by Rebecca Sherratt, Features Editor, International Aquafeed

 

Sea lice are a pest that many fish farmers are no doubt more familiar with than they would like to be. Parasites of the sea, they latch onto fish and feed on their bodies relentlessly, causing a variety of issues for both fish and fish farmers.

Despite the wide range of solutions that are on the market to both prevent and treat lice, there is no solution that guarantees 100 percent effectiveness and the issue continues to still plague members of the industry. If your farm is falling victim to these pesky critters, then the best way to manage them is to first gain a comprehensive understanding of the variety of different solutions that are readily available on the market.

So, what exactly are sea lice?

Sea lice are a family of salt-water small crustaceans that are also considered to be marine ecoparasites. Approximately 162 Lepeophtheirus and 268 Caligus sub-species of sea lice have been discovered, both of which wreak havoc on fish farms when an outbreak of them occur in a cage system.

These parasitic pests attach to fish and feed upon the blood, mucus and epidermal tissue of their unfortunate hosts. Sea lice are known to be particularly common occurrences amongst farmed salmon, but they aren't especially picky eaters and all varieties of farmed fish can fall victim to their gluttonous ways.

Certain species of sea louse may target specific fish species, such as the Pacific strand of Lepeophtheirus salmonis which has been recorded to target the three-spined stickleback (Gasterosteus aculeatus), whereas the Atlantic L. salmonis is known to stick solely to Atlantic salmon (Salmo salar), sea trout (Salmo trutta), Arctic char (Salvelinus alpinus) and other such forms of Pacific salmon.

Once a host has been located, the copepodid stage louse will begin feeding and soon develops into the initial chalimus stage. Female sea lice, as well as fully-grown adults and pre-adults are known to be especially aggressive feeders. Their feeding often results in serious damage to fish, including lesions, hyperplasia and inflammation. As a result of this, fish also experience heightened stress levels, which often results in various bodily changes. These changes then result in reduced growth and performance, disease susceptibility and decreased immune responses.

Surprisingly, the instigating factors for infection are not yet known. It is suspected that smolt become infected with sea lice in their larval stage when they enter estuaries in the spring. Adult stages of Lepeophtheirus spp can also transfer under specific laboratory conditions, whilst Caligus spp can transfer much more frequently and easily between fish species.

With limited space for aquaculture production on land, open net-cage farming and development has grown in popularity amongst many farmers, leaving fish vulnerable to infestation from sea lice.

Technological solutions

Whilst antiparasitic drugs have been produced that help resolve infestations of sea lice, these can also come with their own complications. Chemical cleanses can result in negative effects on the affected fish such as reduced growth and appetite. Salmon also cannot be sold for several weeks following chemical cleanses, and there is a possibility of lice building up a resistance to the chemicals used.

In recent years the technological innovations of various fish farming companies have also enabled farmers to take better preventative measures to keep their precious stock safe from these aquatic nuisances.

Sea lice skirts: Sea lice skirts, traditionally constructed from PVC, can be mounted around the top of salmon pens. This skirt acts as a shield, preventing lice from entering the pen. Skirts are a popular method of lice prevention for many farmers as they do not overly restrict fish movement or involve any regular maintenance.

Skirts enable both water and oxygen to move freely in the pen, whilst also promoting sustainability as they can be recycled and reused again and again. Sea lice skirts traditionally have a depth of six metres, the widely considered optimal length for lice prevention. Certain skirts may also offer special coatings which prevent cage damage due to arctic temperatures and other extreme weather conditions.

Snorkel nets: Another technological innovation intended to prevent initial sea lice infection is snorkel nets for salmon. Lice larvae are commonly found nearer the surface of the water, so placing a net above your stock to encourage them to remain in deeper waters can reduce the likelihood of contact with lice.

This solution is a much more environmentally friendly method of preventing infection, but also can cause issues with salmon. Salmon are physostomes, meaning that they lose buoyancy if they do not return to the surface to refill their swim bladder with air. Confining your fish to deeper waters can result in reduced appetite and growth.

Some snorkel nets have, however, adopted snorkel-like tubes that allow fish to reach the surface for a brief period of time, in order to refill their swim bladders before returning to the watery depths.

Thermal treatments: Thermal treatments take advantage of sea lice's naturally low tolerance for sudden shifts in temperature. The thermal treatment device is usually affixed to a boat beside the pen, which then feeds the fish through a system of tubes, submerging them in lukewarm water which kills the lice. Following this, the fish are returned to their pens.

This method, on average, eliminates between 75-100 percent of lice on infected fish and can process up to 80 tonnes of infected fish-per-hour. Cold water treatments have also been used, but studies suggest that cold water treatment can result in damage to the skin and eyes of the treated fish.

Flushers: Jets of water are also capable of washing away sea lice that are affixed to fish. Flushing treatment involves moving salmon through a system with water jets, in order to flush off the lice, which are then collected and disposed of following the procedure.

This system requires experienced personnel who best know how to delicately handle the fish being treated but also has the advantage of being a much less stressful form of treatment for the afflicted salmon, when compared to other options.

Lasers: Another very unique method of sea lice removal is through the use of lasers. Specialised camera systems are placed in pens which quickly and accurately detect where sea lice are present on fish, through AI programming that identifies abnormalities in colour and texture on their skin. Once the camera detects louse present, the device pulses diode lasers at 550nm directly at the louse, killing them efficiently.

Thanks to the very unique mirror-like skin of the salmon, these lasers merely reflect off them harmlessly, ensuring that they remain unharmed by the process. Lasers are also able to pick up lice that cleaner fish might not be able to see. As our technological capabilities continue to rapidly evolve, our AI systems will only further grow more and more effective.

These are only a small selection of the technological solutions available to farmers in the ongoing war against sea lice, but it is crucial for farmers to properly educate themselves on the various ways in which to prevent and treat these troublesome parasites.

Certain methods may work better for certain farms, depending on the level of human labour/maintenance required, time of treatment and available financial budget, but the best ways to discover the solution for you is to speak to the professionals who offer these solutions to the market.

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