4. Discussion
Most farmed salmonids sink to the bottomof the cage when they die,
and these fish will be removed during daily husbandry. In some cases,
gases will build up in the cavities of the fish as it decomposes, and the
fish will float. In other cases the dead fish will “hang on” the cage nets,
because the net is not stretched tight enough and/or the currents are
unfavourable. If the weather conditions are too hard it is not always
possible to perform the daily dead fish removal procedure. During a
disease outbreak with high mortalities it can also be a challenge to
remove all dead fish. In this study we have assessed the infection risk
that these dead fish may represent to the remaining fish at the site.
In Norway, mortality removal is in most cases often done after two
principles; LiftUp® systems or dead fish landing nets. The LiftUp® is
an automatised system that collects the dead fish at the bottom of the
cage using compressed air in order to pump the fish up in a tube, outside
the cage, and into a container placed at the farming construction. This
container contains a sieve where the dead fish remain and from which
wastewater is removed by a tube and led to deep water. The landing
net is an open system inside the cage, where the dead fish are lifted
from the bottom, through the fish cage, manually collected at the surface
and brought into appropriate containers. The dead fish that do
not sink to the bottomwill not be collected in any of these two systems.
In Chile, removal of dead fish is most often carried out by divers who
swim inside the cages and pick the dead fish manually in to landing
nets. After the collection of dead fish, the material is brought to the surface
and placed in tanks on a separate platform located in proximity to
the farming site. In both countries the dead fish are kept are in tanks
without runoff. The dead fish material is then minced and ensiled to a
pH less than 4 for inactivation. This routine should be sufficient to inactivate
the ISA virus (Falk et al., 1997), but to our knowledge no one has
examined how long infectious virus particles may be retrieved from