Nitrogen released. Feed is taken at a conversion ratio of 1.2 and a nitrogen content of 7.2 percent, of which 45 percent leaves as dissolved inorganic nitrogen, the only fraction a seaweed can use. That gives about 39 kg of dissolved N per tonne of salmon produced. Checked against the published basin figure: 1.02 million tonnes of salmon releases roughly 50,600 tonnes of N in total, and these coefficients reproduce it.
The plume. Steady-state advection and dilution. The farm releases into a moving body of water whose cross-section grows with distance, faster sideways than vertically because stratification holds the plume in a layer. Concentration is the release rate divided by current speed times cross-section. The model is calibrated so that concentrations fall back to reference near 500 m, which is what the field studies report.
Copper. Net antifouling is near-field: copper is particle-associated and settles, so it decays over a much shorter length scale than dissolved nitrogen spreads. That difference between the two length scales is the entire siting problem, and it is why an optimum band exists at all rather than "as close as possible".
What this is not calibrated on. No measurement from any specific farm. Every coefficient is stated below and each one is replaced by a fortnight of sampling at a real concession.
| Coefficient | Value used | What replaces it |
|---|---|---|
| feed conversion ratio | 1.2 | The operator's own FCR, which they already track daily |
| feed nitrogen content | 7.2% by mass | The feed supplier's specification |
| DIN fraction of feed N | 45% | Published, and stable across studies |
| lateral plume spreading | 0.10 m per m | A drogue or dye release on site, one tide |
| copper decay length | 150 m | Tissue copper along a transect, one sampling round |
| minimum standoff | 30 m | The operator's mooring plan. Without a floor the band's inner edge reaches the cages on a low-copper net, which is an artifact rather than a siting answer |
| tissue N content | 3.2% DW (kelp), 4.2% (Ulva) | Analysis of the first harvest |
| buyer copper spec | 25 µg/g DW | A buyer specification, not a statute. For the purified excipient a pharmacopoeial number does exist: the Ph. Eur. sodium alginate monograph carries a heavy-metals limit of 20 ppm, governing the extracted product rather than the crop (ledger T20) |
One honest gap. The statutory limits that exist for seaweed are for arsenic, cadmium, lead and mercury, not copper. Copper is modelled here because it is the metal a salmon farm actually adds, through the nets. A real qualification has to carry the statutory four as well, and those come from the site rather than from the farm.