The optimal density in net pens is a critical factor in farm profitability. Getting more fish out of every volume of growing space increases throughput in the same way that increasing a machine’s capacity increases a factory’s productivity. Stocking density can be limited by several factors including restrictions from legislation or certification bodies, density-dependent stress responses, or exceeding the carrying capacity of the environment. Usually, density is limited by the environment. With too many fish in too small of a water volume, oxygen can deplete, waste will accumulate in the sediments, and parasites can transmit too readily.
Open ocean environments offer some valuable characteristics that can dramatically increase a farm’s bottom line by increasing the safe stocking density. Ocean currents are often stronger and do not show the tidal back-and-forth pattern typical of bays or fjords. The stronger water exchange increases the effective oxygen availability and disperses wastes over a wider area which, combined with oligotrophic conditions (low background nutrients), creates a very high potential to assimilate farm wastes into the natural food chain.
These effects are explored using a deposition model (TROPOMOD) and a farm-scale financial model to compare the financial returns of two potential farms, one in a high energy environment, and one in protected waters. The results are supported using two empirical examples from commercial farms and discussed in the context of farm planning.