Increased mortality using warm water delousing

It is the conclusion in an academic article, “Fish welfare has to be secured when removing lice,”  in The Norwegian Veterinary Journal no. 3/2018, that better documentation for delousing using warm water, so-called thermal delousing, is necessary in order to find out how many fish are harmed or die.

The use of warm water to remove lice has increased dramatically in a relatively short period of time. Lice does not tolerate warm water as well as salmon. Because the lice is insignificantly in size compared to salmon, it will warm up to damaging temperatures and let go from the salmon.

Figures shows extensive use of thermal delousing in 2017. While the number of thermal treatments of salmon was just three in 2014, it increased to 1,308 in 2017.

Professor Emeritus Trygve T. Poppe at the Norwegian Veterinary Institute, and working for Pharmaq Analytiq, is one of the authors of the article, says that the mortality rate increased after thermal treatments.

Thermal treatment, where salmon is held in water at 28-34 C for 30 seconds, is currently the most common for none medial treatment of salmon and rainbow trout. The article “Termisk behandling av salmon” (Thermal Treatment of Salmon – available in Norwegian only) document that there is serious damage to the fish after thermal delousing. In addition, it is also detected damage to organs which have not been investigated earlier related to delousing methods. This includes organs such as nose, thymus (gland) and brain. The likelihood is high that thermal delousing is painful for the fish according to studies referenced in the article.

The material that was investigated came primarily from sick fish and therefore may not reflect the average occurrence of damage. Details about the execution of each individual treatment, as well as the time between the thermal treatment and the test samplings are often not known. It also includes how may thermal treatments the fish has been through. This makes assumptions about causalities uncertain.

Trygve T. Poppe states that due to the above, it is not possible to use the original documentation for thermal delousing of salmon to say that the fish welfare has been safeguarded.

For further information, contact:  Trygve T. Poppe,


By Tor-Eddie Fossbakk