Metabolically, animals are pretty inefficient, and each trophic level (layer of the food web) only keeps about 10% of its caloric intake as edible flesh for the next layer (
https://en.wikipedia.org/wiki/Ecological_efficiency#Ten_perc...).
So if algae feed smaller fish, the smaller fish feed salmon, and the salmon feed humans, to double our salmon, we'd need 20 units of smaller fish and 200 units of algae (edit: oops, math is hard).
That's a lot. Climate change can increase algae productivity, but not by that much unless we want to barbecue the world. Of course there are other avenues (like simply harvesting more of it, or growing crops in areas of land/ocean not suitable for other uses). But there are always knock-on effects too, like algae blooms can be poisonous or deplete nutrients in an area.
The on-land agricultural revolution and improved/modified genetics gave us access to much more edible food, but a lot of that is only 1-2 trophic levels (we eat grains directly or feed grains to livestock that we then eat). The ocean tends to have a lot more layers, and is also not as easy for us to till and fertilize, and not all of it is equal... much of the edible parts are concentrated in certain areas, with a lot of empty water in between.
In general we probably shouldn't be hunting apex predators in the ocean anymore than we hunt them on land. It took 'em millions of years of evolution to reach a relatively stable carrying capacity, but we can drive them to extinction in a few years.
But, hell, realistically there's probably no stopping any of this. One day our kids will read about whales alongside dodos in the history books =/