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Weather and Mysterious Fish Deaths

I would like to expand on the earlier post about flow rates for oxygen requirement in aquaculture and aquaponic fish tanks.  In the article “Oxygen, Water Flow and Your Fish” I included reference to atmospheric pressure and showed how a drop in air pressure, such as a low pressure weather system, can adversely affect the amount of oxygen available in your fish tanks. This is really relevant for those in Northern Australia at this time.  We have a tropical low over inland Queensland and a cyclone on the north coast of Western Australia.  These systems combined with the humidity this time of year and warmer weather can cause mysterious fish deaths in tanks. In fact whole farms of pond cultured fish have been known to stop feeding completely during these weather systems which can be related to lower oxygen availability and reduced energy in the fish. Take the following weather pattern here in Australia today : I have added color to highlight different pressures across Australia and of interest in the red zone that is travelling east and effecting the northern tropics and may, in the next few days, effect South East Queensland (where I live).  If you were currently in that red area, you will also have high temperatures this time of year as shown by the map below.  These higher air temperatures will be pushing that tank water temp. up around 30oC, if they have not already done so. To illustrate how the weather will impact on your backyard or commercial fish farming I have put together yet another fine table to break down the differences in flow rate requirements based on todays estimates of temperature (30 to 36oC) and the current weather. Weather zone colors on the top map from BOM Height Above Sea Level (M) Red Orange Green 0 21.10 20.20 19.70 500 22.40 21.50 21.00 1000 24.00 23.00 22.40 Flow rates (LPM) required to maintain 5mg/L oxygen at 30oC per 100 grams of fish feed per day You can see there is a small difference (1LPM) from the red to the green zones on the map.  60 liters per hour per 100 grams of feed is nothing to get too concerned about and start changing out pumps and reconfiguring your set up.  We are purely providing a visual example of what happens to give you food for thought. Example: Imagine we have our 100kg of fish again and we are feeding our fat 500 gram fish their 1kg per day (1%bw/d) and we are on the Atherton Tablelands in Far North QLD with that low pressure heading our way.   I have my 20kg per 1000 liters so I have a culture tank volume of 5000 liters. I had been quite clever and planed my pumping flow rates for normal sea level air pressure (1013.25mbar) and running my 11,820 liters per hour and exchanging my tanks at least 2.4 times per hour.  Above two we are sitting pretty. Now I have this weather coming in on me and it will be here for at least a week.  Because I am 1000 meters above sea level and that nasty looking red zone is heading over the top of me and every day the air pressure is dropping, my pumping requirements changed a bit to 14,400 liters per hour.  This means my exchange rate will need to be somewhere near 2.9 timer per hour. This really is not much of a concern, just an extra bit of pumping volume.  I may get away with turning the air up a bit and keeping the tanks as cool as I can.  However, imagine if I was running one tank exchange per hour.  That could paint a very smelly story.  Ones we often see. Food for thought Paul…

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