Ok, no one can confirm yet, but if you look at other sudden mass die offs of fish, the cause has been temperature or salinity changes. The idea that they got chased into exhaustion seems a little absurd.

I would posit that they're keeping it quiet because the implications of structural cataclysmic dieoffs could mean disrupting fishing season with catch quotas or even outright cancellation.

However, the fact that they've all washed up on a beach is a bit different to other mass dieoffs, which are recorded by catch numbers at sea. Time will tell what the reason for this one actually was. A similar thing happened in Orkney, Scotland recently, which they put down to natural causes