Build ships which vaccuum up jellyfish, puree them, and use the proteins as feed stock for 3D printing of food. The stingers can get filtered out, or just left into the low-grade product used in prisons and orphanages.
I'm sure that Red Lobster can come up with some clever marketing term for this stuff. After the actual lobsters, cod, and king crabs die off they'll have plenty of motivation.
Interesting Geek-culture historical note: In the 1973 movie "Soylent Green," the titular product is supposed to be made from krill scooped from the oceans. The underlying horror of the movie isn't that the crackers are made of dead people, but that the ocean ecosystem has collapsed due to pollution. The movie also has Edward G. Robinson bitching about how the greenhouse effect has made it hot and damp year-round.