User blog comment:Unownshipper/Flora and Fauna of Rapture/@comment-92.40.60.43-20131019204004/@comment-6024608-20131020031447

The problem with sea life is that it moves about and most of it is near the surface (in deep ocean areas) where sunlight lets algae/plankton grow. Thats why Rapture has fishing fleet submarines to chase them. The mammals are too rare or usualy exist at coastlines.

Since The lighthouse Island breaks the surface (an Rapture is supposed to be built on the side of a old volcano (sea mount) then there would be some very shallow spots for kelp farms using sunlight

20000 people (Novel number) is still alot for just seafood and with abundant power from Geothermal to keep warm and enough light  (and cheap space like tunnels in the seabed)  then all kinds of large farms could exist (hydroponics can produce many time sthe yield as conventional cropland and is not limited to being seasonal).