Livestock & aquaculture
Aquaculture and fish farming
The physiology and water chemistry that govern aquaculture — why poikilothermy gives fish their feed-conversion advantage, why dissolved oxygen and ammonia set stocking density, and how the nitrification loop and the two siting externalities work.
Farmed fish convert feed into body mass better than any farmed terrestrial vertebrate, and the reason is thermodynamic rather than genetic. A poikilotherm spends nothing on maintaining body temperature against ambient, and a neutrally buoyant animal in water spends little on postural support. Both terms are large in a pig or a broiler and near zero in a salmon, so more of the ingested energy is available for growth. Nitrogenous waste reinforces the advantage: a fish excretes ammonia straight across the gill rather than paying the metabolic cost of making urea or uric acid.
Oxygen sets the ceiling
Everything the fish saves is spent back against the medium. Water holds far less oxygen than air — a few milligrams per litre against roughly 280 in the same volume of air — and oxygen solubility falls as temperature rises, at exactly the point where the fish’s metabolic demand is rising with it. That inverse pairing is the central constraint of the industry: stocking density is not a husbandry preference but a statement about how much oxygen can be delivered and how fast. It explains why warmwater species farmed near their thermal optimum sit closer to their limit than coldwater salmonids, and why aeration or oxygenation is the first capital item in any intensification.
Ammonia and the nitrification loop
In a recirculating system the second constraint arrives quickly. Excreted nitrogen enters the water as total ammonia, which exists in equilibrium between ionised NH4+ and un-ionised NH3. Only the un-ionised fraction crosses the gill readily and it is the toxic species; that fraction rises steeply with pH and with temperature, so the same total ammonia concentration is harmless at pH 7 and damaging at pH 8. A recirculating farm therefore runs a biofilter: chemoautotrophic bacteria oxidise ammonia to nitrite and nitrite to nitrate, an obligate two-step in which the intermediate is itself toxic — nitrite crosses the gill and oxidises haemoglobin to methaemoglobin. A biofilter that is immature or has been chemically knocked back stalls at nitrite, which is why start-up and post-treatment periods are the dangerous ones. Nitrate accumulates and is removed by water exchange or denitrification.
What decides siting
Two externalities, not feed cost, generally decide where a farm can be. Sea lice are ectoparasitic copepods whose free-swimming stages disperse on currents; a dense farmed population raises local infective pressure on wild salmonids, and successive chemical treatments have selected resistance, which is what pushed the industry towards cleaner fish, freshwater and thermal delousing, and physical barriers. Effluent is the other: dissolved nitrogen and phosphorus plus settling organic solids from faeces and uneaten feed, which under a net pen consume oxygen in the sediment and change the benthic community. Both scale with local flushing rather than with production alone, so the same tonnage is benign in one fjord and not in another.
Land-based recirculation internalises both — and substitutes a hard energy and biosecurity problem, since a closed system’s own microbial community is now the farmer’s responsibility.