It was probably - but not certainly by any means - furunculosis
Is there any significance in it - yes and no.
Yes in that close contact with other fish (ie - spawning)
may see the disease spread.
No in the hope that it was not severely infected. More of a problem in a confined environment (eg Fish farm).
Furunculosis is relatively common in aquaculture and in the wild, I'm digging into the memory banks here a bit but if I recall correctly in the wild varying water temperature and damage to a weakened fish (which in this instance may have been in the river for a while) cause the infection to start up. Quite often a white jelly type spongy infection - nasty
