I've seen sturgeon in aquariums -- the Tennessee aquarium in Chattanooga has a pair. Haven't seen any in the wild yet, at least of any size. The Everglades have these prehistoric gar that are very cool and fun to watch. Their scales are square; very, very old configuration. I wouldn't call gar affectionate, though ;)

Could be dumping. There are dead zones in the Rockies where methane and CO2 leaks up from vents and kills off stuff in the area. I'd guess something like that, but the Gulf of Mexico isn't very geologically active.