I don't find myself worrying too much about "how they evolved". Tiny, nervous fish with exceptionally hard scales and skulls. Makes sense to me! And tasty roe. All is in order here.
These fish can be all sorts of fun use as an encounter on their own (with the cultural tid-bits thrown in), or as a surprise addition to some other aquatic melee. The PCs could be fighting something else, when suddenly a school of panicked skirrin bum-rush the boat, complicating matters. Also like Val's flying skirrin idea.
Tundra, if you ever read this comment, I'd like to see the Akaat written up too!Go to Comment
There's a large draw to this submission. A vastid-contaminated water source would be a perfect test for those of druidic or clerical sensibilities, and could be a coveted research location for a wizard or scholar. My only suggestions are to clean up the formatting (as PoisonAlchemist suggested) and to add some plot hooks at the end to help a GM immediately throw this into his/her game as a side quest. That's would just be icing though -- well done!