Jesus picked up the "Fishers of Men" theme from the Old Testament

Whenever someone becomes a Christian they also become a fisher of human
beings. This was not merely a cute metaphor which fit the setting. If you look at
Jeremiah 16:16: Ezekiel 29:4f; 38:4; Amos ·4:2; Habakkuk 1:14-17, you will see
there is a wealth of Old Testament material which serves as the background for
Jesus' use of the phrase "fishers of men".
In the Old Testament God was the fisherman and the image stressed divine
judgment (i.e. if you don't repent you are likely to get a hook in your mouth). By taking up this metaphor Jesus stressed his message of the nearness of the Kingdom and the necessity of responding in radical repentance. The cost of following Jesus involves the willingness to take that message of judgment upon our lips. --from the Mark Study Guide produced by Redeemer Church NYC