Fish Biology

For engineering purposes, a fish is a temperature-dependent converter. It takes in feed and dissolved oxygen and produces new tissue, ammonia, carbon dioxide and faecal solids. Its rates are governed by water temperature, because fish do not regulate their own body temperature, and by size, activity and feeding state.

The design loads in a recirculating system are biological outputs. Oxygen demand, ammonia production, solids production and carbon dioxide production all come from the same physiology, which means they rise and fall together and cannot be treated as independent design cases.

• Temperature governs metabolic rate, so it governs every load in the system simultaneously.
• Feeding drives a post-prandial rise in oxygen demand and ammonia excretion, so the design case is the hours after feeding rather than the daily average.
• Stress diverts energy away from growth and suppresses immune function, so handling practice has a direct production cost.
• Growth rate determines how the standing biomass, and therefore the treatment load, changes over a production cycle.

The gill is the organ that couples the fish to the water. It is where oxygen is taken up, where carbon dioxide is released, where ammonia is excreted and where osmoregulation is performed. This is why gill health is central, and why a single tissue is affected by suspended solids, ammonia, carbon dioxide and pathogens alike.

• Temperature sets the rate of everything. A system specified at one temperature carries different loads at another.
• Design oxygen and ammonia capacity for the post-feeding peak, not the daily mean.
• Osmoregulation is an energetic cost that varies with the match between water ionic composition and the species' requirements.
• The stress response is adaptive in the short term and damaging when sustained, which is why chronic mild stressors cost more production than occasional acute ones.

• Design around the species' physiological requirements — temperature, oxygen, ionic composition, light and velocity — rather than fitting the species to a generic system.
• Size treatment for the life stage with the highest specific rates, which is usually the smallest fish held.
• Provide water velocities appropriate to the species and size, giving exercise without imposing continuous energetic cost.
• Design handling, grading and transfer facilities to minimise the stress events that a production plan requires.
• Consider light regime and tank colour, which affect behaviour, feeding and stress in ways that are easy to overlook and expensive to retrofit.

• Observe the fish daily. Behaviour, feeding response and distribution in the tank are the earliest available indicators, ahead of any instrument.
• Record feed intake against offered feed. A drop in appetite precedes almost every problem.
• Minimise handling, and where handling is required, plan for the recovery period afterwards rather than resuming normal feeding immediately.
• Sample for weight on a schedule, using a method and sample size that produce a mean worth acting on.
• Interpret any physiological observation together with the water quality record from the same period.

• Record mortality daily by tank, with cause where it can be established.
• Sample weights at a fixed interval using a consistent method, so figures are comparable over the cycle.
• Log feed offered and feed consumed separately.
• Observe and record behaviour at the same time each day, when conditions are comparable.
• Keep the biological record and the water quality record in the same system, so they can be read against each other.

• Designing treatment capacity for average load rather than for the post-feeding peak.
• Assuming metabolic rates scale linearly with biomass, and under-treating nursery systems as a result.
• Treating handling as free, and absorbing the resulting growth loss as normal.
• Interpreting behavioural changes without checking water quality from the same period.
• Sampling too few fish and acting on a mean weight that is mostly sampling error.
• Applying published growth or excretion figures from one species, diet or temperature to another.

Energy budgeting formalises the picture: ingested energy is partitioned between maintenance, activity, waste and growth. It makes explicit why an environmental stressor reduces growth without any change in feeding, and it is the framework within which temperature, velocity and water quality decisions can be compared against one another.

Chronic low-grade stress is the most consequential and least visible production factor. It suppresses immune function and growth simultaneously, and it usually originates in something continuous — a marginal parameter, an ongoing noise or light disturbance, a velocity slightly too high — rather than in any discrete event.

Fish physiology generates every load the engineering exists to handle. Temperature governs the rates, feeding produces the peaks, smaller fish are more intensive per unit weight, and the gill is where the fish meets every water quality parameter at once. Observation of the fish remains the earliest and most sensitive instrument on the farm.