Oxygen

Dissolved oxygen (DO) is the most important chemical parameter in aquaculture. Because of the low amounts of oxygen saturation in water, fish must ventilate a volume of water which is 10-30 times greater than the volume of air used by most terrestrial animals in order to obtain the same amount of oxygen. The amount of oxygen required for respiration depends on the species, size, feeding rate, activity level, and temperature. Due to their higher metabolic rate, small fish consume more oxygen than larger fish. The amount of oxygen that can be dissolved in water decreases as water temperature decreases with increases in salinities and altitudes. Therefore cold-water fish require higher dissolved oxygen levels compared to warm water fish.

In order to measure the ambient pressure of dissolved oxygen in water, a sensor is used to measure its consumption of oxygen. An oxygen meter relies on a voltage produced by a submersible dissolved oxygen probe. This probe consists of a gas permeable membrane, a cathode and an anode. When a polarizing voltage is passed across the sensor, oxygen diffuses across the membrane and reacts were it is consumed at the cathode. Because of the pressure produced by diffusion and consumption of the oxygen, a current is produced. This current is proportional to the amount of oxygen outside the membrane and generated in either mg L-1 or % saturation.

DO probe

 

(Figure 6: Oxyguard Polaris Probe. Enco Environmental Suppliers, 2009. Viewed 20th October 2014, (http://www.envco.com.au/catalog/product/ysi-handhelds/proodo-handheld-optical-dissolved-oxygen-meter.html).)

Calibration and use of the Oxyguard Polaris probe: for dissolved oxygen & temperature

  • Check membrane, cable and connection
  • Dip in water to moisten membrane
  • Calibrate with air
  • Adjust to 101% saturation
  • Adjust for salinity if need
  • Immerse the electrode
  • Record DO in mg L-1 or % saturation
  • Record temperature

Temperature:

After oxygen, water temperature may be the single most important factor affecting the welfare of fish. Fish are Ectotherms so the temperature of the water affects the animal’s behaviour, feeding, growth and reproduction. Fish can be separated into four categories; cold water, cool water, warm water and tropical. The temperature of the water also has an affect on the amount of dissolved gases (Nitrogen, Oxygen and Carbon dioxide), the cooler the water the more soluble the gas. Temperature can be easily measured with a thermometer (glass or digital), reversible numeric liquid crystal strips or thermocouples.

pH:

pH can be measured with a colourimetric test kit or a pH probe. In order to use the pH probe, it must be calibrated first by using an uncontaminated standard solution and the electrode must be free from biofouling. The acceptable range for fish culture is normally between:

  • pH of 6.5-9.0 for freshwater
  • pH of 7.0-9.0 for estuarine &  marine

The optimal range for pH is:

  • pH of 6.5-7.5 for freshwater
  • pH of 8.0-8.5 for estuarine &  marine

Nitrogen & Ammonia:

Nitrogen and its intermediate forms have the potential to become toxic to aquatic organisms (especially in recirculating systems). Fish excrete ammonia and small amounts of urea into the water. Two forms of ammonia occur in aquaculture systems, ionised and un-ionised. The un-ionised form of ammonia (NH3) is extremely toxic while the ionised form (NH4+) is not. Both forms are can be grouped together as “total ammonia” (TAN).

pH, and to a lesser extent temperature, controls the relative  percentage of total ammonia that is in the toxic un-ionised form.  Through biological processes, toxic ammonia can be degraded to harmless nitrates. The amount of un-ionised ammonia that is detrimental to fish varies within species,  the lethal limits for finish between 0.2-0.5 mg L-1 of un-ionised ammonia in freshwater. This key parameter is easily measured using a colorimetric test or a probe. Ammonia-selective electrodes can be used with a gas permeable membrane; similar to the DO probe. The colourimetric tests use either a colour chart or absorbance and is measured on a spectrophotometer.

NO3 test

(Figure 7: Example of a colourimetric water testing kit. Riddle, D. Advanced Aquarist, 2007. Viewed 20th October 2014, (http://www.advancedaquarist.com/2007/4/review).)