I could add just a further point on this. It's partly related to the biological parameters that the fish need to grow. In closed containment systems, you have much more control over the parameters in which you raise your fish. To be able to stock fish at 50 kilograms per cubic metre, you need to have much more control over your system in terms of being able to remove the solids and the dissolved wastes. You also need to have systems to introduce oxygen into the water to maintain fish at those densities.
Alistair is right. From an economic perspective, you need to have that density for it to be economically viable. But to have those densities, you need to have the technology to support growing fish at those densities. In the wild, the natural tides and currents flush the water in and out of the cage and provide oxygen to the fish—and that is the density they're able to maintain at in a wild environment.