Scispry

Aliens colonize a planet and people that knows itself only as Terran.

Decades after the Terrans cast off the alien invaders, toxins begin leaking from the aliens' decaying artifacts, poisoning the fisheries upon which the Terrans depend. Thus begins a desperate interplanetary search for new sustainable fisheries.

Science fiction short. Dark/thrilling/deep. Not since Douglas Adams have fish been more important to the future of the human race.