Pen nets are having objections in many countries as are blamed for contamination, spread and infection of diseases (from farmed to wild species), escapes threatening other aquatic fish and seafood, etc.
Under this frame the RAS and HFS are presented as solutions, or part of the solution. While RAS (Recycling Aquaculture System) uses a close circuit of water (treated and cleaned from time to time) the HFS (Hybrid Flow-through System) uses subsurface, cold water to pump it to maintain the flow in the ponds, and then return it to the ocean. Both have good conversion rates (1.1 they claim) and lower mortality (than traditional pen nets), but both need big investments and construction job.
Today the first ones are already under production, and apparently could be an alternative to reduce the pen nets in certain areas where authorities are imposing restrictions for salmon, trout and other species.