Qeya River subsistence

Veteran wandered over to the bank as the stranger emerged from the shallows. "What's roe?" he questioned. He followed his question immediately with, "Can I see?" The stranger seemed friendly enough. Of course the budding Guardian wouldn't drop his guard completely, but some of his mom's lessons on ambasssadorship were getting through too. He knew that he wanted to portray his pack as strong and well-defended, but not so much so that he chased away potential joiners. 

"I don't think anyone fishes much in my pack, Duskfire Glacier. Fish don't keep well in the caches," he explained. "But they make a pretty good snack, huh? There's a lake in our territory-- it feeds this river upstream. I think that's where the fishes are going." But why were all the fish trying to get upstream to the lake? What were they running away from? What purpose did this frenzied fish-stampede serve?