That's a great question and, obviously, people are very interested in the answer. From the people I've talked to who are doing research on density effects on fish performance, it seems the biggest determiner of density effects is actually indirect, through water quality. So it seems that as long as you can keep the water quality good, you can have quite high densities. Open net pen systems, because you don't have control over the incoming water, tend to be at lower densities. In closed containment the number that seems to recur at a lot of the workshops that we go to is around 40 kilograms per cubic metre. But at the Freshwater Institute, for example, they've been rearing fish at even 80 kilograms per cubic metre and finding really quite good performance.
So it seems that as long as you have good control over the water quality, density per se may not be an issue at those sorts of densities.