The fishermen mourn because the fish have died due to the drying Nile River. There are three parallel lines in this verse, which New Revised Standard Version shows clearly:

Those who fish will mourn;
all who cast hooks in the Nile will lament,
and those who spread nets on the water will languish.

The three Hebrew verbs rendered mourn, lament and languish all begin with the same letter and are similar in sound. This further heightens the parallelism, but these two features of the Hebrew text almost certainly cannot be mirrored in other languages.

The fishermen will mourn and lament, all who cast hook in the Nile: Revised Standard Version places the verb lament with the first line, but it should go with the second one (see New Revised Standard Version above). The fishermen is parallel to all who cast hook in the Nile. Good News Translation partly combines these two phrases by saying “Everyone who earns a living by fishing in the Nile.” It expresses hook in the second half of the verse. All who cast hook may be rendered “all who cast hook and line.”

And they will languish who spread nets upon the water adds to the description of the dejected fishermen. The method of fishing here is by casting nets, whether from a boat or from the shore. The Hebrew verb rendered languish often means “be exhausted,” but here it means “be discouraged” (Contemporary English Version). New Jerusalem Bible and Revised English Bible say “lose heart,” which is a good model.

For the translation of this verse consider the following examples:

• The fishermen will mourn;
all those who cast hook and line into the Nile will lament,
and those who cast nets upon the water will lose heart.

• Fishermen will mourn;
all who fish with hook [and line] in the Nile will lament,
and those who fish with nets will give up.

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .