Fighting salmon lice with “cleaner fish”

by Thomas Jensen
The salmon louse Lepeophtheirus salmonis causes considerable damages in the net cages of a lot of salmon farms.

Salmon lice are probably one of the biggest threats to salmon farming at the moment. The available control methods are often not sufficient to effectively reduce lice infestation. In Norway radical measures are sometimes even considered, such as slaughtering whole salmon stocks or concerted action to leave farms unstocked. But might it be possible to solve the salmon louse problem biologically using “cleaner fish”?

Probably everyone has seen pictures in films about coral reefs where small cleaner fishes swim around the mouths and bodies of large fishes, freeing them from bothersome ectoparasites and dead skin. It is known that about 45 fish species and some shrimps display this kind of cleaning behaviour. And these species do not only exist in the tropics but also in temperate and colder seas. For example, they include wrasse species which live along the coasts of Norway and North America. This raises hopes that it might be possible to solve the salmon louse problem on salmon farms naturally using biological control measures. Wrasse could act as a “green weapon” against the parasite problem, rendering the use of chemicals unnecessary.

In trial tests along the Atlantic coast of Canada researchers are currently investigating whether cunners (Tautoglabrus adspersus) might be suitable as health custodians for controlling salmon lice on salmon farms. Should this prove possible, cunners – which have so far had no commercial significance and have at most been used as bait – could suddenly become immensely important since salmon lice cause severe economic losses on Canadian salmon farms. During the first phase of its life the sea louse is pelagic and does not attach itself to a host animal until the copepodit stage when it meaures 0.7 mm. It clings to the salmon skin with its antennae and a chitin hook for several weeks before, later on, when it has reached a size of 5 to 10 mm, it begins to move freely on the body surface or to jump onto other salmon. 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. Extreme parasite infestation can even lead directly to the salmon’s death.

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. Up to now Canada’s salmon farmers were almost helpless in the face of salmon lice due to the fact that in their country hardly any viable remedies are permitted. In desperation they sometimes then clutch at even the thinnest straw to gain at least partial control of the situation. 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 2011 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. Although a scientific study carried out by MacKinnon in the year 1995 doubted the efficiency of the cunner (the study with 30 wrasse and 2,000 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. Among other things they want to find out how many cunners are needed to delouse the salmon in the cages effectively.

Demand for wrasse cannot be satisfied

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. With today’s production figures, however, this is no longer a feasible option. 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. 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. Depending on their origins there are possibly also seasonal differences in the fishes’ cleaning activity. 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 100 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. At least 15 million wrasses would currently be necessary to meet the Norwegian salmon industry’s requirements.

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 fi
shermen 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). Goldsinny wrasse is the most widespread species along Norway’s coasts and probably the most frequently used wrasse on farms. 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. However, goldsinny wrasses stop eating altogether at temperatures of below 6°C so that they only effectively reduce salmon lice numbers during the warmer months. In contrast, ballan wrasse grow to a larger size and are thus suitable for delousing large salmon, particularly since they do not stop eating until temperatures fall below 4°C. 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

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. The spectrum of opinions ranges from useful, effective, and reasonable to unnecessary and ineffective. When a number of years ago reports appeared that the wrasse did not only attack salmon lice but also the salmon’s eyes and some farmers lost several tonnes of large salmon on this account it seemed that the end of this kind of parasite control had come. 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. However, the real problem involved in the “chemical hammer” revealed itself already early on with the first pesticides like Nuvan and Neguvon which were not only harmful to salmon lice but also to the salmon and even the farmers themselves – so dangerous if fact that they had to wear special protective clothing, rubber gloves and gas masks. Overdoses of the substance led to panic reactions and increased mortality among the salmon. The biggest problem, however, was that the salmon lice very soon became resistant to these chemotherapeutants. Today the substances are less aggressive and very reliable in their efficacy but the problem of resistance still exists.

When in the mid-1990s Ectobann and Lepsidon came onto the market they had one substantial advantage in that they could be administered orally and just had to be mixed in with the feed. This was a significant step forward. They are only suitable for young salmon, however, because it takes a long time for all the residues to be completely broken down within the fish’s body. Apart from that, Norwegian environmentalists criticised the substances vehemently arguing, for example, that they could be carcinogenic. Today nearly all of them have disappeared almost completely from the market and are only used – if at all – very occasionally. To make up, Emamectin (also called “Slice”) has gained strongly in significance as a feed additive for fighting salmon lice. Originally this substance, too, had to be stopped 120 days prior to the fish’s death but after the EU directive on permitted maximum residue quantities of chemicals and medication in aquaculture fishes came into effect in October 2001 the time span was shortened to 175 “day degrees”. In water temperatures of 15°C this means for example that the substance has to be stopped at the latest 12 days before the fish is killed. Farmers can also use other substances such as Ivermectin or Diflubenzuron which combat salmon lice effectively at all stages of their development.

Here, too, however it can be seen that the salmon lice quickly become resistant to the substances. The recommendation to change medication as frequently as possible during any necessary treatment only postpones the problem but cannot solve it. The use of chemicals is also detrimental to the image of salmon farming because they remain in the fishes’ excrement and can harm crustaceans within the ecosystem when they feed on it.

Wrasse now being specifically produced in aquaculture

This situation can certainly be seen as contributing to the fact that we are currently experiencing a revival of cleaner fish in salmon farms. And because wild fish catches alone are not sufficient to satisfy demand from salmon farms attempts are now being made to grow the fishes in aquaculture. The increase in demand for wrasse could be met more easily with farmed fish since the fishes have to be available in sufficient numbers at the required time and in good quality. Whilst supply from fisheries is mainly limited to the relatively short fishing season aquaculture can guarantee year-round supply. Salmon farmers can purchase the cleaner fish in accordance with the mesh size of their net cages and the size of the salmon. Farmed wrasses are more uniform in age, size and condition than wild catches and their health can be specifically controlled. And farming also serves to protect the natural wrasse stocks better from overfishing.

During recent years, several companies have been developed in Norway alone whose business model is based on farming wrasse: Marine Harvest Labrus, Profunda, Nordland Leppefisk and Cleanfish. Interest in farming this species is growing in Ireland and Scotland, too. In February this year the Scottish Salmon Company and the Morpol Group’s Meridian Salmon Group co-operated with the Scottish Salmon Producers Organization to set up a project for farming wrasse. A three-year contract with the Scottish fish farmer Otter Ferry Seafish provides for the production of more than 250,000 cleaner fish for Scottish salmon farms. In Canada, too, researchers were overjoyed when test cunners spawned and reproduced for the first time in the lab in August 2011.

Their pleasure is understandable for up to now little is known about the biology and reproduction of wrasse. Farming these fishes is a real challenge, and much still has to be left to chance. Ballan wrasse (Labrus bergylta), for example, is a protogenic hermaphrodite, i.e. all the fishes are initially female and later on become male. Apart from that, the males are territorial and not very compatible with one another. Bites which occur during conflicts can become infected and cause the injured fish’s death. The fishes need special soft feed and sufficient places to hide if they are to reproduce. The larvae are only 3.5 m
m long when they hatch and initially have to be fed on enriched rotatoria. Wrasse farmers benefit from the wealth of existing experience in hatching cod larvae. Despite certain advancements, however, they still have to cope with poor growth, deformations and high mortality rates among the young wrasses. There is still no good commercial feed available and it takes several months for the fishes to grow to 30 to 70 grams (10-14 cm) and thus a sufficient size for them to act as lice pickers. But the effort could prove worthwhile in the long run if salmon louse can ultimately be combated using biological means with the result that less chemicals would be required.

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