Choosing the size of fly for cols water.

Hibernicus

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A question for the experienced catchers!
How much bigger should the salmon fly be where one is fishing the same conditions in very different cold water temperatures.
For example if you successfully fish say a size 8, cast at a certain angle, on a cool day in late Spring with a sink tip in a nice run of say 3 or 4 feet depth and a water temperature of say 55 degrees Fahrenheit (c. 12.5 degrees Centigrade); what size fly would you expect to have used two months earlier in the same conditions, save the water temperature was say 40 degree Fahrenheit (c. 4.5 degrees Centigrade)?
H
PS. I meant to say cold water in the title!
 

Sash

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@Stevieob76 VERY kindly sent me his old Hardy “Arkless” salmon fly and lure calculator a few weeks ago.

And so I set out to use it with your data: the results were really interesting:
- assuming clear water, average flows in both cases, your datum of 55 degree temperatures indeed point straight to a size 8 fly, or a 3/4” tube.
- the lower temperature gives a 1/0 single hook (!), or a 1.5” tube

To my mind, a doubling of tube fly length (I assume the length is the tube itself, so the actual fly could be up to an inch longer with hook(s) and buck tail wing) seems quite conservative: I would happily use a longer fly, whether double with a long flowing tail, or a bigger tube.

But, then, that’s why I got the Arkless in the first place, to provide a check to my assumptions!
 
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