There are no specific studies that I can provide now off the top of my head, but certainly I would refer you to some of the standards I mentioned. The Royal Society for the Prevention of Cruelty to Animals and Compassion in World Farming both have standards that deal with fish farming. They reference a number of projects and where they've come to those numbers of 17 to 22. That's what they consider their maximum. They also deal with a large number of other welfare parameters that can be measured.
On December 1st, 2011. See this statement in context.