Barb wire stretching...
if it were me i would put a few turnbuckles in there
1/4" X 4" Galvanized Eye & Eye Turnbuckle
now, im guessing there is a certain amount of slack per foot that needs to be taken out, for wich im sure there is a formula that i dont know
id find that out before purchasing the turnbuckles to ensure they will draw them far enough