I'm a second year grad student in Immunology who's also an avid
recreational salmon fisherman. I have a question that combines those
two interests. That is: why do Pacific salmon, when they reenter fresh
water become infected with all the various nasties that their particular
home streams have to offer. I've come up with two explanations, but I
have problems with both (besides the fact that they're way too simplistic).
1) Upon entering fresh water, Pacific salmon undergo a shutdown of immune
function. This could explain how the adults succumb to massive
infections that they must have successfully resisted as fry. It also
makes a bit of sense in that the adults undergo very dramatic physical
changes as they enter the fresh water - an immune system shutdown might
be one of those. The problem I have with this one is the tremendous
selective pressure against this sort of thing. Fish that wouldn't shut
down their immune systems would have at least an opportunity to get back to
the ocean and make it back upstream another year - like Atlantic salmon
and steelhead.
2) The trip for the fry is easier than the trip for the adults. Fry
literally "go with the flow." Provided that they don't get eaten or
chewed up in the turbines of a hydroelectric dam, they make it down the
river relatively unscathed. The trip upstream for the adults is a little
more difficult. They get dashed up on rocks, cut themselves up, lose their
slime, and get infected with various things. The problem I have with
this one is that ALL the adults die. If it were just a case of getting
beaten up, you'd expect that one end of the bell curve, by sheer luck
would make it upriver none the worse for wear and escape.
So the big questions I need answered:
Are the above ideas completely ludicrous?
Does immune shutdown occur in the adults?
Is death the necessary result of the physiological changes that occur
upon reentry into fresh water and opportunistic infection just a
sidelight?
Do the physiological changes that occur happen in landlocked salmon as
well?
Enough for now, thanks for reading this far.
Jeff