High Test Eternium Fishing Line

The High Test Eternium Fishing Line is an enchanted fishing line that will add +5 fishing to a fishing pole. This effect will not stack with other enchantments on the item.

Source
The High Test Eternium Fishing Line is a quest reward from the quest, which starts at the in Booty Bay in Stranglethorn Vale. To complete the quest, you must turn the following item to the :
 * 1x Dezian Queenfish

You can also get it from from the daily fishing quests given by Marcia Chase in Dalaran.