Understanding Aeration for Marine Livewells & Baitwells

O2 Solubility - The common currency of all aeration systems

Air is about 21% Oxygen 78% Nitrogen and the rest various stuff that fish really don’t care about.
Water solubility of pure oxygen at 25°C and pressure = 1 bar is ~ 40 mg/L water. In air with a normal composition the oxygen partial pressure is 0.2 atm. This results in dissolution of 40 x 0.21 ≈ 8.4 mg O2/L in water that comes in contact with air.
Oxygen solubility is strongly temperature dependent and decreases at higher temperatures. Oxygen solubility is negatively correlated with the amount of dissolved solids.
Consequently, oxygen solubility in freshwater exceeds that in seawater by 1-3 mg/L, depending on temperature.
Bottom line – in freshwater, we can achieve a baseline of ~8.4 mg/l oxygen at 25°C in water. This is our maximum replenishment goal

Air Derived Saturation in mg/l (ppm)

Air = ~21% Oxygen

Air derived saturation = ~8.4 mg/l Oxygen @25°C

Fish O2 Consumption: Oxygen use in livewells

Various studies dating back to the 1970’s and earlier have investigated the amount of oxygen used by aquatic animals. The amount of oxygen consumed by these aquatic animals varies with species, size, temperature, time since feeding, degree of physical activity and other factors. One recent study from Auburn University’s Department of Fisheries and Allied Aquacultures determined the average oxygen consumption rates for adult fish usually are between 200 and 500 mg oxygen/kg fish/hour.

Auburn University goes on to additional oxygen use assumptions and results. They make comparison with game fish vs. bait fish. Large 500-gram channel catfish were reported to use 480 mg oxygen/kg fish/hour. In contrast, 10-gram channel “bait” catfish consume 1,050 mg oxygen/kg fish/hour.

Oxygen consumption is also strongly dependent upon temperature – the rate roughly doubles with a 10 degrees-C increase in temperature within the temperature tolerance of the species (confirmed by Schramm & Heidinger 1988). Although less data are available on oxygen consumption by crustaceans, it appears that shrimp consume oxygen at rates similar to those of fish.

500 mg O2/kg fish
per hour for calm fish
&
1000 mg O2/kg fish
per hour for stressed fish
as a benchmark for assessment

Results from recent fisheries research publications:

Aquatic species research has further refined oxygen consumption rates needed to maintain healthy livewell and baitwell environments.

Fish O2 Consumption: Recommended O2 range in live wells:

Recent Texas Parks and Wildlife (TPWD) studies and corresponding recommended
dissolved oxygen levels can be summarized thusly:

The most important factor relative to minimizing stress and mortality on livewellheld
bass is dissolved oxygen. Warmer water naturally holds less oxygen than
cooler water. At a moderate water temperature of 70˚F (21°C), 100% air derived
oxygen saturation is 8.9 mg/L (ppm). Saturation rates drop at higher temperatures
of 80 (26.6) and 90˚F (32.2°C), to 8.0 and 7.3 mg/L.
Oxygen levels less than 5.0 mg/L are harmful to bass, especially if allowed to
remain this low over extended periods.
TPWD defines sufficient dissolved oxygen levels to be 7.0 mg/L or greater.
Therefore, the attainable and acceptable range of air supplied dissolved oxygen is
between 8.4 and 5 mg/l at 25°C (standard pressure and temperature) [see graph].

Air derived saturation = ~8.4 mg/l Oxygen @25°C

Minimum Allowable = 5mg/L O2 @ 25°C

The Maths of Livewells: Recommended O2 range in live wells:Fish Consumption of O2 within a Livewell

30 gallon live well = 113.5 liters

9.25 mg/liter O2 @ 20°C (1 atm – solubility) [Chemistry]

Minimum allowable O2 level = 5 mg/liter [TPWD]

Available O2: (9.25 – 5) x 113.5 = 482.4 mg

6 (21-inch bass) * 6 lbs = 16.33 kg / bass

O2 consumption 500 mg/kg fish – hour [Biology]

6 ave. bass have ~3.5 minutes of oxygen w/out replenishment before dropping below 5 mg/L O2 (in a 30 gallon livewell).

3.5 minutes before a dangerous situation is presented to the six, captive fish!

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