Best depth?

[nottskev]

I think it's the combination of factors, as some have said. It won't be hard to guess correctly how many bream I caught that day

There are some good rules of thumb about conditions - temperatures, depths, light levels, wind direction etc - but fish can still surprise us. Feed a swim for a bit and there are often small fish above bigger ones. But sometimes the bigger fish are higher up in the water.

 

[Philip]

Hydrostatic pressure is much more intense than air pressure due to water being considerably denser than air. So, if a fish changes its depth even a couple of feet up or down it will go through more of a pressure change in seconds than any of us will ever experience on dry land. How could the barometric pressure above the surface affect a fish that goes through equal to a surface pressure change if they move only a few inches up, or down in the water.

Exactly & well said ...thats why its more likley the conditions associated with high air pressure cause any changes in the way they feed rather than the air pressure itself.

In terms of water pressure, I recall fishing a deep lake in the Alps that it took a count of 15 for a 4oz lead to hit bottom ...with a stringer which would have slowed it down a bit.. but the fish where quite happy to feed there.

 

[fishface1]

In winter particularly, high pressure allows a thermocline to develop. This could also lead to oxygen depletion below the warmer water, if persistent enough. Both good reasons why certain fish would then prefer the upper layers.

Not so obvious in weedy lakes in the summer, but fish like carp just seem to enjoy the sun on their backs!

 

[no-one in particular]

Interesting replies, thanks, I think all sorts of science can be associated with fishing and it gives us something to think about now and then.

 

[108831]

Preferring the upper layers doesnt equate to feeding there,roach often sit below the surface heads down,tails up,just waiting for the appropriate times,or conditions to feed.

 

[Keith M]

It appears that there are a couple of things that might be being missed in some of the above posts:

1. Air pressure lying above water can and often does have an effect on the water pressures below it.
2. Warm water holds a lot less oxygen than cold water does.
3. Water is at its densest/heaviest when it reaches 39°F (or 3.98°C to be exact) which is why it’s possible to get warmer water lying underneath colder water during the winter months. Although the suns rays; after being reflected off a surface (such as a fish) can start to radiate heat so fish can feel warmer nearer the surface on a cold winters day when the sun is shining.
4. Thermoclines have little or no effect on shallowish lakes.

NB: I studied Meteorology and Oceanography and served in the RN as a Meteorologist and Oceanographer all over the globe back in the 1970s, however I’ve probably forgotten a lot more than I remember after all these years away from that job.

Keith