Fish physiology governs how fish respond to water quality, temperature, oxygen levels, salinity and the culture environment. Understanding gills, osmoregulation, metabolism and stress response makes it easier to optimise both fish health and production results.
Gills
The gills are the fish’s most important organ for contact with surrounding water. They are responsible not only for respiration but also for salt regulation, acid–base balance and excretion of nitrogenous waste.
The gills are used among other things for:
- Uptake of oxygen (O₂).
- Excretion of carbon dioxide (CO₂).
- Ion regulation (Na⁺, Cl⁻, K⁺, Ca²⁺ and others).
- Regulation of acid–base balance.
- Excretion of ammonia (NH₃/NH₄⁺).
Because gill tissue is very thin to allow efficient gas exchange, it is also sensitive to environmental disturbance.
Common causes of gill damage
- High particle levels (TSS).
- Ammonia (NH₃).
- Nitrite (NO₂⁻).
- Residual ozone.
- Hydrogen peroxide.
- High CO₂ levels.
- Gas supersaturation.
- Algae and organic fouling.
- Bacteria and parasites.
When the gills are damaged, oxygen uptake deteriorates, which often leads to reduced growth, poorer FCR and higher disease sensitivity. See also dissolved oxygen, ammonia, nitrite, CO₂ and turbidity/TSS.
Osmoregulation
Osmoregulation is the fish’s ability to control water and salt balance in the body. It is an energy-demanding process that runs continuously.
Freshwater fish
Freshwater contains far fewer salts than fish body fluids. Water therefore diffuses into the fish and salts diffuse out; the fish produces large volumes of dilute urine and the gills actively take up ions from the water. Species such as Arctic char, rainbow trout, brown trout and perch use large amounts of energy to maintain salt balance.
Seawater fish
In seawater the situation is reversed: the fish loses water to the surroundings, salt diffuses in, the fish drinks seawater and excess ions are pumped out via the gills. Examples include sea bass, sea bream and turbot.
Brackish water
Brackish water sits between fresh and seawater and can in some cases mean lower energy demand for osmoregulation. For several species the right salinity can reduce physiological stress, improve growth, improve feed utilisation and increase survival. See salinity.
Minerals and ions
Besides salinity, ion composition matters. Important ions include calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), chloride (Cl⁻) and bicarbonate (HCO₃⁻). These affect osmoregulation, nerve function, muscle function, skeletal development and egg/embryo development. Very soft waters may therefore need remineralisation for sensitive species or life stages. See mineral and ion management and hardness.
Metabolism and energy
Fish are ectothermic — body temperature follows water temperature. As temperature rises, metabolism, feed ration, oxygen consumption and growth increase up to the species optimum. If temperature becomes too high, stress rises, oxygen demand increases sharply, appetite may fall and mortality may rise. Each species therefore has a specific temperature range where growth is best. See temperature.
Stress
Stress is a natural physiological reaction, but prolonged chronic stress is one of the most common causes of production problems in aquaculture.
Common stress factors
- Poor water quality.
- Oxygen shortage.
- High ammonia levels.
- High CO₂ levels.
- Overcrowding.
- Handling and transport.
- Rapid temperature changes.
- Incorrect salinity.
- High noise or light disturbance.
Effects of chronic stress
- Reduced growth.
- Poorer FCR.
- Suppressed immune system.
- Increased disease sensitivity.
- Increased mortality.
- Impaired reproduction.
Many of the economic losses in aquaculture are directly or indirectly linked to chronic stress.
Reproduction and maturation
Reproduction is controlled by complex environmental signals. Important factors include photoperiod (day length), temperature, nutrient availability, salinity and hormonal signals. Many farms use programmed lighting to control maturation, spawning periods and growth.
Broodstock
Broodstock often need different environmental conditions than production fish. Typical measures include controlled light programmes, special temperature regimes, specialised feeds, lower stress levels and careful health control. Good broodstock management affects egg quality, fertilisation rate, hatch rate, larval survival and growth of the next generation.
Why physiology matters in aquaculture
Almost all technical systems on a farm ultimately aim to support fish physiology. Oxygenation, biofilters, temperature control, degassing, UV systems and feeding are about giving the fish optimal conditions for gas exchange, osmoregulation, growth and reproduction. When physiology works well, more energy goes to growth and less to coping with stress, which gives better fish health, higher survival and lower production cost.
See also