I am not sure if I can answer your question completly because of some
things that make absolutes in this case difficult. There are a few
possibilities that I will just describe briefly.
Osmotic shock reduction/reduced energy expenditure. Doubtful that this is
the means that helps the fish fight disease, although it would be
important in handling shock. It is however possible that the gill flukes
are induced to let go of the gills through sheer osmotic shock and
stress, the fish being larger would take longer to reach the level of
stress that a gill fluke would be.
Salt stimulates the production of the slime coat. This may lead to
sloughing of the existing one and the removal of a good percentage of the
offending microbes. Assuming then that the water quality etc improved
this would give the fish a good chance to get rid of the infection.
It has been shown that salt can be an immunostimulant in fish (I can not
remember the reference off hand for this one so it is not an absolute
ok). This would help the fish fight off secondary non pathogenic
infections and this would then help the fish to heal the primary
infection. It is not much of a help against true pathogens because they
are by definition able to cope with the hosts immune system.
Salt itself can be toxic for a number of reasons. Each cell in our bodies
spends a great deal of its energy (somewhere in the order of 40%) getting
rid of the Na that makes its way accross the cell membrane. Adding more
Na to the water makes the concentration gradient greater and thus means
that cells have to spend even more energy getting rid of the stuff. This
can lead to the death of individual organisms unable to cope with the
energy drain. This is probably what gets most of the pathogens that we
are talking about, most single celled organisms are not so tolerant of
high salt loads because of the primary structure of their plasma
membranes. The scales of a fish represent some protection from this,
where they have problems is in the gills.
I think that the important thing is that it does work for some things and
where it does work it is a good thing to do. Adrians post shows a good
general plan of attack and what to do and in this case I think that how
the salt kills is not really able to be disected from some of the other
things that could be going on. Mode of action toxicology is not so much
fun to do because there are so many effects of some compounds that the
primary effect gets lost in all the other things that go on.
Peter Hughes
ANGFA ACT