The methane explosion postulated to have whiped out 80% of life 250 million years ago by Gregory Ryskin in Feb 2000 corresponded with a sharp increase in global atmospheric temperature. Our temperature is rising gradually, so I don't think this is the case. It could very well be that fish are being killed by smaller, less catastrophic methane explosions, however they would have to be triggered by some sort of high energy impact or heat release. Since these pockets rest at the bottom of the ocean, a very large meteor, earthquake, or volcanic eruption would need to take place to ignite these pockets of gas. Our global seismic instruments are very sensitive and would likely pick up something large enough to cause a significant fish kill. It's my belief that anthropogenic influences pose a far greater threat to the survival of the oceans fishes and all life on earth than the methane deposition cycle punctuated by cataclysmic explosions that were somehow undetected by any human instruments.