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Total gas pressures (TGP, or total dissolved gases, TDG) include the partial pressures of oxygen, nitrogen, carbon dioxide, and argon in the water. The solubility of each gas depends on its properties as well as temperature, salinity, and total pressure. High total gas pressures (supersaturation) occur when the total of the partial pressures is greater than the atmospheric pressure. Supersaturation can cause gas emboli and associated pathology in the vasculature, eyes, or subcutaneous spaces in fish (Smiley et al. 2011). While all gases can show increased partial pressures, nitrogen is the most likely to cause gas emboli because its insolubility makes it more likely to come out of solution within the fish.

Frequency of testing: It is useful to obtain baseline gas pressures for every system; this provides a comparison if issues are seen. Gas pressures may be monitored continuously in intensive aquaculture systems.

Sampling: Gas pressures must be measured on site in the system and represent only that moment in time.

Testing: TGP meters, also known as saturometers, are used (e.g. Point Four™ Tracker, Pentair Aquatic Eco‐Systems, Cary, NC). TGP meters must be kept in the water for >30 minutes as it takes time for the gases to diffuse across the membrane and produce an accurate result; the initial results need to be discarded (Figure A2.3). It is most useful to track results over at least a 24‐hour period to look for transient increases.

Units: TGP is provided in milligrams per liter (mg/L). It is simultaneously reported as percentage saturation (%) relative to atmospheric pressure of air at the water surface.

Figure A2.2 Changes in dissolved oxygen (DO), carbon dioxide (CO₂), and pH across the day due to respiration and photosynthesis.

Figure A2.3 Total gas pressure meter readings from a fish system showing the initial values that are related to calibration and should be discarded.

Target values: TGP should ideally be 90–100%, but <105% is a common target (Table A2.2). Values >110% are likely to cause pathology.

Practical considerations:

 High TGPs are problematic as they can cause gas emboli in fish.

 High TGPs are often due to entrainment of air into a pump or valve due to a crack or leak upstream, turbulent water flow (particularly when at depth), or poorly designed gas‐exchange towers (e.g. poor water distribution, poor selection of gas‐exchange media, flooded gas‐exchange media, or air entrainment leaving the tower).

 Single TGP values are less useful than serial monitoring over time (e.g. over 24 hours); supersaturation events can be transient and may be missed with a sporadic sampling schedule. The impact of corrective action is also assessed using serial monitoring.

 Low TGPs are unusual. They may be associated with low dissolved oxygen or low water flow and warrant investigation.

Further discussion of gas supersaturation is available in Chapter C1.