Tale of fish behavior

This is a question for you guys in Alaska.  We were surveying for a bridge over a small inlet coming into Sarkar Lake on Prince of Wales Island.  (Google Maps now shows "N. Island Rd." crossing over the spot we were surveying.)  It was a calm quiet sunny day, no wind, and the water was flowing calmly.  I was busy taking measurements when I heard a distant noise.  I looked down the inlet and was stunned.  About 100-150 yards downstream, it looked like somebody had dropped a giant Alka-Seltzer tablet into the water.  It was thrashing and boiling but far enough away I couldn't see the cause.  It slowly came closer until it was on us and it was a huge school of salmon that looked like they had gone crazy.  They were leaping and splashing in every direction and so densely packed that we didn't think we could even cast a lure and retrieve it.  I had heard that salmon will thrash when entering fresh water to get rid of sea lice but didn't know if that was true.  My bud and I decided to give flies a try so we tied on some big red shrimpy looking flies and cast those in all directions, which the fish completely ignored.  Finally, the whole school passed by and entered the lake.  A couple days later we took our boat up to one of the inflowing streams and the salmon were stacked up.  My question is, why were the fish acting like this?  Is it really to get rid of sea lice or other parasites? 

I bet they saw you and were like... hey guys, let's play a trick on those humans over yonder!   

No idea if the lice theory holds, but it sure would have been cool to see in person.