This is how ROVs contribute to safeguarding fish health and equipment during delousing operations

There are essentially five different delousing methods in use in Norway

Salmon lice are monitored and counted weekly at the fish farm. Daily measures are also taken against salmon lice, such as cleaner fish, lice lasers and lice skirts. During certain periods of the year, however, the lice thrive better and reproduce faster, and the fish farmers then have to resort to more extensive methods where the fish are handled to keep the salmon lice away.

The most common delousing operations today are:

• Thermal delousing
• Mechanical delousing
• Freshwater treatment in a well-boat
• Freshwater lid in a pen with a skirt
• Chemical delousing

We use the Blueye ROV to follow up and check on the farmed fish. The underwater drone makes it possible to check fish welfare and fouling on the nets, and to keep a general eye on what is happening in the pen. -Johannes Schrøder, Salmar

Thermal delousing

Thermal delousing is the most common delousing method according to FHF (the Norwegian Seafood Research Fund), where salmon are exposed to seawater at 28-34 °C for approximately 20-30 seconds on board a vessel or barge (fhf.no). The lice are removed by rapidly heating the mobile lice so that their muscles are paralysed; the lice lose their ability to cling to the skin surface and fall off. The exposure to warm seawater must be so brief that the fish do not heat up, so that the thermal effect works primarily on the lice (fhf.no).

Mechanical delousing

Three commercial methods are mainly used for mechanical delousing: SkaMik, the FLS delousing system and Hydrolicer (fhf.no). With SkaMik, for example, the fish are pumped on board a vessel (such as a well-boat) and sieved off before passing through a flushing chamber. The fish are then passed through a chamber with rotating brushes before being returned to the pen.

Freshwater treatment in a well-boat

Today, a lot of farmed fish are exposed to fresh water in well-boats for delousing. This treatment usually lasts four to eight hours before the fish are sieved off and returned to the pen.

The method is used to disrupt the osmotic balance of the salmon lice, which are essentially iso-osmotic with their surroundings. This causes paralysis and eventually death for the lice (fhf.no).

Freshwater lid in a pen with a skirt

The aim of this method is to control parasites using fresh water, in the same way as freshwater treatment in a well-boat (fhf.no). The freshwater bath takes place directly in the pen: fresh water is pumped into a pen enclosed by a watertight skirt. The fresh water naturally drifts to the surface and forms a "lid". The method involves no medicines or chemicals, and it also reduces the handling of the fish as no pumping is needed.

Chemical delousing

A final form of delousing, used to a lesser extent today, is chemical delousing. A good deal of research has been done on the environmental effects of the chemicals, and research carried out by NORCE, Akvaplan-niva and the Institute of Marine Research, among others, has shown that chemical lice treatments (bath treatments) also affect species other than salmon lice (akvaplan.niva.no). Delousing agents have, for instance, been shown to cause mortality among shrimp.

In Norway, medicinal treatment is carried out in two ways: by bath treatment with the lice agent, and by adding the lice agent to the feed. After chemical treatment of the salmon, the bath water is discharged into the sea, while lice agents from feed settle on the seabed together with surplus feed.

This is how you ensure optimal use of the ROV during delousing operations

According to a major study carried out by SINTEF Ocean, holes in the net as a result of operations, storms and the like were the cause of escaped salmon in as many as 75% of the escape incidents in the period 2010-2018 (SINTEF, Knowledge and methods to prevent escapes, 2019). One of the five measures proposed in the report is paying close attention during crowding and delousing operations, which is important for preventing escapes.

An ROV can be an important contribution to preventing damage to nets. During operations such as delousing, it is therefore important to carry out a visual inspection of the facility afterwards, precisely to uncover any damage that could lead to fish escaping.

With ROVs, you can observe the behaviour of the fish during and after delousing, and when they are released back into the pen. You can also observe how many fish die and sink to the bottom of the net during and after the delousing operation.

Underwater ROVs are also used to assess behaviour and fish health before a delousing operation, and to check that any cleaner fish have been removed from the pen before the delousing operation starts.

In conversations with various players in the aquaculture industry, we have put together these tips for optimal use of ROVs during delousing operations:

Preparation:

• Check that all the equipment, including the phone or tablet, is fully charged, that the software on the phone and the drone is up to date, and that there is free storage space on the ROV.

• Plan which dives you will do, where and when. How long do you expect to spend? Where should the ROV be in the water for the best overview of the delousing process? How will you avoid obstacles and hazards such as float lines, crowding nets, pumps and return hoses? And finally, find a safe place to pilot the ROV from.

• Agree with a colleague to help keep an eye on the different screens, especially if more than one camera is used on the ROV. This spreads the inspection work and minimises the risk of overlooking damage.

• Check that the ROV and the equipment work as expected. Do you need to recalibrate the compass?

• Inspect the pen and the equipment in the water before delousing starts, to verify that everything looks fine and that any cleaner fish have been removed. Here it is particularly important to look for foreign objects that could get caught in the delousing equipment during crowding, for example, and make holes in the net.

During delousing:

• Note which side you pass obstacles such as ropes on, so you can return more easily without risking tangling the tether, and keep track of equipment that is moved around inside the pen during the operation.

• Take short video clips and photos for reference. Short video clips are easier to process afterwards for reporting.

• Note how the fish behave during delousing and keep an eye on how the various equipment sits in the pen. Do you see any deviations, or can the operation continue as normal? Are the fish harmed by the delousing, or are they behaving normally? Video footage is easy to copy or delete after the operation, so remember to record if you see behaviour that can help you assess the situation better later on, or that could be used to train others.

After delousing:

• Carry out a new round of inspections of the net and other equipment in the pen, with a particular focus on areas that may have been in contact with boats or barges and where heavy equipment has been used. Check that you have enough battery, and swap in a fresh one if necessary before you start on a new pen. As holes in nets often appear after major operations such as delousing, this part is particularly important.
• Rinse the ROV and tether in fresh water and disinfect if necessary. This extends the life of the product. We recommend submerging the ROV in clean fresh water and running all the thrusters for 20-30 seconds to flush salt water and particles out of the water-cooled motors.
• Log the dive and upload the media files to your dive report.
• Put all the equipment back on charge so it is ready for new assignments at short notice.

Would you like to learn more about using ROVs in fish farming? Feel free to contact us using the contact form below.

SINTEF Ocean, Kunnskap og metoder for å forebygge rømming, 2019.