It's usually a case of mass suffocation - too many fish crammed into a cove and too little oxygen. Happened once with menhaden while I was living in Maine - it was the middle of the tourist season and the town (I can't remember which one) stunk so bad that people were packing up and leaving. The merchants were tearing their hair out and begging the state to haul the rotting fish away. I've also seen it happen in RI back in the days when we still had bunker. Don't have to worry about that any more.
