Gormaganders (xenologic classification: Federation species 283732) were a non-atmospheric organism capable of living in the vacuum of space. Not technically a fish, but described as essentially a "space whale", as of 2256, they were on the Endangered Species List.
Although this species was a target for hunting, their numbers dwindled due to other reasons. Gormaganders spent their lives feeding off alpha particles in solar winds. Often so consumed by this task, they ignored all their instincts, including reproduction.
Run-ins with this species were considered extremely rare, with only fifty-seven near-miss encounters by starships recorded between 2246 and 2256. In late 2256, the USS Discovery came across a female gormagander which appeared to be injured. In an effort to aid the creature and transport it to a xenologic facility, as was required by the Endangered Species Act, the crew unknowingly transported aboard a saboteur, Harcourt Fenton Mudd, who was hiding inside the creature and who planned on taking the ship for himself. (DIS: "Magic to Make the Sanest Man Go Mad")
Some species consumed the meat of gormaganders. Skewers of gormagander meat were available at the Orion embassy outpost on Qo'noS. In 2257, Sylvia Tilly ate some and enjoyed the taste, though she spat it out upon being told what it was. (DIS: "Will You Take My Hand?")