QTL for lice susceptibility

Grey-DNA-fish-LICEUpdated documentation, per March 2019


Salmon lice (Lepeophtheirus salmonis) is the most resource consuming single problem for the aquaculture industry today.  Besides the injuries lice inflict salmon, handling in connection with counting and treatment against lice has become a serious health risk to the salmon.  Many different instruments must be used in the fight against this parasite, and genetic selection is one of the few preventive methods that reduce the fish’s susceptibility to lice throughout the production cycle.  QTL-innOva® LICE will be an important contribution in combination with the use of cleaner fish, lice skirts and other non-drug measures.  Starting with the 2016/2017 egg season, all AquaGen products for salmon will contain QTL-innOva® LICE.

Results from lice challenge tests

Figure 1. Challenge with lice in tanks where fish with 1 copy of the lice susceptible QTL had more lice than the average for all the fish in the group.

Based on genetic testing of 4,000 lice-challenged fish, AquaGen has found a QTL for lice susceptibility (QTL-innOva® LICE).  This QTL is over-represented in fish with high levels of lice, so-called “lice-attractors “. Through infection experiments with lice we have repeatedly seen that fish with this marker for lice susceptibility have at least 50% more lice than Fish

that lack this marker (Figure 1).  By genotyping brood fish and remove fish with the undesired QTL from egg production, we are getting a lower proportion of particularly susceptible fish in the cages.

Understanding the significance of the QTL for lice susceptibility, in typical farming conditions

AquaGen has since the discovery of the QTL for lice susceptibility worked to obtain data from fish farms to investigate the effect of the QTL under commercial farming conditions.  Based on a field material comprising 625 fish from 11 different locations in central and western Norway, it was documented that fish with the unfavorable QTL had higher levels of lice also under commercial field conditions.  Fish with 1 copy of the adverse marker had on average 11% more lice than fish without the marker, while fish with 2 copies had on average 28% more lice compared to fish without the marker for lice susceptibility (Figure 2).

Considerable variation in the frequency of the gene for lice susceptibility in farmed populations

Figure 2. Results from the field validation of QTL for lice susceptibility. Amount of lice for fish that had the favorable marker (QQ) is set to 100%. When the 11 locations were evaluated together, we found that fish with 1 copy of the adverse marker had 11% more lice, while fish that had 2 copies (qq) of the adverse marker had 28% more lice.

An extension of this field survey showed that the number of fish that had the gene for lice susceptibility varied a lot, and the marker was detected up to 37% of surveyed fish from a population.  It is clear that such groups will be predisposed to serious lice problems and that it is very important to reduce the proportion of the unfavorable gene variant.  As an example we can mention a location where fish with the marker on average had 1.9 lice, while fish without the marker had 0.8 lice on average.

Eggs from brood fish without the adverse marker was delivered to hatcheries the first time in 2015.

Products with lice reducing effect:

  • AquaGen® Atlantic QTL-innOva® PRIME
  • AquaGen® Atlantic QTL-innOva® SHIELD
  • AquaGen® Atlantic GEN-innOva GAIN

Benefits of QTL-innOva® LICE:

  • Lice susceptible brood fish will not be used in egg production
  • When “lice attractors” are taken out, the infection pressure is further reduced in the population