There are two ways to produce seafood: catch fish in the wild or farm your own. Seafood farming is often called “aquaculture”. Aquaculture is dominated by the farming of fish, but also includes other organisms, such as crustaceans and aquatic plants.
From other sources, one third of aquaculture (in weight) is algae.
Is algae for fish food or is it something I didn’t know it was in?
The Nori you eat with sushi is farmed, but quite a lot of aquafulture goes to the production of Carrageenan. That is used in tons for things from milk and meat products, to sex lubes, and toothpaste.