The folks I met were really cool about allowing me to shoot video, explaining their actions as part of a broader campaign called the Danube Study Path. The effort seeks to inspire interest in the river's changing social and natural ecosystem. I guess the idea here is that you see a sturgeon on the sidewalk and wonder why you rarely see them in the water.
I'm not entirely sure whether spray-painting sturgeon will produce any results (or whether this was a "sanctioned" part of the Study Path), but I appreciated the artists' friendly tolerance of a stranger shooting video. Oh, and given the weird lighting conditions and my choice to use a relatively cheap camera, don't be surprised that the footage suffers from excess noise. Social activism isn't always pretty.