes cannot be reused in subsequent salmon farming cycles. These two factors lead to a tremendous demand for wrasse which is currently far from being satisfied.

In reality, however, there are hardly more than 2 million wrasses available and they are mostly caught along the coast by local fishermen. Wrasses are often landed as by-catch but some fishermen have specialised in catching this species, as a rule using baited basket traps to avoid skin damages and other injuries to the fishes.

In the meantime there are even official guidelines for the fishing, handling and transportation of wrasse.

Norwegian salmon farms make use of various different wrasse species for combating salmon louse, particularly ballan wrasse Labrus bergylta , goldsinny wrasse Ctenolabrus rupestris and corkwing wrasse Symphodus melops. Because this species is relatively small it is especially suitable for young salmon during their first year in the marine pens. It could be eaten itself by larger salmon. Unfortunately this species is not found so frequently off the Norwegian coast so that fisheries are unable to satisfy demand from the salmon farms. Corkwing wrasse can also be used for larger salmon but the cage nets should be kept clean since this species prefers foraging fouling organisms than salmon lice.

Many salmon farmers prefer to rely on chemical agents.

Salmon lice treatments and salmon mortality in Norwegian aquaculture: a review

But no matter which species of wrasse is used to combat salmon louse it would be wrong to expect very quick effects from biological parasite control. Salmon lice in young and smaller development stages on the fish skin are hardly attacked at all. It is only later on when they have grown to a larger size that the cleaner fishes feel tempted to eat them. And accordingly, salmon farmers assess the efficiency of the wrasses differently.

Disturbs salt and water balance

The spectrum of opinions ranges from useful, effective, and reasonable to unnecessary and ineffective. And it is true that occasional cases of eye picking do occur from time to time although it is often unclear whether the wrasse attack the living or dead salmon. It has often been noted that an eye was only missing on one side of the body which would seem to imply that the salmon already lay dead on the bed of the net cage at the moment of attack.

But this is why a lot of farmers prefer to use chemical as opposed to biological means for combating salmon lice. Although the stress to which the salmon are thereby exposed is not inconsiderable and the substances are not environmentally friendly, chemicals are always available, their usage is relatively cheap, and they are a reliable combatant. Breiland stresses that there is no harm in eating fish that do have lice on them.

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Salmon lice

This is backed up by the food authorities on the website matportalen. Sources: Lakselus , Norwegian Veterinary Institute. In Norwegian Dalvin, S. Mette Wesmajervi Breiland's profile.

Current research:

New results show that there are two sub-species of salmon lice. The salmon louse has become ever more resistant to drugs. A salmon louse is very tiny when it attaches itself to a fish. This photo shows a fully grown female louse. The long, beige anterior threads contain eggs, which she releases into the water.


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New lice hatch and float around until they come across a host to attach to. But what direct effect do salmon lice have on consumers? A scientist with expertise in marine parasites thinks if a louse were to make it to the fresh fish counter in a retail market it would most likely have to be in this nearly microscopic stage.

Photo: R. Skern-Mauritzen and C.