Is the fly angler likely to be fishing at the same depth as the spin fisherman / troller? I'd reckon that on a river like the Tay, the answer would be probably not. If going deep, really deep I'd probably go bigger too, think about how Pollock tend to lie up / swim, where are the biggest ones? Makes sense to me really.
Fair comment, but I would say that Pollock (and Pike for that matter) are predators. They lay deep because thats a good ambush point. I also believe that a large shilouette above a predator will cause a response (its worth eating that because its a moutfull) whilst a smaller "lure" would be ignored.
Salmon are not there feeding, they are there to spawn and defend there redds. Again, a large "threat" entering their lie will be treated with a "snap" and a warning bite. Something with more of a threat, ie, bigger and closer, might, just might invoke more of a reaction.
Somewhere on YouTube, and I can't find it just now, there is some very interesting footage of Atlantic Salmon laying in a pool. There are minnows and small fish swimming around nearby, often laying out the flow behind the Salmon. On several occassions something bigger swims into its view and it leaves the lie to chase it away, then drops back to where it was, followed by the smaller fish. One Brown Trout, I guess of about 6oz pops into view and the Salmon grabs it, holds it for perhaps 2-3 seconds and then lets it go.
If that were a 6oz sized fly, would that have been a take?
Needless to say, I am tying up "normal" salmon flies, because they work, but I think I will try a few "different" flies out of interest on the more fly unfriendly water. WHo knows what might happen, could be fun...might even start to hook proped fish like Pike..lol