Fish Biology

The gill is the organ around which fish biology and aquaculture engineering meet. It is a very large surface area, only a few cells thick, held in direct contact with the water so that oxygen can diffuse in. That same thinness makes it the route by which ammonia leaves, ions move in or out, and dissolved toxicants enter. A gill cannot be selectively permeable to only the useful things, so anything a farm allows into the water reaches the bloodstream quickly.

The second organising fact is ectothermy. A fish does not maintain its own body temperature; it adopts the water's. Enzyme activity, digestion rate, immune function, growth and oxygen consumption therefore all move with water temperature. This is why temperature appears in every other section of this knowledge base and why a farm's thermal regime is a biological specification rather than a comfort setting.

Operators who know only the rules follow them until conditions fall outside the situation the rules were written for, and then they are guessing. Operators who understand the physiology can reason about a novel situation: why appetite has fallen at an unchanged oxygen reading, why a pH shift matters more than the number suggests, why fish that looked fine at transfer died three days later.

Biology also explains why so many problems are delayed. The stress response mobilises energy quickly and suppresses immune function slowly, so the consequence of a rough handling event appears days afterwards as disease rather than immediately as mortality. Without that model the two events are never connected, and the same handling practice continues.

Respiration and the countercurrent gill

Water passes over the gill lamellae in the opposite direction to blood flow within them. This countercurrent arrangement maintains a diffusion gradient along the whole length of the exchange surface, and it is why fish can extract a large fraction of the oxygen from water despite water holding far less oxygen than air. It also means anything that damages lamellar structure, coats it with mucus, or reduces water flow across it cuts oxygen uptake sharply.

Nitrogen excretion

Fish are ammonotelic: they excrete the nitrogen from protein catabolism mostly as ammonia, directly across the gill, rather than converting it to urea at metabolic cost. This is efficient for the animal and inconvenient for the farm, because ammonia enters the culture water continuously and in proportion to protein fed. In water, total ammonia exists in equilibrium between the ionised ammonium form and the un-ionised ammonia form, and only the latter crosses membranes freely. The position of that equilibrium shifts towards the more toxic un-ionised form as pH and temperature rise, which is why the same total ammonia measurement can mean quite different things in two ponds.

Osmoregulation

Freshwater fish are saltier than their surroundings, so water enters them constantly and salts leak out; they respond by producing large volumes of dilute urine and actively taking up ions at the gill. Marine fish face the reverse and drink seawater, excreting excess salt through specialised chloride cells. Both processes cost energy, and both are disrupted by gill damage, sudden salinity change and stress. This is the physiological basis for gradual acclimatisation and for the use of salt as a supportive treatment in some freshwater situations.

The stress response

Stress in fish follows a recognisable sequence: an immediate catecholamine release that raises heart rate, gill perfusion and blood glucose; a slower cortisol response that mobilises energy reserves; and, if the stressor persists, a tertiary phase in which growth slows, osmoregulation is impaired and immune competence declines. Short, resolved stress is survivable and normal. Chronic stress is what turns a healthy population into a susceptible one.

Both formulas are diagnostic instruments. Oxygen consumption per kilogram rises with temperature, with feeding and with activity, and falls as fish grow larger; watching how it moves over a cycle tells an operator more about the population than any single water reading does.

The most useful practical skill in fish biology is reading behaviour, because behaviour changes before any laboratory result does. Fish gathering at an inflow or at the surface indicates that oxygen at the point they have left is inadequate. Rapid opercular movement indicates respiratory difficulty, whether from low oxygen, gill damage or a raised carbon dioxide concentration. Flashing against surfaces suggests irritation. Loss of appetite is the broadest early signal there is, and it almost always precedes visible disease.

The practical implication is that the interval between the event and the consequence must be held in mind. When investigating a mortality, the question is not only what changed today but what happened to this population over the preceding week.

Carbon dioxide is the consideration most often overlooked. Fish excrete it continuously, it is highly soluble, and in systems with limited gas exchange it accumulates. Elevated dissolved carbon dioxide lowers blood pH, reduces the affinity of haemoglobin for oxygen, and therefore causes respiratory distress even when the dissolved oxygen reading looks satisfactory. It also depresses water pH, which incidentally shifts ammonia towards its less toxic form and can mask a rising nitrogen load. Any intensive system that adds oxygen without stripping carbon dioxide is solving half a problem.

Gas supersaturation is the mirror-image risk. Air entrained on the suction side of a pump, or water warmed after being saturated at a lower temperature, can produce total dissolved gas pressure above equilibrium. Gas then comes out of solution inside the animal, causing bubble formation in fins, eyes and gills. It is an engineering fault with a purely biological expression, and it is invisible in a dissolved oxygen reading alone.

Alkalinity links biology to treatment. Nitrification consumes alkalinity, and as alkalinity falls the water loses its ability to resist pH change. Fish experience the resulting pH instability as an osmoregulatory and respiratory stressor, and the nitrifying community itself performs poorly at low alkalinity, so the load rises at the same moment the treatment weakens.

• Change conditions slowly. Fish tolerate a wide range of temperature, salinity and pH far better than they tolerate rapid movement within that range.
• Treat appetite as the primary biological instrument on the farm, and record intake against offered feed rather than only the amount delivered.
• Withhold feed before handling, so that the digestive tract is empty and oxygen demand is at its lowest during the event.
• Handle in cooler water where the option exists, since oxygen solubility is higher and metabolic demand lower.
• Protect the mucus layer during any handling by using wet, smooth equipment and minimising air exposure.
• Interpret every water quality reading alongside temperature and pH, because both alter what the same number means to the animal.
• Allow a recovery interval after any stressful procedure before applying the next one, rather than combining grading, transport and treatment into one day.

• Record temperature with every other water measurement, since almost every biological rate and chemical equilibrium in the system depends on it.
• Measure dissolved carbon dioxide, or infer it from pH and alkalinity, in any system operated at high density.
• Keep a written behavioural baseline for each unit so that a deviation is recognised as a deviation.
• Design handling procedures around minimising duration and air exposure rather than around convenience.
• Examine gills first in any health investigation, as they carry the earliest evidence of both water quality and parasitic problems.
• Correlate mortality events with the preceding week's operations log, not only with the day's water readings.
• Train staff in the physiological reasoning behind each rule, so that a novel situation is met with judgement rather than with a rule that does not apply.

Aerobic scope is a useful advanced framing. The difference between an animal's maximum sustainable oxygen uptake and the uptake required simply to stay alive is the capacity available for digestion, growth, activity, immune function and recovery from stress. Every environmental burden — elevated temperature, raised carbon dioxide, osmotic mismatch, gill irritation — consumes part of that scope. Growth and immunity are what remain after the compulsory costs are paid, which explains why a fish in marginal conditions can appear healthy while growing poorly and being unusually susceptible to disease.

A second advanced theme is thermal history. Fish acclimate physiologically to the temperature regime they have experienced, adjusting enzyme expression, membrane composition and metabolic rate over days to weeks. A population moved to a new temperature performs differently from one that has been held there, and performance data gathered without recording thermal history is difficult to interpret afterwards.