There are good plant based alternative for maki at least. https://asian-veggies.com/products/vegan-zeastar-sashimi-10-...

For sushi novergian are buildling "in ocean" fish farm that are ridiculously big to tackle problems with fishing and existing fish farms (polution, feed, sea lice etc...)

That thing is ridiculously big -> Ocean Farm 1 https://www.salmar.no/en/offshore-fish-farming-a-new-era/

Another good thing is implementing fish directly in aquaponic farms, where you use water from fish tank as fertilizer in the water loop of you hyroponics. Might work with salmon.... but not on huge scale to meet current demand.