Thermocline notes

[Gerrit Viljoen]

Elandskloof for example is more than 30 metres deep at the dam wall, and it is a known fact that all the big trout congregate there, in the cool oxygen rich water.

I'm confused now,surely by going deeper the oxygen levels starts to decrease?

Please correct me if I'm wrong.

Thanks

[Gerrit Viljoen]

Elandskloof for example is more than 30 metres deep at the dam wall, and it is a known fact that all the big trout congregate there, in the cool oxygen rich water.

I'm confused now,surely by going deeper the oxygen levels starts to decrease?

Please correct me if I'm wrong.

Thanks

[Chris Shelton]

I'm confused now,surely by going deeper the oxygen levels starts to decrease?

Please correct me if I'm wrong.

Thanks

It depends on various factors like time of year (season), wind action, etc. but basically cold oxygen rich water is denser than warm water. Wind action causes the surface temperature to drop and this colder water then slowly sinks (circulates) to the bottom. In a nutshell then, you would have a layer of cold oxygen rich water above and below warmer water which is seperated by the thermocline. If there is no cooling wind action on the surface, the warmer oxygen depleted water would rise to the surface, trapping the colder heavier oxygen rich water underneath it.

[Jasper]

Evening Chris, may i ask is there an depth limit for trout or can they be fished to any depth, if there is a limit does this change with the seasons(i.e hotter = deeper) or is it constant.

In the days of Ida's Valley fishing .. many trout (big ones) were caught by (unnamed) float tube anglers letting ALL the line off their reels - fly line as well as backing .. often with some lead-core line at the tip ... and there were lots of fish down there!!

There is definitely an observable thermocline in the deep areas (off the cliffs) at Lakenvlei .. if you can find the right depth that's where lots of trout concentrate.

[Chris Shelton]

the following is Wikipedia's take on thermoclines. I find this stuff fascinating.

Thermoclines can also be observed in relatively shallow lakes. In colder climates, this leads to a phenomenon called turnover. During the summer, warm water, which is less dense, will sit on top of colder, denser water that sinks to the bottom; with a thermocline separating them. Because the warm water is also exposed to the sun during the day, a stable system exists, and very little mixing of warm water and cold water occurs. One result of this stability is that as the summer wears on, there is less and less oxygen below the thermocline, as the water below the thermocline never circulates to the surface. As winter approaches, the temperature of the surface water will also drop until it approaches 4 °C (39 °F), which is the temperature at which water is densest. 4 °C is, generally speaking, the temperature of the water below a thermocline. When the entire body of water is at or close to 4 °C, 'fall turnover' begins - the thermocline disappears, (or, to say a different way, it reaches the surface) and the water from the bottom of the lake can mix freely with the water from the top. This process is aided by wind or any other process that agitates the water. This effect also occurs in Arctic and Antarctic waters, enriching surface nutrients and producing blooms of phytoplankton, making these very rich and diverse ecosystems.

As the temperature continues to drop, in those locations where it does, the water on the surface begins to get cold enough to freeze and the lake begins to ice over. A new thermocline develops where the densest water (4 °C) sinks to the bottom, and the less dense water (water that is approaching the freezing point) rises to the top. Once this new stratification establishes itself, it lasts until the water warms enough for the 'spring turnover,' which occurs after the ice melts and the surface water temperature rises to 4 °C.

Waves can occur on the thermocline, causing the depth of the thermocline as measured at a single point to oscillate (usually as a form of seiche). Alternately the waves may be induced by flow over a raised bottom, producing a thermocline wave which does not change with time, but varies in depth as one moves into or against the flow.

[chris williams]

Thermocline I think we discussed before, but as per chats with Andre, it plays a huge factor in deep big dams. In England we used to chuck out a pair of thermometers, the one near the surface and the other on the bottom. In simple terms when we hit the thermocline we'd often do poor man's CZNyphing with Hi-D lines and a short leader with say a leprechaun (green matuka) or orange Whisky fly for a daphnia simulation and jig the thing up and down - not v sporting but lethal in competitions...

[Chris Shelton]

Thermocline I think we discussed before, but as per chats with Andre, it plays a huge factor in deep big dams. In England we used to chuck out a pair of thermometers, the one near the surface and the other on the bottom. In simple terms when we hit the thermocline we'd often do poor man's CZNyphing with Hi-D lines and a short leader with say a leprechaun (green matuka) or orange Whisky fly for a daphnia simulation and jig the thing up and down - not v sporting but lethal in competitions...

hi Chris. Tell me, how do you go about measuring the temperature down there? Remote device?

[chris williams]

I stick my willy down there.

Seriously, thermometer on a length of Maxima and put permament marker every metre and count it off on the backhaul. I must admit I havn't been in the appropriate conditions in South Africa where I've had to do this, but it's a useful tip if you're fishing deep water.

As an aside, we had an Essex boy (Pom equivalent of Ferdi) who used to fish Hanningfield reservoir. They had big trout cages in very deep water, and the uneaten pellets used to filter down into the depths below. This guy used to fish with a leaded 'pellet fly' with about a 100 foot leader down to the thermocline and attach a strike indicator to the bit next to the floating fly line. Not pretty fishing but he certainly caught more than his fair share of ten pounders!!

[Jasper]

hi Chris. Tell me, how do you go about measuring the temperature down there? Remote device?

Hi Chris
The good boat shops sell thermometers .... and like Chris W says .. mark the line (I used to do it in 5m intervals)

[nicholas]

I stick my willy down there on a length of Maxima and put permament marker every metre and count it off on the backhaul.Not pretty fishing but caught my fair share of 10 pounders

is'nt this what you really wanted to say chris.