High-tech and conventional methods for sea lice control

Salmon industry fi ghting its most persistent problem

Parasitic sea lice or, to use the simpler and more common term “salmon lice”, are a problem for salmon farms all over the world. Great efforts are being made to fi nd a control method that meets the requirements of salmon farming, is environmentally friendly, and also suits consumer demands for food safety. The current strategies for sea lice control alleviate the problem but do not provide complete protection.

The Thermolicer makes use of the sea louse’s low tolerance of sudden changes in water temperature. Infested fi sh are placed in a lukewarm bath for a few seconds which eliminates between 75% and 100% of the parasites.

Sea lice are a bane for salmon aquaculture and a real challenge for parasitologists. Salmon farms are in a constant struggle against the onslaught of naturally occurring parasites. Anyone who slackens their controls for just a few days usually has a high price to pay because the invasive mobile ectoparasitic copepods can infest the fi sh population very quickly. Th e sea lice settle on the fi shes’ scales and feed on their tissue, mucus and blood, which endangers the well-being and health of the host. Th e salmon become chronically stressed, they lose their appetites and no longer thrive, and the skin that is damaged by the lice can become infl amed. Bleeding and oedema often occur, the immune system of the fi sh is weakened, and

secondary pathogens can penetrate the body once the natural skin barrier is injured. In the case of extreme infestation mass mortality can even occur. Th e small lice are in fact to blame for huge losses, and it seems that the problem has continued to build up over the years since the start of salmon aquaculture. In Norway, the number of registered treatment measures increased 1.4 times between 2012 and 2017 according to the Food Authority, although salmon production hardly increased at all. It is in the salmon industry’s own interests to fi nally solve the persistent problems around salmon lice (in the North Atlantic mainly Lepeophtheirus salmonis, in the South Atlantic off Chile mostly Caligus rogercresseyi). At the same time, external pressure on the industry is also increasing. Th e announcement by the Canadian liberals around Justin Trudeau to abolish salmon farms in the sea by 2025 in order to protect wild fi sh stocks may have been infl uenced by the parasite problem. In July 2019, Norway reduced the limit for salmon lice in organic farms (“green licences”) from 0.5 to 0.25 lice per salmon. In September 2019, the Chilean government announced tighter regulations for their sea lice control and eradication programme, and a farm belonging to the Icelandic salmon breeder Arnarlax failed to gain ASC certifi cation in July 2018 due, among other things, to excessive infestation with salmon lice. According to the audit report, the auditors found 10.78 female salmon lice

per fi sh: the ASC standard permits only 0.1 parasites per fi sh.

Salmon lice are not only an ecological but also an economic problem. Th e marine research institute Nofi ma estimates that Norway’s salmon industry loses more than 560 million euros per year due to the cost of control and eradication measures for sea lice. A simple treatment for removing the lice can cost up to EUR25,000 (NOK200,000) per net enclosure. And losses have to be taken into account, too: in

The Tubenet from Akva Group keeps the fi sh at a depth which sea lice do not inhabit. The fi sh can come to the surface to fi ll their swim bladders with air using a snorkel net that is impermeable to sea lice. The system can reduce infestation by up to 80%.

Norway, 20 per cent of salmon die before they reach harvesting weight and this is mainly due to sea lice and ineffi cient monitoring of fi sh welfare. Th ese losses correspond to a billion salmon portions worth over 1.5 billion US dollars. In Chile’s salmon industry production costs have increased by about 1.23 euro/kg as a result of control measures against Caligus rogercresseyi salmon lice. In the face of such fi gures it is hardly surprising that the salmon farming industry is doing all it can to eff ectively combat sea lice infestation. At the same time, it is looking for innovative methods to tackle the problem proactively rather than reactively, i.e. when the fi sh have already been infected by the parasites. Th at means guarding against or at best stopping sea louse invasion in advance.

Th e fi rst step in any measure to combat sea lice is precise measurement of their number and distribution in the aff ected areas and on the aff ected fi sh. What previously necessitated manual counting is now performed by intelligent image recognition systems that use high-resolution cameras to photograph salmon in net enclosures and evaluate the images on the basis of sophisticated algorithms. Th is advanced technology not only counts the sea lice but can also detect their development stage quite precisely. Salmon lice go through eight life stages before they become “adult” and sexually mature. Th e infectious larvae are smaller than one millimetre, and adult female lice measure about 12 millimetres. Ecotone’s SpectraLice System counts and classifi es sea lice according to their size. Th e camera records a hyperspectral image of individual salmon and the intelligent software analyses the colour signature of each pixel. Based on the intensity of the colour spectrum the system recognizes the sea lice and also identifi es their growth stage. SpectraLice can evaluate between 300 and 2,000 salmon within 24 hours and is more accurate than manual counts. MSD Animal Health’s intelligent Falcon system also provides valuable snapshots of the salmon lice data within the salmon stock, and these are then transmitted directly from the net enclosure to the desk computer. Th is makes it possible to plan treatments more precisely and effi ciently.

Chemotherapeutants gradually losing importance

Initially, the salmon industry relied mainly on chemical substances to control the sea lice problem. Th ese are indeed eff ective but also have a negative impact on the fi sh and the

environment. Th ey curb salmon appetite, reduce growth and also damage marine ecosystems. It can take weeks before the fi sh can be sold after treatment. Another particular disadvantage is that over time the parasites develop resistance to the treatments. Chemotherapeutants against salmon lice can be in the form of bath treatments (e.g. organophosphates, pyrethroids, hydrogen peroxide) or as additives in fi sh feed (e.g. emamectin benzoate, difl ubenzuron). For bath treatment, the net enclosure is either lined with a tarpaulin and the enclosure volume reduced, or the fi sh are transferred to a wellboat. Until 1995 more than 80 of all delousing operations in Norway were carried out with water-soluble organophosphates. When sea lice became resistant to dichlorvos, farmers switched to azamethiphos (which was ten times more eff ective) in the mid-1990s. Sea lice are already developing resistance to emamectin benzoate (“SLICE”) which was introduced in 2000, which is why the dosage has had to be increased about fi vefold since then in order to eff ectively repel the ectoparasites. Researchers at the Sea Lice Research Center (SLRC) of the University of Bergen warned as early as 2013 that some lice strains had become resistant to all available drugs. So there was an urgent need for new eff ective drugs.

One focus of current research is on substances that inhibit the moulting and thus the development of sea lice. (Like all crustaceans, parasitic copepods have to change their exoskeleton regularly in order to grow.) However, using these drugs involves the risk that the moulting phases of shrimps, lobsters and

other crustaceans in the vicinity will also be aff ected. Baths in hydrogen peroxide (H 2 O 2 ), on the other hand, are less risky with regard to the natural environment. After use, H 2 O 2 decomposes into water and oxygen and is therefore considered non-toxic and not environmentally persistent. Due to growing resistance to organophosphates and pyrethroids in some coastal regions, hydrogen peroxide has been increasingly used again for delousing since 2009. It is assumed that H 2 O 2 , as a strong oxidizing agent, leads to the formation of gas bubbles in the haemolymph, which paralyses the sea lice so that they ultimately fall off their host fi sh. Hydrogen peroxide is a registered drug and its use must be monitored and documented.

In October 2019, Pharmaq presented a new bath treatment for the control of sea lice in salmonids in Chile. It is based on the chemotherapeutic agent Alpha Flux with the active component hexaflumuron which inhibits the synthesis of chitin, the main component of the sea louse cuticle. The effectiveness of the treatment has been confirmed in trials. Since 2016, Chile has also been using the delousing agent Imvixa, which contains 10 of the active ingredient lufenuron, whose use is not permitted in some European countries, however. In addition, the first vaccine against sea lice was launched in Chile in November 2015, but little is known about its effectiveness in controlling the parasites.

Alternative control methods are favoured

More recently, a paradigm shift in sea lice control – away from chemotheraputants towards thermal and mechanical treatment methods – has taken place in almost all salmon producing countries. In Norway, chemical agents dominated more than 80 of treatments between 2012 and 2015, while 74 of treatments in 2017 were thermal or mechanical. Non-chemical control methods are mainly based on three characteristics of sea lice. Firstly, the parasites – and particularly during the infectious young stages – prefer to stay in the upper water layers to a depth of about 10 metres. Secondly, they do not tolerate freshwater: when wild salmon ascend into the rivers the lice separate from the host animal). And thirdly, they are relatively sensitive to heat.

A simple but effective protective measure against sea lice is therefore to place impermeable tarpaulins across the upper layers of the net enclosures. Although they cannot prevent salmon infestation completely, they reduce the number of invading parasites. The same basic idea is followed by “snorkel nets”, in which a horizontal roof within the net enclosure keeps the salmon at greater depths, which is additionally supported by lighting and feeding in the lower part of the net. Lights and feeding systems lure the salmon into the deep water, which is avoided by the sea lice. The snorkel net allows the salmon to swim to the surface unharmed by sea lice. There they can absorb air and transfer it from the front intestine into the swim bladder via a special connecting duct (ductus pneumaticus). Trials by the Norwegian Akva Group, which calls its snorkel net “Tubenet”, have shown that this simple procedure can reduce lice infestation by about 80 per cent.

Thermal delousing makes use of the sea louse’s low tolerance of sudden changes in water temperature. Among the systems commonly used in practice are the Thermolicer and the Optilicer, which – apart from small technical details – work very similarly: the fish is put briefly into a lukewarm water bath (usually for about 20 to 30 seconds at 28°C in spring and 33 to 34°C in late summer) which eliminates between 75 and 100 per cent of the lice, depending on the conditions. Thermal delousing is thus very effective but can also pose a risk for the salmon because the upper limit of heat tolerance of the host and the parasite is almost identical. The shorter survival time of the lice is mainly due to their smaller size but the salmon are at great risk in the case of application errors.

Delousing measures with freshwater are similarly effective. Sea lice react sensitively to freshwater and die quickly if the salinity of the water is too low. If salmon are temporarily exposed to freshwater the sea lice fall off and can be removed. However, this alternative treatment also poses certain risks for the salmon. The Norwegian salmon farmer Lerøy, for example, lost more than 150,000 salmon (633 tonnes) in a freshwater delousing operation in July 2016.

Another method is to use “flushers”, in which the lice are mechanically removed from the fish body as in a shower with lowpressure water jets (0.2-0.8 bar)

or with rotating brushes (as used, for example, in Hydrolicer, SkaMik, Flatsetsund). For this treatment the salmon should not be too small (the recommended size is 4 kg), and with the brushing method scale losses and skin injuries are possible. Sea lice traps (e.g. Capture & Contain) are also used. They are usually placed outside the net enclosures around the salmon farm and attract mobile lice stages with flashing lights to keep them away from the salmon. Stingray’s Optical Delousing System, which combines stereo cameras, advanced image recognition software and precision lasers, is a high-tech solution. Lice are identified on the salmon as they swim by and killed by a precisely directed laser pulse without damaging the fish (the parasite coagulates in milliseconds, the salmon’s scales reflect the laser beam). Air bubble curtains that rise from the lower edge of the net enclosure to the water surface are also effective. Such bubble curtains act as a barrier that prevents a lot of sea lice from entering the interior of the enclosure.

Innovative ideas open up new possibilities

An approach that the US company Prospective Research is pursuing is not yet ready for practice. Prospective Research hopes to use bacteria to combat the sea lice that are known to quickly become resistant to chemical agents. The idea is to enrich salmon feed with suitable bacteria that stimulate the formation of natural bioactive compounds in the fish intestines, which then have a paralyzing or repulsive effect on lice. Prospective Research

is currently searching for bacterial strains that meet these requirements. A successful method for the biological control of salmon lice is the use of “cleaner fish”, which eat the annoying parasites directly from the body surface of the salmon. The number of such cleaner fish on Norwegian salmon farms has risen sharply since 2008. According to the Norwegian Fisheries Directorate more than 50 million cleaner fish were used in two thirds of all salmon farms in 2017. The cleaner fish mainly consist of five species: Labrus bergylta, Symphodus melops, Ctenolabrus rupestris, Centrolabrus exoletus and Cyclopterus lumpus. Demand for the species is in the meantime so great that it can no longer be met by wild catches from nature but requires regular reproduction. Because this needs special know-how, Mowi Scotland purchased Ocean Matters in April 2019, which produces around 4 million cleaner fish annually.

The management sector of the salmon aquaculture industry can also make a contribution to sea lice control. Since infestation rates increase with the spatial density of the salmon farms it would be possible to distribute the farms more widely across the sea. New, larger farms that are currently being built on land in Norway are also expected to alleviate the situation. They make it possible to keep salmon on land longer and shorten the time at sea when they are exposed to sea lice. Perhaps even the entire life cycle of salmon will soon be transferred to recirculating aquaculture systems (RAS) on land. However, this would increase rearing costs and make salmon more expensive.

Sea lice management and control is currently focusing on highly complex hydrodynamic computer models that can predict the probability and intensity of lice outbreaks in certain regions off the Norwegian coast, taking into account variable environmental conditions, in particular currents, temperatures, salinity and wind. Th is will enable companies located there to initiate countermeasures in good time. Th e “traffi c light system” introduced a few years ago, for which the areas off the Norwegian coast were divided into 13 production zones, has also proved very successful. On the basis of environmental indicators, which include salmon lice infestation as an important criterion for the sustainability of farming, the production zones are classifi ed as either “green” (further growth of farming is possible), “yellow” (freezing of production at current level) or “red” (production must be reduced).

In addition, there are some new ideas and approaches on how to better manage the sea lice plague. Researchers at Tromsø University believe that control strategies must be more genetics-based because the lice adapt very quickly to their local environment. That is why salmon lice often develop a genetically different type in each net enclosure, and this then has to be taken into account when initiating control measures. The “environmental DNA” method, in which tiny traces of DNA that enter the water for example with skin cells, excrement and blood, are analysed and compared with databases (“metabar coding”), also opens up interesting possibilities. Researchers are currently working on a database containing DNA sequences of all parasites, diseases and microalgae that can be dangerous for salmon. This would provide an early warning system that would enable identification of emerging risks through water analysis. Other considerations are based on the use of ultrasound, which repels or kills sea lice at certain frequencies without disturbing the salmon. mk

Strict monitoring of salmon production and sea lice infestation

The 2008 Norwegian Regulation on the Operation of Aquaculture Production Sites makes it compulsory for fi sh farmers to report the number and age of their fi sh, mortality-related losses and any fi sh removals and feed consumption to the Fisheries Directorate every month. This information is stored in a database together with the IDs of the farms and net enclosures. In addition, the Ordinance on the Prevention of Salmon Lice in Aquaculture requires farmers to regularly report the number of sea lice (broken down into sessile and mobile stages as well as egg-bearing females). Any sea lice treatment carried out must be reported to the Norwegian Food Authority stating the method and substance used. Between 2012 and 2017 the database covered an average of 807 (788 to 828) marine farm locations.