Fish (cod and salmon) spawn in waterfalls located underground, specifically at levels below y=0, but possibly others as well. They then sometimes fall from the waterfall, killing themselves on the cave floor far below. In this one cave alone, 4 waterfall streams generate about 5 dead fish per minute, per waterfall, for a total of just under 20 fish per minute.
Note: the biome is listed on the f3 screen as "Cold Ocean," however the ocean in question is located well above the ceiling of the cave, and is not leaking into it at any location.