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If sea lice will be a challenge for fish farmers forever, can we learn how to live with it?

-  It has become much more difficult to fight salmon lice because of resistance to the medicines we use, says professor Tor Einar Horsberg. Picture: Tellef Øgrim.

-  It has become much more difficult to fight salmon lice because of resistance to the medicines we use, says professor Tor Einar Horsberg. Picture: Tellef Øgrim.

The billion dollar question i salmon farming is to develop a sustainable sea lice solution. Resistance in the louse against medical treatments is the biggest challenge.

Tellef Øgrim

Tellef Øgrim

Tor Einar Horsberg has worked on fish diseases and their treatment since 1986. At present, his main focus is on the development of resistance in sea lice against medical treatments.

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We are never going to get rid of lice in fish farming.

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The grave status of the spread of salmon (or sea) louse resistance in the Norwegian fish farming industry was soberly, and gravely, described in the annual report of "The surveillance programme for resistance to chemotherapeutants in salmon lice", published in March 2014: "Reduced sensitivity and resistance to the medicines tested in biological assessment are generally widespread along the coast."
The surveillance programme aims to summarize the use of various chemotherapeutants in salmon farming and to describe the resistance status against the most important of these chemotherapeutants in L. salmonis in Norway.

Professor Tor Einar Horsberg research the sea lice. Picture: Tellef Øgrim.

Professor Tor Einar Horsberg research the sea lice. Picture: Tellef Øgrim.

Loss of goodwill and income

The threat that resistant sea lice represents to the aquatic industry cannot be underestimated. But severe challenges to goodwill and income can also fuel entrepreneurship and inventive research. The proposals for groundbreaking genetic solutions, as well as more mechanically oriented measures, abound. But one thing stands; the problem with salmon lice will not disappear.

- We are never going to get rid of lice in fish farming, says professor Horsberg.

Not only is resistance to medicines "generally widespread along the coast". The analyses that Horsberg and his research fellows have conducted show that "In many places around the country we have full resistance in lice against all the different medicines in use".

- It has become much more difficult to fight salmon lice because of resistance to the medicines we use. 

The rate of resistance varies of course from medicine to medicine. But for the majority of the pharmaceuticals in use, the situation is what Horsberg calls "relatively serious".

 
- For instance, there is more than 50 percent resistance in lice against the medicines Azamethiphos, Salmosan, Alpha Max and Betamax vet.

One area where the resistance report, published in March 2015, showed a comparably sensitive salmon lice populations was in the county of Finnmark, in the far north of Norway. Although, also here reduced sensitivity to different chemotherapeutants exists.

The bottle test

The test being used to find out if lice are resistant or not was developed by Horsberg and his colleagues at the Norwegian Veterinary Institute around 2012. It is a straight forward method, suitable for testing in the field.  Lice are put on bottles containing certain medicines in certain amounts. The number of lice still clinging to the inner wall of the bottle indicates the level of resistance.

The regulations say that when more than one grown up female louse is found for every other fish, the farm has to take action. There is no rule saying which means the farm shall take to reduce the number of lice. When a fish farmer is in a situation where he has to take action, the natural thing to do is either to combine different medicines or to increase the amount of the medicine used. 

- Higher dosages contribute directly to the level of resistance. It is not forbidden to increase the dosage, even if there is no doubt that it leads to a downward spiral. Everyone knows that it is an unhealthy method. But if the alternative is to slaughter all the fish, maybe before they have reached the normal age for slaughter, the choice of using more medicine can become irresistible, says Horsberg.

Non-medical newcomers

With this sad development as backdrop, the growing list of inventive alternatives to medicine becomes a natural development. The list contains such measures as closing the lice out by the use of a tight mesh,  lightening systems that force the fish to stay deeper in the water, a laser that removes lice rinsing the fish in warmer water, to using other fish, often lumpsuckers, to clean lice from the salmon.

- If the lice problem will not go away, how can the fish farming industry learn to live with it?

- Both in fish and human health we have seen many times how a decease has been fought back and brought under control, even if it has not disappeared completely. I think that to achieve this in fighting lice, the farms must prepare for some adjustments in how they conduct their operations. For instance, if you manage to feed the fish deeper in the water, thereby preventing the fish from coming up for food, you will reduce the lice threat considerably. We should also adjust the feed itself so that the fish will taste bad for the lice.

Sea Lice

Horsberg list of measures also includes developing a salmon species that attracts fewer lice, as well as some of the non-chemical methods listed above. One he mentioned is the mesh keeping the lice out off the farm. 

- An alternative method is to administer common fallow periods in areas suffering from lice. After two months without fish to feed on, the sea lice dies.

In other words, a combination of measures can, according to the experiences veterinary scientist, learn the industry to live with the lice threat.

A more dynamic regime?

- What can Norwegian authorities do to improve the current regime for lice prevention?

- They should take another look at the flat limit of 0.5 grown, female louse per fish. This rule does not take into consideration that different areas differ with regards to for instance the salt-grade in the water. In a fjord where there is less salt in the water, the lice population will never grow as in other areas. Regardless of such variations, a test that reveals more lice on the fish than the limit will often result in more medication, regardless of how nessecary this is, the salt-grade taken into consideration.

 
Areas where the distance between the farms is short and the collected bio-mass is high; lice can take of like a tsunami.
A more diversified lice limit is the way to go, according to Horsberg. The rule that actions always have to be taken when more lice than that are registered, can drive resistance since the non-resistant lice will die and resitant will live.

Horsberg har earlier pointed to how important the Patogen-lab in Ålesund is in providing information about the genetic resistance profile in a lice population at the farm level. 

These new genetic analyses make it possible administer the use of medicine at farm-level. 

The area where the resistance report shows a comparably sensitive salmon lice populations was in the county of Finnmark, in the far north of Norway. Although, also here reduced sensitivity to different chemotherapeutants exists.