Cornell is still unsure...parasitic disease is quite unevident in any organs or filets of collected troubled fish. But gill lice are strongly evident... B1 defficiency seems to be their best guess. Several healthy lake erie trib fish have been detained and given same injections as Ontario trib fish that were detained to broaden the experimental results for observation... Google it for specifics

If it is a nutritional disease that is not good...

Nutrition comes from food... But that's just my opinion

Question: are the weakened steelhead that are nutritionally imbalanced more prone to acquire gill lice?

Another thing to consider is that although there were salmon in the river there were far less than usual. These fish usually gorge on eggs and they were just  not there this year,  so more feeding on other stuff and possibly less nutrition overall  from the  lack eggs. I know this year egg patterns were less successful for me than nymphs. I do not think they ( weakened fish) are more prone to get the lice, but they would definitely be more apt to suffer ill consequences because of them.  The lice are just another nail in the coffin..............
 

http://hotspotoutdoors.com/forum/ubbthreads.php/topics/2997824/The_Gill_Lice_Epidemic_Expands
 
On gill lice

I agree about another nail in the coffin clint... But just like a sickened or weakened human or animal... Once weakened its a domino effect... Am I saying gill lice are the problem??? No I am not... Are they latching on to potentiallu diseased weakened steelhead??? yes.... But only my opinion... I would think that the lack of salmon has something to do with it also for the sake of the beating the early run steelhead took from the salmon crowd... Lack of eggs may be effecting it to... Domino effect could be taking place. Why such a lack of salmon though??? Questions with no answers thus far... My recent findings are that biologists are leaning towards nutritional disease for these dying steelhead. That's scary in my book

That is a bad scenario, Andy. It's more system collapse than a specific pathogen . I have noticed that the steelhead I've caught and the ones I've seen are fairly thick and do not appear to be undernourished. The swirlers i saw seemed to be thinner. Just a very limited observation.  Clint, I thought the egg fly non-productivity was just my lousy technique, I fished them way less. Especially since  I didn't see  scattered eggs on the bottom like I have in the past. My best fly has been a bunny flesh fly and larger wooly buggers are  producing for the non-ripping fly guys.  Hopefully, this forum isn't experiencing system collapse. It does have a virus from Ohio that is hard to kill off, but hopefully most will be immune.

According to the Boat survey report that came out yesterday, salmon and trout numbers were up for the year.  The report does mention a drop off in caches from June to mid August, but then notes a return to normalcy for the remainder of August and September.
 
The Gill lice Article that was posted , it should be noted, is still discussing and outbreak in Wisconsin inland brook trout in 2009.
 
 

It will be interesting to see what comes from DEC when they get the lab reports.