for emergency survival situations in my view the following criteria is applicable:- long life as you say, very lightweight as you don't want to hump something around that may or may not be used, effective and dependable and at your disposal for your immediate use at any moment instantly, and the quicker the 'catch' the better - a small gill net is brilliant, it doesn't need to be baited so will catch whether the fish are feeding or not and you don't have to grub around for bait, it can be used instantly, it weighs very little and it packs small and unnoticed in your pack, easily placed anywhere from a canoe and moved around at will, gill nets are seriously effective, the mesh size is important and a full mesh size (knot-to-knot) of 2.5'' will catch for example trout of between roughly 8oz to 24oz, of course different fish different shape but about 2.5'' give or take a bit either way is a good average to cover, bigger if you want bigger fish but there are generally more smaller than bigger fish, smaller if you want smaller fish, 3'' will catch trout from 16oz to about 40oz for example. In length just what you are happy carrying, of course the bigger the better but you will want to limit the size obviously, 30' x 8' is not too big for emergency but smaller is ok if you feel that too big, even 15' x 6' will be sufficient in emergency and that will weigh next to nothing in monofilament.