Well there is a luck of the draw kinda thing. Pretent that the games script has created alot of numbers. From 1 all the way to 100 and everytime a fish is caught, the script executes and chooses a random number. 1 - 60 is tuna and 61- 100 is swordies. The probability of you getting swordies 2 times in a row is about 1 in a hundred. But in reality, swordies are always randomly mixed with other fishes and its very realistic. With nets, people catch salmon,tuna,and other verieties of fish in real life. Besides, Swordies arnt worth the effort.It would be nice of Jagex to make an amulet or a harpoon have a 100% catch rate of swordies.