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Water Temperature and Oxygen saturation

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Florian Rossmark
(@frossmark)
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Joined: 2 years ago
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Why is it that during the hot summer the fish don't bite as much anymore, you see them almost lethargic swimming around, if you can find them. 

Ever wondered why they bite after a heavy shower for a few days, and it stops again during that time?

It all boils down to the oxygen saturation of the water you are fishing.

Fish need oxygen, as we do. The amount of oxygen that the water can hold is directly related to the water temperature. This is also called solubility.

A good graph can be found here: Temperature affects dissolved oxygen concentrations | U.S. Geological Survey (usgs.gov)

You can clearly see as the temperature rises over the month, the oxygen goes down until the temperature falls again and the oxygen goes back up again. Of course, there are fluctuations, these are mainly impacted by weather like strong winds and rain, that bring additional oxygen or might temporarily cool down the water. 

These graphs can NOT be applied 1:1 to our fishing. 

But they can help. 

In general, not every depth of the water will have the same temperature. The deeper you go, the cooler the water will be in summer, in winter it is actually warmer further down than on the surface - hence the surface freezes. 

If you are using e.g., a sonar like a deeper or similar, you only measure the surface temperature of the water. Depending on the depth, the temperature further down might be significantly different. You need a depth to temperature measurement device, like Fish Hawk TD (Amazon.com: Triple S Sporting Supplies, In Fish Hawk Electronics TD Digital at-Depth Water : Electronics) - this device can be cast out, sinks to the bottom and reads about every meter / yard the temperature and lets you read it once you have it back in your hands. 

This also will clearly show the thermocline, if there is one. The whole thermocline is a beast on its own, not going to discuss this today.

As a matter of fact, in theory the oxygen saturation can be better below a certain depth level, because the water above simply is not able to hold as much oxygen as it is too warm. This can now mean that fish stand way deeper and might feed way deeper than usual during these times. Often the bite slows down in general. But deep areas might be your best chance at that point.

On a lake this mostly is straightforward. Of course, there can be additional hotspots if for example a river is feeding water in it and might bring cooler water and oxygen with it.

Rivers are moving water bodies, and the water is generally a bit cooler than a lake during the heat of the summer. The fact that the water is moving also increases the chances that obstacles increase the oxygen levels. Samples would be big and small waterfalls - any white water etc. below these there clearly is going to be an increased oxygen level and you can expect fish to be more active.

The same principle applies to canals and channels or a river that is damned for whatever reason. Any kind of ship lift or damn will have an increased oxygen amount around the area.

If you have issues finding the fish during the hottest time of the year, these are areas to look out for and possibly target. 

There are plenty other samples out there where people saw the fish, but they didn't feed, fishing way deeper than normal often was the only way to catch fish, though the bite is still slow, but not completely off. 

Conclusions

Be aware of where you fish and the weather and other environmental conditions. Keep in mind that the surface temperature is not everything, the temperature changes with depth. Higher temperatures hold less oxygen and can make with uninterested in feeding. You might need to find an oxygen richer area or depth in order to improve your chances.

Florian Rossmark
Germany / USA - Carp Angler


   
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Jeff Bourns
(@jeff-bourns)
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Joined: 2 years ago
Posts: 51
 

Your explanation of the dam effect, spillway agitation increasing oxygen levels, probably explains why I am not catching at my current location. Upstream from where I fish there is a spillway 200 yards or so. I would guess that that is where the buffalo are concentrated currently due to higher water temperature this year versus last. The behavior in Fall last year seemed to be more of a patrolling the canal north to south. I am hoping the cooler temps as of late will find me success soon. 

Great post! Thanks for the info. 


   
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Florian Rossmark
(@frossmark)
Member Moderator Vetted Member
Joined: 2 years ago
Posts: 63
Topic starter  

The Germans have two rivers that had been interesting facts all over in 2022. 

The Main, a river with many damns and water flow control, more like a channel therefor.. 

And the Rhein / Rhine, a shipped river from lake Constance / Bodensee to the northern sea.. fast flowing.

2022 was exceptionally hot and dry in the summer, i was there and tried my luck on a canal i know well and caught a single fish.

As the Main is as mentioned similar to a canal or channel with all the barriers, people normally caught good there but not that year.. it was almost dead...

The Rhein had low water as most others as well, extreme low.. how ever, the catch rate was still up.

In general public the conclusion was it must have been the oxygen levels and to warm water, as the Main warmed easier then the Rhein, wo also flows unhindered. 

Another issue is the Rivers flow one east west and the other south north, but still, this was a generic observation throughout a bigger group of people who didn't catch at the normally good catch rate Main with notice that the Rhein people seemed to have no issues and catch pretty good. 

Main slow, Rhein fast.. water temp on Main likely higher then Rhein, oxygen saturation going along with it.. I don't have fact data at hand, but it was something that was discussed that year over there..

Observe, analyze, understand and apply. Simple principles that work very well 😉 just hoping this all helps here.. think different, think like the fish, understand their needs and habitat and you will succeed. 

Florian Rossmark
Germany / USA - Carp Angler


   
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