Mass die-offs of fish are a frighteningly common occurrence. My understanding is that the most common cause is related to algae blooms; The algea either releases toxins that are deadly to the fish, or uses up all the free oxygen in the water suffocating the fish. I obviously don't know if that's the case here, but given my very limited understanding of how radioactivity works, and the situation at Fukushima, I think it's worth considering causes other than fish being killed directly by radioactive material in the water.

Algal blooms, with resulting toxic areas or areas with no oxygen, have been on the rise, exacerbated by warmer waters due to global warming.