After catching a 60", roughly 140 lb. tuna the other day, I checked over all my equipment for wear or problems. The leader I used was 80# Seaguar fluorocarbon connected to a 250# Spro barrel swivel, using a 4-turn Uni knot. The drag I used throughout the fight, which lasted 8 minutes, was the initial setting of 18 pounds. That means the maximum load on the line, after 150 yards was taken on the fist run, was 25-30 pounds. The Uni knot held fine and is no sign of damage or slipping. (The knot had been pretested to 40 pounds or so.) However, the area just above the tag of the knot had some damage that you can see in the photo below (when you feel the area, it is noticeably thinned). I don't know if the damage was caused by fighting the fish or by casting for several hours but it is clear that the tag did cause the fraying. There was also some thinning of the leader in the area of the fish's gill plate which leads me to feel that I should be using a heavier leader. Interesting that the Uni knot tag put more wear on the main line that the fish's gill plate. It seems that fluorocarbon can cut fluorocarbon. (I don't know if this would happen with mono.) I usually prefer a knot where the tag exits parallel to the main line, if the knot will be going through the guides, so that the tag doesn't catch as it moves through the guides. Contact with the guides can weaken a knot and I've seen shock leaders fail during casting from knot to guide contact. However, I now feel that for my terminal tackle knot I should use a knot that has the tag exiting perpendicular to the main line, or at least the tag is not touching a critical point. Lastly, I have read before that a leader should be replaced after each good fish. I think that is advice I will follow.