String

String is as close to nothing as the Desolate Deep gets. A Trash catch, it weighs 0.275 kg on average (0.1 to 0.4 kg) and sells for about C$1.20. Among the 742 documented fish, only four have a lower average value, and its maximum weight ranks 651 of 663. The wiki gives no bait, time or weather requirement, so you can pick it up with whatever you are already using.

It does give 1 base XP, which is the floor of what a catch can give. In practice, you will receive it as a junk result while fishing for something else, not as a target.

The one useful comparison is Stalactite, the other Trash entry in the same zone. Stalactite weighs about thirty times as much and pays roughly twenty times as much, so if you are choosing between junk items, there is no contest. Reinforced Rod and Trident Rod are both sold in the same area, and either one is a better use of your attention than a piece of string.

Verdict: sell it if it lands in your net, and do not waste a single bait on purpose. It is a filler item with no hidden upside in the data.