Bear in mind that sometimes you may need that 15lb line because of gravel bars etc. Lighter line may be more susceptible to fraying. The sensible thing to do is that whatever breaking strain of line you use make sure the clutch on your reel gives when the rod is almost at its 90 degree curve limit. The pressure exerted on a fighting fish is quite tremendous from a 2lb TC rod. Don't forget it has nothing to do with the actual weight of the fish as you obviously would not be doing a dead lift with the rod.
I may be wrong, but due to displacement don't fish weigh 1/3 less in water? That being the case 15lb line would give you an upper limit of 45lb..... It would need a fairly clear water to bring that in on a 2lb TC rod
Glyn - you should also be aware of the casting weight limitations of a 2lb TC rod (I'm sure one of our more scientific colleagues will give you the formula) but basically you are more likely to overload (break) the rod with excessive casting weight than you ever are playing and landing a fish
There are a lot more factors that go into this. For example, too much pressure on a fish can merely end up in a hook pull or even straightened. Some may disagree with me but I feel that when you start going over the 2lb test curve range in rods the higher power is more for achieving casting dsitance than playing a fish. Yes, with a 3lb TC rod and suitably high breaking strain line you could just haul in most fish like a crane. If, of course, that hook did not pull But where is the fun in that? By making a rod of that test really work you'd be applying an awful lot of pressure. Not only on the fish but also on yourself. In effect, to really make the rod work - I mean utilise the inbuilt spring power of the rod - constantly would wear you out. It is easier to let a rod of say 2lb TC bend into a fish and work than it is to do the same with a 3lb TC. It's all about applying pressure at the right time in the right way. I took my PB barbel of 10lb on an old Shakey amber Alpha ledger. These rods were only about 1lb TC at most. The swim did hold some snags but the fight was not overly long - 8 minutes max - and was enjoyable. At no time did I feel out of control. I used all the old advice like applying pressure in opposite directions to urge the fish to go the way I wanted it. Ok, I know this does not always go to plan but more often than not it works for me.
Hook pulls and hook bending/breaking are not likely to happen if you get a good hookhold. Look at the power in a feeder rod that can cast 6oz of loaded feeder across a big river. These are often used with 18's hooks. Compare that with a 3lb rod and a 6's or 4's hook and there's much more pressure on the weaker hook with the feeder rod. Any hook that isn't bedded up to the bend will pull or break/bend. Of course, you have to be sensible and use hooks made from thicker wire, such as Drennan Super-Spades and Super Specialists.
It also depends on what you mean by most fish. I was recently catching carp and catfish from snags to over 30lb with a 3lb rod and 18lb line. With that tackle I turned over a 50lb-plus cat after its first run and only lost it because the barbless hook pulled when it's tail came down on the line and gave it a split second of slack.
And believe me, it was fun. If you call a racing heart, aching arms and skidmarks fun, that is!
It's all relative.
On the Dove last season I lost one or two fish from snags that were bottoming out a 1.5lb rod on 10lb line and breaking the line. Stepping up the rod, with the same line, was the answer. Hook pulls were not a problem.
Glyn I've read (in a number of sources) that a rough guide is to multiply the test curve by 4 to find a rod's lower line limit & by 6 to get the highest strength line.
i.e. take 2lb test curve;
4 x 2 = 8lb line
6 x 2 = 12lb line
the optimum would be;
5 x 2 = 10lb line.
Again this has to be taken into consideration with rod & fishing conditions.
People who use braid & super fine lines will often use braking strains outside the guidelines for that rod's test curve.