El Nino tends to send a lot of that stuff into Texas. So maybe it's not so bad after all. Just because El Nino is sitting out there off the coast of South America, and fish-killing freezes tend not to happen when it's there, that doesn't mean the Texas coast is bulletproof against such a catastrophe this winter. It could happen. But we do understand how they kill fish.
Cold weather kills Texas' inshore fish by suffocation - they die from lack of oxygen.
Fish breathe by extracting oxygen from water via their gills. The colder the water, the harder that is for them to accomplish. At some point, the situation reaches critical mass. The fish's metabolism slows to a point where it can't extract oxygen and thus can't breathe. The results are fatal.