Yeah, you are right, though adding some seaweed to the fish can help a lot! And if you have seaweed you can eat some kinds (more than most people might think), and given enough seaweed and rock to spread it on and at least a few months of warmish weather, you can grow vegetables. My gran used to grow amazing crops of healthy potatoes in beds made by piling seaweed a couple of feet deep, weighting it with rocks until it began to rot down, planting seed potatoes into the first layer of seaweed and when they sprouted continuing to pile seaweed on top, heaping itdeeper as the haulms of the potatoes began to poke through the surface. In Autumn you harvested your spuds by peeling back the layers - they came out white and clean, firm and delicious, with never a trace of blight.
The next year you planted leaf vegetables, then turnips or beets, and eventually barley. Also, a lot of common weeds are edible……